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linux/kernel/futex.c
Thomas Gleixner 51cf94d168 futex: Make syscall entry points less convoluted 
The futex and the compat syscall entry points do pretty much the same
except for the timespec data types and the corresponding copy from
user function.

Split out the rest into inline functions and share the functionality.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210422194705.244476369@linutronix.de
2021-05-06 20:19:04 +02:00

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// SPDX-License-Identifier: GPL-2.0-or-later
/*
* Fast Userspace Mutexes (which I call "Futexes!").
* (C) Rusty Russell, IBM 2002
*
* Generalized futexes, futex requeueing, misc fixes by Ingo Molnar
* (C) Copyright 2003 Red Hat Inc, All Rights Reserved
*
* Removed page pinning, fix privately mapped COW pages and other cleanups
* (C) Copyright 2003, 2004 Jamie Lokier
*
* Robust futex support started by Ingo Molnar
* (C) Copyright 2006 Red Hat Inc, All Rights Reserved
* Thanks to Thomas Gleixner for suggestions, analysis and fixes.
*
* PI-futex support started by Ingo Molnar and Thomas Gleixner
* Copyright (C) 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
* Copyright (C) 2006 Timesys Corp., Thomas Gleixner <tglx@timesys.com>
*
* PRIVATE futexes by Eric Dumazet
* Copyright (C) 2007 Eric Dumazet <dada1@cosmosbay.com>
*
* Requeue-PI support by Darren Hart <dvhltc@us.ibm.com>
* Copyright (C) IBM Corporation, 2009
* Thanks to Thomas Gleixner for conceptual design and careful reviews.
*
* Thanks to Ben LaHaise for yelling "hashed waitqueues" loudly
* enough at me, Linus for the original (flawed) idea, Matthew
* Kirkwood for proof-of-concept implementation.
*
* "The futexes are also cursed."
* "But they come in a choice of three flavours!"
*/
#include <linux/compat.h>
#include <linux/jhash.h>
#include <linux/pagemap.h>
#include <linux/syscalls.h>
#include <linux/hugetlb.h>
#include <linux/freezer.h>
#include <linux/memblock.h>
#include <linux/fault-inject.h>
#include <linux/time_namespace.h>
#include <asm/futex.h>
#include "locking/rtmutex_common.h"
/*
* READ this before attempting to hack on futexes!
*
* Basic futex operation and ordering guarantees
* =============================================
*
* The waiter reads the futex value in user space and calls
* futex_wait(). This function computes the hash bucket and acquires
* the hash bucket lock. After that it reads the futex user space value
* again and verifies that the data has not changed. If it has not changed
* it enqueues itself into the hash bucket, releases the hash bucket lock
* and schedules.
*
* The waker side modifies the user space value of the futex and calls
* futex_wake(). This function computes the hash bucket and acquires the
* hash bucket lock. Then it looks for waiters on that futex in the hash
* bucket and wakes them.
*
* In futex wake up scenarios where no tasks are blocked on a futex, taking
* the hb spinlock can be avoided and simply return. In order for this
* optimization to work, ordering guarantees must exist so that the waiter
* being added to the list is acknowledged when the list is concurrently being
* checked by the waker, avoiding scenarios like the following:
*
* CPU 0 CPU 1
* val = *futex;
* sys_futex(WAIT, futex, val);
* futex_wait(futex, val);
* uval = *futex;
* *futex = newval;
* sys_futex(WAKE, futex);
* futex_wake(futex);
* if (queue_empty())
* return;
* if (uval == val)
* lock(hash_bucket(futex));
* queue();
* unlock(hash_bucket(futex));
* schedule();
*
* This would cause the waiter on CPU 0 to wait forever because it
* missed the transition of the user space value from val to newval
* and the waker did not find the waiter in the hash bucket queue.
*
* The correct serialization ensures that a waiter either observes
* the changed user space value before blocking or is woken by a
* concurrent waker:
*
* CPU 0 CPU 1
* val = *futex;
* sys_futex(WAIT, futex, val);
* futex_wait(futex, val);
*
* waiters++; (a)
* smp_mb(); (A) <-- paired with -.
* |
* lock(hash_bucket(futex)); |
* |
* uval = *futex; |
* | *futex = newval;
* | sys_futex(WAKE, futex);
* | futex_wake(futex);
* |
* `--------> smp_mb(); (B)
* if (uval == val)
* queue();
* unlock(hash_bucket(futex));
* schedule(); if (waiters)
* lock(hash_bucket(futex));
* else wake_waiters(futex);
* waiters--; (b) unlock(hash_bucket(futex));
*
* Where (A) orders the waiters increment and the futex value read through
* atomic operations (see hb_waiters_inc) and where (B) orders the write
* to futex and the waiters read (see hb_waiters_pending()).
*
* This yields the following case (where X:=waiters, Y:=futex):
*
* X = Y = 0
*
* w[X]=1 w[Y]=1
* MB MB
* r[Y]=y r[X]=x
*
* Which guarantees that x==0 && y==0 is impossible; which translates back into
* the guarantee that we cannot both miss the futex variable change and the
* enqueue.
*
* Note that a new waiter is accounted for in (a) even when it is possible that
* the wait call can return error, in which case we backtrack from it in (b).
* Refer to the comment in queue_lock().
*
* Similarly, in order to account for waiters being requeued on another
* address we always increment the waiters for the destination bucket before
* acquiring the lock. It then decrements them again after releasing it -
* the code that actually moves the futex(es) between hash buckets (requeue_futex)
* will do the additional required waiter count housekeeping. This is done for
* double_lock_hb() and double_unlock_hb(), respectively.
*/
#ifdef CONFIG_HAVE_FUTEX_CMPXCHG
#define futex_cmpxchg_enabled 1
#else
static int __read_mostly futex_cmpxchg_enabled;
#endif
/*
* Futex flags used to encode options to functions and preserve them across
* restarts.
*/
#ifdef CONFIG_MMU
# define FLAGS_SHARED 0x01
#else
/*
* NOMMU does not have per process address space. Let the compiler optimize
* code away.
*/
# define FLAGS_SHARED 0x00
#endif
#define FLAGS_CLOCKRT 0x02
#define FLAGS_HAS_TIMEOUT 0x04
/*
* Priority Inheritance state:
*/
struct futex_pi_state {
/*
* list of 'owned' pi_state instances - these have to be
* cleaned up in do_exit() if the task exits prematurely:
*/
struct list_head list;
/*
* The PI object:
*/
struct rt_mutex pi_mutex;
struct task_struct *owner;
refcount_t refcount;
union futex_key key;
} __randomize_layout;
/**
* struct futex_q - The hashed futex queue entry, one per waiting task
* @list: priority-sorted list of tasks waiting on this futex
* @task: the task waiting on the futex
* @lock_ptr: the hash bucket lock
* @key: the key the futex is hashed on
* @pi_state: optional priority inheritance state
* @rt_waiter: rt_waiter storage for use with requeue_pi
* @requeue_pi_key: the requeue_pi target futex key
* @bitset: bitset for the optional bitmasked wakeup
*
* We use this hashed waitqueue, instead of a normal wait_queue_entry_t, so
* we can wake only the relevant ones (hashed queues may be shared).
*
* A futex_q has a woken state, just like tasks have TASK_RUNNING.
* It is considered woken when plist_node_empty(&q->list) || q->lock_ptr == 0.
* The order of wakeup is always to make the first condition true, then
* the second.
*
* PI futexes are typically woken before they are removed from the hash list via
* the rt_mutex code. See unqueue_me_pi().
*/
struct futex_q {
struct plist_node list;
struct task_struct *task;
spinlock_t *lock_ptr;
union futex_key key;
struct futex_pi_state *pi_state;
struct rt_mutex_waiter *rt_waiter;
union futex_key *requeue_pi_key;
u32 bitset;
} __randomize_layout;
static const struct futex_q futex_q_init = {
/* list gets initialized in queue_me()*/
.key = FUTEX_KEY_INIT,
.bitset = FUTEX_BITSET_MATCH_ANY
};
/*
* Hash buckets are shared by all the futex_keys that hash to the same
* location. Each key may have multiple futex_q structures, one for each task
* waiting on a futex.
*/
struct futex_hash_bucket {
atomic_t waiters;
spinlock_t lock;
struct plist_head chain;
} ____cacheline_aligned_in_smp;
/*
* The base of the bucket array and its size are always used together
* (after initialization only in hash_futex()), so ensure that they
* reside in the same cacheline.
*/
static struct {
struct futex_hash_bucket *queues;
unsigned long hashsize;
} __futex_data __read_mostly __aligned(2*sizeof(long));
#define futex_queues (__futex_data.queues)
#define futex_hashsize (__futex_data.hashsize)
/*
* Fault injections for futexes.
*/
#ifdef CONFIG_FAIL_FUTEX
static struct {
struct fault_attr attr;
bool ignore_private;
} fail_futex = {
.attr = FAULT_ATTR_INITIALIZER,
.ignore_private = false,
};
static int __init setup_fail_futex(char *str)
{
return setup_fault_attr(&fail_futex.attr, str);
}
__setup("fail_futex=", setup_fail_futex);
static bool should_fail_futex(bool fshared)
{
if (fail_futex.ignore_private && !fshared)
return false;
return should_fail(&fail_futex.attr, 1);
}
#ifdef CONFIG_FAULT_INJECTION_DEBUG_FS
static int __init fail_futex_debugfs(void)
{
umode_t mode = S_IFREG | S_IRUSR | S_IWUSR;
struct dentry *dir;
dir = fault_create_debugfs_attr("fail_futex", NULL,
&fail_futex.attr);
if (IS_ERR(dir))
return PTR_ERR(dir);
debugfs_create_bool("ignore-private", mode, dir,
&fail_futex.ignore_private);
return 0;
}
late_initcall(fail_futex_debugfs);
#endif /* CONFIG_FAULT_INJECTION_DEBUG_FS */
#else
static inline bool should_fail_futex(bool fshared)
{
return false;
}
#endif /* CONFIG_FAIL_FUTEX */
#ifdef CONFIG_COMPAT
static void compat_exit_robust_list(struct task_struct *curr);
#endif
/*
* Reflects a new waiter being added to the waitqueue.
*/
static inline void hb_waiters_inc(struct futex_hash_bucket *hb)
{
#ifdef CONFIG_SMP
atomic_inc(&hb->waiters);
/*
* Full barrier (A), see the ordering comment above.
*/
smp_mb__after_atomic();
#endif
}
/*
* Reflects a waiter being removed from the waitqueue by wakeup
* paths.
*/
static inline void hb_waiters_dec(struct futex_hash_bucket *hb)
{
#ifdef CONFIG_SMP
atomic_dec(&hb->waiters);
#endif
}
static inline int hb_waiters_pending(struct futex_hash_bucket *hb)
{
#ifdef CONFIG_SMP
/*
* Full barrier (B), see the ordering comment above.
*/
smp_mb();
return atomic_read(&hb->waiters);
#else
return 1;
#endif
}
/**
* hash_futex - Return the hash bucket in the global hash
* @key: Pointer to the futex key for which the hash is calculated
*
* We hash on the keys returned from get_futex_key (see below) and return the
* corresponding hash bucket in the global hash.
*/
static struct futex_hash_bucket *hash_futex(union futex_key *key)
{
u32 hash = jhash2((u32 *)key, offsetof(typeof(*key), both.offset) / 4,
key->both.offset);
return &futex_queues[hash & (futex_hashsize - 1)];
}
/**
* match_futex - Check whether two futex keys are equal
* @key1: Pointer to key1
* @key2: Pointer to key2
*
* Return 1 if two futex_keys are equal, 0 otherwise.
*/
static inline int match_futex(union futex_key *key1, union futex_key *key2)
{
return (key1 && key2
&& key1->both.word == key2->both.word
&& key1->both.ptr == key2->both.ptr
&& key1->both.offset == key2->both.offset);
}
enum futex_access {
FUTEX_READ,
FUTEX_WRITE
};
/**
* futex_setup_timer - set up the sleeping hrtimer.
* @time: ptr to the given timeout value
* @timeout: the hrtimer_sleeper structure to be set up
* @flags: futex flags
* @range_ns: optional range in ns
*
* Return: Initialized hrtimer_sleeper structure or NULL if no timeout
* value given
*/
static inline struct hrtimer_sleeper *
futex_setup_timer(ktime_t *time, struct hrtimer_sleeper *timeout,
int flags, u64 range_ns)
{
if (!time)
return NULL;
hrtimer_init_sleeper_on_stack(timeout, (flags & FLAGS_CLOCKRT) ?
CLOCK_REALTIME : CLOCK_MONOTONIC,
HRTIMER_MODE_ABS);
/*
* If range_ns is 0, calling hrtimer_set_expires_range_ns() is
* effectively the same as calling hrtimer_set_expires().
*/
hrtimer_set_expires_range_ns(&timeout->timer, *time, range_ns);
return timeout;
}
/*
* Generate a machine wide unique identifier for this inode.
*
* This relies on u64 not wrapping in the life-time of the machine; which with
* 1ns resolution means almost 585 years.
*
* This further relies on the fact that a well formed program will not unmap
* the file while it has a (shared) futex waiting on it. This mapping will have
* a file reference which pins the mount and inode.
*
* If for some reason an inode gets evicted and read back in again, it will get
* a new sequence number and will _NOT_ match, even though it is the exact same
* file.
*
* It is important that match_futex() will never have a false-positive, esp.
* for PI futexes that can mess up the state. The above argues that false-negatives
* are only possible for malformed programs.
*/
static u64 get_inode_sequence_number(struct inode *inode)
{
static atomic64_t i_seq;
u64 old;
/* Does the inode already have a sequence number? */
old = atomic64_read(&inode->i_sequence);
if (likely(old))
return old;
for (;;) {
u64 new = atomic64_add_return(1, &i_seq);
if (WARN_ON_ONCE(!new))
continue;
old = atomic64_cmpxchg_relaxed(&inode->i_sequence, 0, new);
if (old)
return old;
return new;
}
}
/**
* get_futex_key() - Get parameters which are the keys for a futex
* @uaddr: virtual address of the futex
* @fshared: false for a PROCESS_PRIVATE futex, true for PROCESS_SHARED
* @key: address where result is stored.
* @rw: mapping needs to be read/write (values: FUTEX_READ,
* FUTEX_WRITE)
*
* Return: a negative error code or 0
*
* The key words are stored in @key on success.
*
* For shared mappings (when @fshared), the key is:
*
* ( inode->i_sequence, page->index, offset_within_page )
*
* [ also see get_inode_sequence_number() ]
*
* For private mappings (or when !@fshared), the key is:
*
* ( current->mm, address, 0 )
*
* This allows (cross process, where applicable) identification of the futex
* without keeping the page pinned for the duration of the FUTEX_WAIT.
*
* lock_page() might sleep, the caller should not hold a spinlock.
*/
static int get_futex_key(u32 __user *uaddr, bool fshared, union futex_key *key,
enum futex_access rw)
{
unsigned long address = (unsigned long)uaddr;
struct mm_struct *mm = current->mm;
struct page *page, *tail;
struct address_space *mapping;
int err, ro = 0;
/*
* The futex address must be "naturally" aligned.
*/
key->both.offset = address % PAGE_SIZE;
if (unlikely((address % sizeof(u32)) != 0))
return -EINVAL;
address -= key->both.offset;
if (unlikely(!access_ok(uaddr, sizeof(u32))))
return -EFAULT;
if (unlikely(should_fail_futex(fshared)))
return -EFAULT;
/*
* PROCESS_PRIVATE futexes are fast.
* As the mm cannot disappear under us and the 'key' only needs
* virtual address, we dont even have to find the underlying vma.
* Note : We do have to check 'uaddr' is a valid user address,
* but access_ok() should be faster than find_vma()
*/
if (!fshared) {
key->private.mm = mm;
key->private.address = address;
return 0;
}
again:
/* Ignore any VERIFY_READ mapping (futex common case) */
if (unlikely(should_fail_futex(true)))
return -EFAULT;
err = get_user_pages_fast(address, 1, FOLL_WRITE, &page);
/*
* If write access is not required (eg. FUTEX_WAIT), try
* and get read-only access.
*/
if (err == -EFAULT && rw == FUTEX_READ) {
err = get_user_pages_fast(address, 1, 0, &page);
ro = 1;
}
if (err < 0)
return err;
else
err = 0;
/*
* The treatment of mapping from this point on is critical. The page
* lock protects many things but in this context the page lock
* stabilizes mapping, prevents inode freeing in the shared
* file-backed region case and guards against movement to swap cache.
*
* Strictly speaking the page lock is not needed in all cases being
* considered here and page lock forces unnecessarily serialization
* From this point on, mapping will be re-verified if necessary and
* page lock will be acquired only if it is unavoidable
*
* Mapping checks require the head page for any compound page so the
* head page and mapping is looked up now. For anonymous pages, it
* does not matter if the page splits in the future as the key is
* based on the address. For filesystem-backed pages, the tail is
* required as the index of the page determines the key. For
* base pages, there is no tail page and tail == page.
*/
tail = page;
page = compound_head(page);
mapping = READ_ONCE(page->mapping);
/*
* If page->mapping is NULL, then it cannot be a PageAnon
* page; but it might be the ZERO_PAGE or in the gate area or
* in a special mapping (all cases which we are happy to fail);
* or it may have been a good file page when get_user_pages_fast
* found it, but truncated or holepunched or subjected to
* invalidate_complete_page2 before we got the page lock (also
* cases which we are happy to fail). And we hold a reference,
* so refcount care in invalidate_complete_page's remove_mapping
* prevents drop_caches from setting mapping to NULL beneath us.
*
* The case we do have to guard against is when memory pressure made
* shmem_writepage move it from filecache to swapcache beneath us:
* an unlikely race, but we do need to retry for page->mapping.
*/
if (unlikely(!mapping)) {
int shmem_swizzled;
/*
* Page lock is required to identify which special case above
* applies. If this is really a shmem page then the page lock
* will prevent unexpected transitions.
*/
lock_page(page);
shmem_swizzled = PageSwapCache(page) || page->mapping;
unlock_page(page);
put_page(page);
if (shmem_swizzled)
goto again;
return -EFAULT;
}
/*
* Private mappings are handled in a simple way.
*
* If the futex key is stored on an anonymous page, then the associated
* object is the mm which is implicitly pinned by the calling process.
*
* NOTE: When userspace waits on a MAP_SHARED mapping, even if
* it's a read-only handle, it's expected that futexes attach to
* the object not the particular process.
*/
if (PageAnon(page)) {
/*
* A RO anonymous page will never change and thus doesn't make
* sense for futex operations.
*/
if (unlikely(should_fail_futex(true)) || ro) {
err = -EFAULT;
goto out;
}
key->both.offset |= FUT_OFF_MMSHARED; /* ref taken on mm */
key->private.mm = mm;
key->private.address = address;
} else {
struct inode *inode;
/*
* The associated futex object in this case is the inode and
* the page->mapping must be traversed. Ordinarily this should
* be stabilised under page lock but it's not strictly
* necessary in this case as we just want to pin the inode, not
* update the radix tree or anything like that.
*
* The RCU read lock is taken as the inode is finally freed
* under RCU. If the mapping still matches expectations then the
* mapping->host can be safely accessed as being a valid inode.
*/
rcu_read_lock();
if (READ_ONCE(page->mapping) != mapping) {
rcu_read_unlock();
put_page(page);
goto again;
}
inode = READ_ONCE(mapping->host);
if (!inode) {
rcu_read_unlock();
put_page(page);
goto again;
}
key->both.offset |= FUT_OFF_INODE; /* inode-based key */
key->shared.i_seq = get_inode_sequence_number(inode);
key->shared.pgoff = basepage_index(tail);
rcu_read_unlock();
}
out:
put_page(page);
return err;
}
/**
* fault_in_user_writeable() - Fault in user address and verify RW access
* @uaddr: pointer to faulting user space address
*
* Slow path to fixup the fault we just took in the atomic write
* access to @uaddr.
*
* We have no generic implementation of a non-destructive write to the
* user address. We know that we faulted in the atomic pagefault
* disabled section so we can as well avoid the #PF overhead by
* calling get_user_pages() right away.
*/
static int fault_in_user_writeable(u32 __user *uaddr)
{
struct mm_struct *mm = current->mm;
int ret;
mmap_read_lock(mm);
ret = fixup_user_fault(mm, (unsigned long)uaddr,
FAULT_FLAG_WRITE, NULL);
mmap_read_unlock(mm);
return ret < 0 ? ret : 0;
}
/**
* futex_top_waiter() - Return the highest priority waiter on a futex
* @hb: the hash bucket the futex_q's reside in
* @key: the futex key (to distinguish it from other futex futex_q's)
*
* Must be called with the hb lock held.
*/
static struct futex_q *futex_top_waiter(struct futex_hash_bucket *hb,
union futex_key *key)
{
struct futex_q *this;
plist_for_each_entry(this, &hb->chain, list) {
if (match_futex(&this->key, key))
return this;
}
return NULL;
}
static int cmpxchg_futex_value_locked(u32 *curval, u32 __user *uaddr,
u32 uval, u32 newval)
{
int ret;
pagefault_disable();
ret = futex_atomic_cmpxchg_inatomic(curval, uaddr, uval, newval);
pagefault_enable();
return ret;
}
static int get_futex_value_locked(u32 *dest, u32 __user *from)
{
int ret;
pagefault_disable();
ret = __get_user(*dest, from);
pagefault_enable();
return ret ? -EFAULT : 0;
}
/*
* PI code:
*/
static int refill_pi_state_cache(void)
{
struct futex_pi_state *pi_state;
if (likely(current->pi_state_cache))
return 0;
pi_state = kzalloc(sizeof(*pi_state), GFP_KERNEL);
if (!pi_state)
return -ENOMEM;
INIT_LIST_HEAD(&pi_state->list);
/* pi_mutex gets initialized later */
pi_state->owner = NULL;
refcount_set(&pi_state->refcount, 1);
pi_state->key = FUTEX_KEY_INIT;
current->pi_state_cache = pi_state;
return 0;
}
static struct futex_pi_state *alloc_pi_state(void)
{
struct futex_pi_state *pi_state = current->pi_state_cache;
WARN_ON(!pi_state);
current->pi_state_cache = NULL;
return pi_state;
}
static void pi_state_update_owner(struct futex_pi_state *pi_state,
struct task_struct *new_owner)
{
struct task_struct *old_owner = pi_state->owner;
lockdep_assert_held(&pi_state->pi_mutex.wait_lock);
if (old_owner) {
raw_spin_lock(&old_owner->pi_lock);
WARN_ON(list_empty(&pi_state->list));
list_del_init(&pi_state->list);
raw_spin_unlock(&old_owner->pi_lock);
}
if (new_owner) {
raw_spin_lock(&new_owner->pi_lock);
WARN_ON(!list_empty(&pi_state->list));
list_add(&pi_state->list, &new_owner->pi_state_list);
pi_state->owner = new_owner;
raw_spin_unlock(&new_owner->pi_lock);
}
}
static void get_pi_state(struct futex_pi_state *pi_state)
{
WARN_ON_ONCE(!refcount_inc_not_zero(&pi_state->refcount));
}
/*
* Drops a reference to the pi_state object and frees or caches it
* when the last reference is gone.
*/
static void put_pi_state(struct futex_pi_state *pi_state)
{
if (!pi_state)
return;
if (!refcount_dec_and_test(&pi_state->refcount))
return;
/*
* If pi_state->owner is NULL, the owner is most probably dying
* and has cleaned up the pi_state already
*/
if (pi_state->owner) {
unsigned long flags;
raw_spin_lock_irqsave(&pi_state->pi_mutex.wait_lock, flags);
pi_state_update_owner(pi_state, NULL);
rt_mutex_proxy_unlock(&pi_state->pi_mutex);
raw_spin_unlock_irqrestore(&pi_state->pi_mutex.wait_lock, flags);
}
if (current->pi_state_cache) {
kfree(pi_state);
} else {
/*
* pi_state->list is already empty.
* clear pi_state->owner.
* refcount is at 0 - put it back to 1.
*/
pi_state->owner = NULL;
refcount_set(&pi_state->refcount, 1);
current->pi_state_cache = pi_state;
}
}
#ifdef CONFIG_FUTEX_PI
/*
* This task is holding PI mutexes at exit time => bad.
* Kernel cleans up PI-state, but userspace is likely hosed.
* (Robust-futex cleanup is separate and might save the day for userspace.)
*/
static void exit_pi_state_list(struct task_struct *curr)
{
struct list_head *next, *head = &curr->pi_state_list;
struct futex_pi_state *pi_state;
struct futex_hash_bucket *hb;
union futex_key key = FUTEX_KEY_INIT;
if (!futex_cmpxchg_enabled)
return;
/*
* We are a ZOMBIE and nobody can enqueue itself on
* pi_state_list anymore, but we have to be careful
* versus waiters unqueueing themselves:
*/
raw_spin_lock_irq(&curr->pi_lock);
while (!list_empty(head)) {
next = head->next;
pi_state = list_entry(next, struct futex_pi_state, list);
key = pi_state->key;
hb = hash_futex(&key);
/*
* We can race against put_pi_state() removing itself from the
* list (a waiter going away). put_pi_state() will first
* decrement the reference count and then modify the list, so
* its possible to see the list entry but fail this reference
* acquire.
*
* In that case; drop the locks to let put_pi_state() make
* progress and retry the loop.
*/
if (!refcount_inc_not_zero(&pi_state->refcount)) {
raw_spin_unlock_irq(&curr->pi_lock);
cpu_relax();
raw_spin_lock_irq(&curr->pi_lock);
continue;
}
raw_spin_unlock_irq(&curr->pi_lock);
spin_lock(&hb->lock);
raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
raw_spin_lock(&curr->pi_lock);
/*
* We dropped the pi-lock, so re-check whether this
* task still owns the PI-state:
*/
if (head->next != next) {
/* retain curr->pi_lock for the loop invariant */
raw_spin_unlock(&pi_state->pi_mutex.wait_lock);
spin_unlock(&hb->lock);
put_pi_state(pi_state);
continue;
}
WARN_ON(pi_state->owner != curr);
WARN_ON(list_empty(&pi_state->list));
list_del_init(&pi_state->list);
pi_state->owner = NULL;
raw_spin_unlock(&curr->pi_lock);
raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
spin_unlock(&hb->lock);
rt_mutex_futex_unlock(&pi_state->pi_mutex);
put_pi_state(pi_state);
raw_spin_lock_irq(&curr->pi_lock);
}
raw_spin_unlock_irq(&curr->pi_lock);
}
#else
static inline void exit_pi_state_list(struct task_struct *curr) { }
#endif
/*
* We need to check the following states:
*
* Waiter | pi_state | pi->owner | uTID | uODIED | ?
*
* [1] NULL | --- | --- | 0 | 0/1 | Valid
* [2] NULL | --- | --- | >0 | 0/1 | Valid
*
* [3] Found | NULL | -- | Any | 0/1 | Invalid
*
* [4] Found | Found | NULL | 0 | 1 | Valid
* [5] Found | Found | NULL | >0 | 1 | Invalid
*
* [6] Found | Found | task | 0 | 1 | Valid
*
* [7] Found | Found | NULL | Any | 0 | Invalid
*
* [8] Found | Found | task | ==taskTID | 0/1 | Valid
* [9] Found | Found | task | 0 | 0 | Invalid
* [10] Found | Found | task | !=taskTID | 0/1 | Invalid
*
* [1] Indicates that the kernel can acquire the futex atomically. We
* came here due to a stale FUTEX_WAITERS/FUTEX_OWNER_DIED bit.
*
* [2] Valid, if TID does not belong to a kernel thread. If no matching
* thread is found then it indicates that the owner TID has died.
*
* [3] Invalid. The waiter is queued on a non PI futex
*
* [4] Valid state after exit_robust_list(), which sets the user space
* value to FUTEX_WAITERS | FUTEX_OWNER_DIED.
*
* [5] The user space value got manipulated between exit_robust_list()
* and exit_pi_state_list()
*
* [6] Valid state after exit_pi_state_list() which sets the new owner in
* the pi_state but cannot access the user space value.
*
* [7] pi_state->owner can only be NULL when the OWNER_DIED bit is set.
*
* [8] Owner and user space value match
*
* [9] There is no transient state which sets the user space TID to 0
* except exit_robust_list(), but this is indicated by the
* FUTEX_OWNER_DIED bit. See [4]
*
* [10] There is no transient state which leaves owner and user space
* TID out of sync. Except one error case where the kernel is denied
* write access to the user address, see fixup_pi_state_owner().
*
*
* Serialization and lifetime rules:
*
* hb->lock:
*
* hb -> futex_q, relation
* futex_q -> pi_state, relation
*
* (cannot be raw because hb can contain arbitrary amount
* of futex_q's)
*
* pi_mutex->wait_lock:
*
* {uval, pi_state}
*
* (and pi_mutex 'obviously')
*
* p->pi_lock:
*
* p->pi_state_list -> pi_state->list, relation
* pi_mutex->owner -> pi_state->owner, relation
*
* pi_state->refcount:
*
* pi_state lifetime
*
*
* Lock order:
*
* hb->lock
* pi_mutex->wait_lock
* p->pi_lock
*
*/
/*
* Validate that the existing waiter has a pi_state and sanity check
* the pi_state against the user space value. If correct, attach to
* it.
*/
static int attach_to_pi_state(u32 __user *uaddr, u32 uval,
struct futex_pi_state *pi_state,
struct futex_pi_state **ps)
{
pid_t pid = uval & FUTEX_TID_MASK;
u32 uval2;
int ret;
/*
* Userspace might have messed up non-PI and PI futexes [3]
*/
if (unlikely(!pi_state))
return -EINVAL;
/*
* We get here with hb->lock held, and having found a
* futex_top_waiter(). This means that futex_lock_pi() of said futex_q
* has dropped the hb->lock in between queue_me() and unqueue_me_pi(),
* which in turn means that futex_lock_pi() still has a reference on
* our pi_state.
*
* The waiter holding a reference on @pi_state also protects against
* the unlocked put_pi_state() in futex_unlock_pi(), futex_lock_pi()
* and futex_wait_requeue_pi() as it cannot go to 0 and consequently
* free pi_state before we can take a reference ourselves.
*/
WARN_ON(!refcount_read(&pi_state->refcount));
/*
* Now that we have a pi_state, we can acquire wait_lock
* and do the state validation.
*/
raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
/*
* Since {uval, pi_state} is serialized by wait_lock, and our current
* uval was read without holding it, it can have changed. Verify it
* still is what we expect it to be, otherwise retry the entire
* operation.
*/
if (get_futex_value_locked(&uval2, uaddr))
goto out_efault;
if (uval != uval2)
goto out_eagain;
/*
* Handle the owner died case:
*/
if (uval & FUTEX_OWNER_DIED) {
/*
* exit_pi_state_list sets owner to NULL and wakes the
* topmost waiter. The task which acquires the
* pi_state->rt_mutex will fixup owner.
*/
if (!pi_state->owner) {
/*
* No pi state owner, but the user space TID
* is not 0. Inconsistent state. [5]
*/
if (pid)
goto out_einval;
/*
* Take a ref on the state and return success. [4]
*/
goto out_attach;
}
/*
* If TID is 0, then either the dying owner has not
* yet executed exit_pi_state_list() or some waiter
* acquired the rtmutex in the pi state, but did not
* yet fixup the TID in user space.
*
* Take a ref on the state and return success. [6]
*/
if (!pid)
goto out_attach;
} else {
/*
* If the owner died bit is not set, then the pi_state
* must have an owner. [7]
*/
if (!pi_state->owner)
goto out_einval;
}
/*
* Bail out if user space manipulated the futex value. If pi
* state exists then the owner TID must be the same as the
* user space TID. [9/10]
*/
if (pid != task_pid_vnr(pi_state->owner))
goto out_einval;
out_attach:
get_pi_state(pi_state);
raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
*ps = pi_state;
return 0;
out_einval:
ret = -EINVAL;
goto out_error;
out_eagain:
ret = -EAGAIN;
goto out_error;
out_efault:
ret = -EFAULT;
goto out_error;
out_error:
raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
return ret;
}
/**
* wait_for_owner_exiting - Block until the owner has exited
* @ret: owner's current futex lock status
* @exiting: Pointer to the exiting task
*
* Caller must hold a refcount on @exiting.
*/
static void wait_for_owner_exiting(int ret, struct task_struct *exiting)
{
if (ret != -EBUSY) {
WARN_ON_ONCE(exiting);
return;
}
if (WARN_ON_ONCE(ret == -EBUSY && !exiting))
return;
mutex_lock(&exiting->futex_exit_mutex);
/*
* No point in doing state checking here. If the waiter got here
* while the task was in exec()->exec_futex_release() then it can
* have any FUTEX_STATE_* value when the waiter has acquired the
* mutex. OK, if running, EXITING or DEAD if it reached exit()
* already. Highly unlikely and not a problem. Just one more round
* through the futex maze.
*/
mutex_unlock(&exiting->futex_exit_mutex);
put_task_struct(exiting);
}
static int handle_exit_race(u32 __user *uaddr, u32 uval,
struct task_struct *tsk)
{
u32 uval2;
/*
* If the futex exit state is not yet FUTEX_STATE_DEAD, tell the
* caller that the alleged owner is busy.
*/
if (tsk && tsk->futex_state != FUTEX_STATE_DEAD)
return -EBUSY;
/*
* Reread the user space value to handle the following situation:
*
* CPU0 CPU1
*
* sys_exit() sys_futex()
* do_exit() futex_lock_pi()
* futex_lock_pi_atomic()
* exit_signals(tsk) No waiters:
* tsk->flags |= PF_EXITING; *uaddr == 0x00000PID
* mm_release(tsk) Set waiter bit
* exit_robust_list(tsk) { *uaddr = 0x80000PID;
* Set owner died attach_to_pi_owner() {
* *uaddr = 0xC0000000; tsk = get_task(PID);
* } if (!tsk->flags & PF_EXITING) {
* ... attach();
* tsk->futex_state = } else {
* FUTEX_STATE_DEAD; if (tsk->futex_state !=
* FUTEX_STATE_DEAD)
* return -EAGAIN;
* return -ESRCH; <--- FAIL
* }
*
* Returning ESRCH unconditionally is wrong here because the
* user space value has been changed by the exiting task.
*
* The same logic applies to the case where the exiting task is
* already gone.
*/
if (get_futex_value_locked(&uval2, uaddr))
return -EFAULT;
/* If the user space value has changed, try again. */
if (uval2 != uval)
return -EAGAIN;
/*
* The exiting task did not have a robust list, the robust list was
* corrupted or the user space value in *uaddr is simply bogus.
* Give up and tell user space.
*/
return -ESRCH;
}
/*
* Lookup the task for the TID provided from user space and attach to
* it after doing proper sanity checks.
*/
static int attach_to_pi_owner(u32 __user *uaddr, u32 uval, union futex_key *key,
struct futex_pi_state **ps,
struct task_struct **exiting)
{
pid_t pid = uval & FUTEX_TID_MASK;
struct futex_pi_state *pi_state;
struct task_struct *p;
/*
* We are the first waiter - try to look up the real owner and attach
* the new pi_state to it, but bail out when TID = 0 [1]
*
* The !pid check is paranoid. None of the call sites should end up
* with pid == 0, but better safe than sorry. Let the caller retry
*/
if (!pid)
return -EAGAIN;
p = find_get_task_by_vpid(pid);
if (!p)
return handle_exit_race(uaddr, uval, NULL);
if (unlikely(p->flags & PF_KTHREAD)) {
put_task_struct(p);
return -EPERM;
}
/*
* We need to look at the task state to figure out, whether the
* task is exiting. To protect against the change of the task state
* in futex_exit_release(), we do this protected by p->pi_lock:
*/
raw_spin_lock_irq(&p->pi_lock);
if (unlikely(p->futex_state != FUTEX_STATE_OK)) {
/*
* The task is on the way out. When the futex state is
* FUTEX_STATE_DEAD, we know that the task has finished
* the cleanup:
*/
int ret = handle_exit_race(uaddr, uval, p);
raw_spin_unlock_irq(&p->pi_lock);
/*
* If the owner task is between FUTEX_STATE_EXITING and
* FUTEX_STATE_DEAD then store the task pointer and keep
* the reference on the task struct. The calling code will
* drop all locks, wait for the task to reach
* FUTEX_STATE_DEAD and then drop the refcount. This is
* required to prevent a live lock when the current task
* preempted the exiting task between the two states.
*/
if (ret == -EBUSY)
*exiting = p;
else
put_task_struct(p);
return ret;
}
/*
* No existing pi state. First waiter. [2]
*
* This creates pi_state, we have hb->lock held, this means nothing can
* observe this state, wait_lock is irrelevant.
*/
pi_state = alloc_pi_state();
/*
* Initialize the pi_mutex in locked state and make @p
* the owner of it:
*/
rt_mutex_init_proxy_locked(&pi_state->pi_mutex, p);
/* Store the key for possible exit cleanups: */
pi_state->key = *key;
WARN_ON(!list_empty(&pi_state->list));
list_add(&pi_state->list, &p->pi_state_list);
/*
* Assignment without holding pi_state->pi_mutex.wait_lock is safe
* because there is no concurrency as the object is not published yet.
*/
pi_state->owner = p;
raw_spin_unlock_irq(&p->pi_lock);
put_task_struct(p);
*ps = pi_state;
return 0;
}
static int lookup_pi_state(u32 __user *uaddr, u32 uval,
struct futex_hash_bucket *hb,
union futex_key *key, struct futex_pi_state **ps,
struct task_struct **exiting)
{
struct futex_q *top_waiter = futex_top_waiter(hb, key);
/*
* If there is a waiter on that futex, validate it and
* attach to the pi_state when the validation succeeds.
*/
if (top_waiter)
return attach_to_pi_state(uaddr, uval, top_waiter->pi_state, ps);
/*
* We are the first waiter - try to look up the owner based on
* @uval and attach to it.
*/
return attach_to_pi_owner(uaddr, uval, key, ps, exiting);
}
static int lock_pi_update_atomic(u32 __user *uaddr, u32 uval, u32 newval)
{
int err;
u32 curval;
if (unlikely(should_fail_futex(true)))
return -EFAULT;
err = cmpxchg_futex_value_locked(&curval, uaddr, uval, newval);
if (unlikely(err))
return err;
/* If user space value changed, let the caller retry */
return curval != uval ? -EAGAIN : 0;
}
/**
* futex_lock_pi_atomic() - Atomic work required to acquire a pi aware futex
* @uaddr: the pi futex user address
* @hb: the pi futex hash bucket
* @key: the futex key associated with uaddr and hb
* @ps: the pi_state pointer where we store the result of the
* lookup
* @task: the task to perform the atomic lock work for. This will
* be "current" except in the case of requeue pi.
* @exiting: Pointer to store the task pointer of the owner task
* which is in the middle of exiting
* @set_waiters: force setting the FUTEX_WAITERS bit (1) or not (0)
*
* Return:
* - 0 - ready to wait;
* - 1 - acquired the lock;
* - <0 - error
*
* The hb->lock and futex_key refs shall be held by the caller.
*
* @exiting is only set when the return value is -EBUSY. If so, this holds
* a refcount on the exiting task on return and the caller needs to drop it
* after waiting for the exit to complete.
*/
static int futex_lock_pi_atomic(u32 __user *uaddr, struct futex_hash_bucket *hb,
union futex_key *key,
struct futex_pi_state **ps,
struct task_struct *task,
struct task_struct **exiting,
int set_waiters)
{
u32 uval, newval, vpid = task_pid_vnr(task);
struct futex_q *top_waiter;
int ret;
/*
* Read the user space value first so we can validate a few
* things before proceeding further.
*/
if (get_futex_value_locked(&uval, uaddr))
return -EFAULT;
if (unlikely(should_fail_futex(true)))
return -EFAULT;
/*
* Detect deadlocks.
*/
if ((unlikely((uval & FUTEX_TID_MASK) == vpid)))
return -EDEADLK;
if ((unlikely(should_fail_futex(true))))
return -EDEADLK;
/*
* Lookup existing state first. If it exists, try to attach to
* its pi_state.
*/
top_waiter = futex_top_waiter(hb, key);
if (top_waiter)
return attach_to_pi_state(uaddr, uval, top_waiter->pi_state, ps);
/*
* No waiter and user TID is 0. We are here because the
* waiters or the owner died bit is set or called from
* requeue_cmp_pi or for whatever reason something took the
* syscall.
*/
if (!(uval & FUTEX_TID_MASK)) {
/*
* We take over the futex. No other waiters and the user space
* TID is 0. We preserve the owner died bit.
*/
newval = uval & FUTEX_OWNER_DIED;
newval |= vpid;
/* The futex requeue_pi code can enforce the waiters bit */
if (set_waiters)
newval |= FUTEX_WAITERS;
ret = lock_pi_update_atomic(uaddr, uval, newval);
/* If the take over worked, return 1 */
return ret < 0 ? ret : 1;
}
/*
* First waiter. Set the waiters bit before attaching ourself to
* the owner. If owner tries to unlock, it will be forced into
* the kernel and blocked on hb->lock.
*/
newval = uval | FUTEX_WAITERS;
ret = lock_pi_update_atomic(uaddr, uval, newval);
if (ret)
return ret;
/*
* If the update of the user space value succeeded, we try to
* attach to the owner. If that fails, no harm done, we only
* set the FUTEX_WAITERS bit in the user space variable.
*/
return attach_to_pi_owner(uaddr, newval, key, ps, exiting);
}
/**
* __unqueue_futex() - Remove the futex_q from its futex_hash_bucket
* @q: The futex_q to unqueue
*
* The q->lock_ptr must not be NULL and must be held by the caller.
*/
static void __unqueue_futex(struct futex_q *q)
{
struct futex_hash_bucket *hb;
if (WARN_ON_SMP(!q->lock_ptr) || WARN_ON(plist_node_empty(&q->list)))
return;
lockdep_assert_held(q->lock_ptr);
hb = container_of(q->lock_ptr, struct futex_hash_bucket, lock);
plist_del(&q->list, &hb->chain);
hb_waiters_dec(hb);
}
/*
* The hash bucket lock must be held when this is called.
* Afterwards, the futex_q must not be accessed. Callers
* must ensure to later call wake_up_q() for the actual
* wakeups to occur.
*/
static void mark_wake_futex(struct wake_q_head *wake_q, struct futex_q *q)
{
struct task_struct *p = q->task;
if (WARN(q->pi_state || q->rt_waiter, "refusing to wake PI futex\n"))
return;
get_task_struct(p);
__unqueue_futex(q);
/*
* The waiting task can free the futex_q as soon as q->lock_ptr = NULL
* is written, without taking any locks. This is possible in the event
* of a spurious wakeup, for example. A memory barrier is required here
* to prevent the following store to lock_ptr from getting ahead of the
* plist_del in __unqueue_futex().
*/
smp_store_release(&q->lock_ptr, NULL);
/*
* Queue the task for later wakeup for after we've released
* the hb->lock.
*/
wake_q_add_safe(wake_q, p);
}
/*
* Caller must hold a reference on @pi_state.
*/
static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_pi_state *pi_state)
{
u32 curval, newval;
struct rt_mutex_waiter *top_waiter;
struct task_struct *new_owner;
bool postunlock = false;
DEFINE_WAKE_Q(wake_q);
int ret = 0;
top_waiter = rt_mutex_top_waiter(&pi_state->pi_mutex);
if (WARN_ON_ONCE(!top_waiter)) {
/*
* As per the comment in futex_unlock_pi() this should not happen.
*
* When this happens, give up our locks and try again, giving
* the futex_lock_pi() instance time to complete, either by
* waiting on the rtmutex or removing itself from the futex
* queue.
*/
ret = -EAGAIN;
goto out_unlock;
}
new_owner = top_waiter->task;
/*
* We pass it to the next owner. The WAITERS bit is always kept
* enabled while there is PI state around. We cleanup the owner
* died bit, because we are the owner.
*/
newval = FUTEX_WAITERS | task_pid_vnr(new_owner);
if (unlikely(should_fail_futex(true))) {
ret = -EFAULT;
goto out_unlock;
}
ret = cmpxchg_futex_value_locked(&curval, uaddr, uval, newval);
if (!ret && (curval != uval)) {
/*
* If a unconditional UNLOCK_PI operation (user space did not
* try the TID->0 transition) raced with a waiter setting the
* FUTEX_WAITERS flag between get_user() and locking the hash
* bucket lock, retry the operation.
*/
if ((FUTEX_TID_MASK & curval) == uval)
ret = -EAGAIN;
else
ret = -EINVAL;
}
if (!ret) {
/*
* This is a point of no return; once we modified the uval
* there is no going back and subsequent operations must
* not fail.
*/
pi_state_update_owner(pi_state, new_owner);
postunlock = __rt_mutex_futex_unlock(&pi_state->pi_mutex, &wake_q);
}
out_unlock:
raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
if (postunlock)
rt_mutex_postunlock(&wake_q);
return ret;
}
/*
* Express the locking dependencies for lockdep:
*/
static inline void
double_lock_hb(struct futex_hash_bucket *hb1, struct futex_hash_bucket *hb2)
{
if (hb1 <= hb2) {
spin_lock(&hb1->lock);
if (hb1 < hb2)
spin_lock_nested(&hb2->lock, SINGLE_DEPTH_NESTING);
} else { /* hb1 > hb2 */
spin_lock(&hb2->lock);
spin_lock_nested(&hb1->lock, SINGLE_DEPTH_NESTING);
}
}
static inline void
double_unlock_hb(struct futex_hash_bucket *hb1, struct futex_hash_bucket *hb2)
{
spin_unlock(&hb1->lock);
if (hb1 != hb2)
spin_unlock(&hb2->lock);
}
/*
* Wake up waiters matching bitset queued on this futex (uaddr).
*/
static int
futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset)
{
struct futex_hash_bucket *hb;
struct futex_q *this, *next;
union futex_key key = FUTEX_KEY_INIT;
int ret;
DEFINE_WAKE_Q(wake_q);
if (!bitset)
return -EINVAL;
ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &key, FUTEX_READ);
if (unlikely(ret != 0))
return ret;
hb = hash_futex(&key);
/* Make sure we really have tasks to wakeup */
if (!hb_waiters_pending(hb))
return ret;
spin_lock(&hb->lock);
plist_for_each_entry_safe(this, next, &hb->chain, list) {
if (match_futex (&this->key, &key)) {
if (this->pi_state || this->rt_waiter) {
ret = -EINVAL;
break;
}
/* Check if one of the bits is set in both bitsets */
if (!(this->bitset & bitset))
continue;
mark_wake_futex(&wake_q, this);
if (++ret >= nr_wake)
break;
}
}
spin_unlock(&hb->lock);
wake_up_q(&wake_q);
return ret;
}
static int futex_atomic_op_inuser(unsigned int encoded_op, u32 __user *uaddr)
{
unsigned int op = (encoded_op & 0x70000000) >> 28;
unsigned int cmp = (encoded_op & 0x0f000000) >> 24;
int oparg = sign_extend32((encoded_op & 0x00fff000) >> 12, 11);
int cmparg = sign_extend32(encoded_op & 0x00000fff, 11);
int oldval, ret;
if (encoded_op & (FUTEX_OP_OPARG_SHIFT << 28)) {
if (oparg < 0 || oparg > 31) {
char comm[sizeof(current->comm)];
/*
* kill this print and return -EINVAL when userspace
* is sane again
*/
pr_info_ratelimited("futex_wake_op: %s tries to shift op by %d; fix this program\n",
get_task_comm(comm, current), oparg);
oparg &= 31;
}
oparg = 1 << oparg;
}
pagefault_disable();
ret = arch_futex_atomic_op_inuser(op, oparg, &oldval, uaddr);
pagefault_enable();
if (ret)
return ret;
switch (cmp) {
case FUTEX_OP_CMP_EQ:
return oldval == cmparg;
case FUTEX_OP_CMP_NE:
return oldval != cmparg;
case FUTEX_OP_CMP_LT:
return oldval < cmparg;
case FUTEX_OP_CMP_GE:
return oldval >= cmparg;
case FUTEX_OP_CMP_LE:
return oldval <= cmparg;
case FUTEX_OP_CMP_GT:
return oldval > cmparg;
default:
return -ENOSYS;
}
}
/*
* Wake up all waiters hashed on the physical page that is mapped
* to this virtual address:
*/
static int
futex_wake_op(u32 __user *uaddr1, unsigned int flags, u32 __user *uaddr2,
int nr_wake, int nr_wake2, int op)
{
union futex_key key1 = FUTEX_KEY_INIT, key2 = FUTEX_KEY_INIT;
struct futex_hash_bucket *hb1, *hb2;
struct futex_q *this, *next;
int ret, op_ret;
DEFINE_WAKE_Q(wake_q);
retry:
ret = get_futex_key(uaddr1, flags & FLAGS_SHARED, &key1, FUTEX_READ);
if (unlikely(ret != 0))
return ret;
ret = get_futex_key(uaddr2, flags & FLAGS_SHARED, &key2, FUTEX_WRITE);
if (unlikely(ret != 0))
return ret;
hb1 = hash_futex(&key1);
hb2 = hash_futex(&key2);
retry_private:
double_lock_hb(hb1, hb2);
op_ret = futex_atomic_op_inuser(op, uaddr2);
if (unlikely(op_ret < 0)) {
double_unlock_hb(hb1, hb2);
if (!IS_ENABLED(CONFIG_MMU) ||
unlikely(op_ret != -EFAULT && op_ret != -EAGAIN)) {
/*
* we don't get EFAULT from MMU faults if we don't have
* an MMU, but we might get them from range checking
*/
ret = op_ret;
return ret;
}
if (op_ret == -EFAULT) {
ret = fault_in_user_writeable(uaddr2);
if (ret)
return ret;
}
if (!(flags & FLAGS_SHARED)) {
cond_resched();
goto retry_private;
}
cond_resched();
goto retry;
}
plist_for_each_entry_safe(this, next, &hb1->chain, list) {
if (match_futex (&this->key, &key1)) {
if (this->pi_state || this->rt_waiter) {
ret = -EINVAL;
goto out_unlock;
}
mark_wake_futex(&wake_q, this);
if (++ret >= nr_wake)
break;
}
}
if (op_ret > 0) {
op_ret = 0;
plist_for_each_entry_safe(this, next, &hb2->chain, list) {
if (match_futex (&this->key, &key2)) {
if (this->pi_state || this->rt_waiter) {
ret = -EINVAL;
goto out_unlock;
}
mark_wake_futex(&wake_q, this);
if (++op_ret >= nr_wake2)
break;
}
}
ret += op_ret;
}
out_unlock:
double_unlock_hb(hb1, hb2);
wake_up_q(&wake_q);
return ret;
}
/**
* requeue_futex() - Requeue a futex_q from one hb to another
* @q: the futex_q to requeue
* @hb1: the source hash_bucket
* @hb2: the target hash_bucket
* @key2: the new key for the requeued futex_q
*/
static inline
void requeue_futex(struct futex_q *q, struct futex_hash_bucket *hb1,
struct futex_hash_bucket *hb2, union futex_key *key2)
{
/*
* If key1 and key2 hash to the same bucket, no need to
* requeue.
*/
if (likely(&hb1->chain != &hb2->chain)) {
plist_del(&q->list, &hb1->chain);
hb_waiters_dec(hb1);
hb_waiters_inc(hb2);
plist_add(&q->list, &hb2->chain);
q->lock_ptr = &hb2->lock;
}
q->key = *key2;
}
/**
* requeue_pi_wake_futex() - Wake a task that acquired the lock during requeue
* @q: the futex_q
* @key: the key of the requeue target futex
* @hb: the hash_bucket of the requeue target futex
*
* During futex_requeue, with requeue_pi=1, it is possible to acquire the
* target futex if it is uncontended or via a lock steal. Set the futex_q key
* to the requeue target futex so the waiter can detect the wakeup on the right
* futex, but remove it from the hb and NULL the rt_waiter so it can detect
* atomic lock acquisition. Set the q->lock_ptr to the requeue target hb->lock
* to protect access to the pi_state to fixup the owner later. Must be called
* with both q->lock_ptr and hb->lock held.
*/
static inline
void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key,
struct futex_hash_bucket *hb)
{
q->key = *key;
__unqueue_futex(q);
WARN_ON(!q->rt_waiter);
q->rt_waiter = NULL;
q->lock_ptr = &hb->lock;
wake_up_state(q->task, TASK_NORMAL);
}
/**
* futex_proxy_trylock_atomic() - Attempt an atomic lock for the top waiter
* @pifutex: the user address of the to futex
* @hb1: the from futex hash bucket, must be locked by the caller
* @hb2: the to futex hash bucket, must be locked by the caller
* @key1: the from futex key
* @key2: the to futex key
* @ps: address to store the pi_state pointer
* @exiting: Pointer to store the task pointer of the owner task
* which is in the middle of exiting
* @set_waiters: force setting the FUTEX_WAITERS bit (1) or not (0)
*
* Try and get the lock on behalf of the top waiter if we can do it atomically.
* Wake the top waiter if we succeed. If the caller specified set_waiters,
* then direct futex_lock_pi_atomic() to force setting the FUTEX_WAITERS bit.
* hb1 and hb2 must be held by the caller.
*
* @exiting is only set when the return value is -EBUSY. If so, this holds
* a refcount on the exiting task on return and the caller needs to drop it
* after waiting for the exit to complete.
*
* Return:
* - 0 - failed to acquire the lock atomically;
* - >0 - acquired the lock, return value is vpid of the top_waiter
* - <0 - error
*/
static int
futex_proxy_trylock_atomic(u32 __user *pifutex, struct futex_hash_bucket *hb1,
struct futex_hash_bucket *hb2, union futex_key *key1,
union futex_key *key2, struct futex_pi_state **ps,
struct task_struct **exiting, int set_waiters)
{
struct futex_q *top_waiter = NULL;
u32 curval;
int ret, vpid;
if (get_futex_value_locked(&curval, pifutex))
return -EFAULT;
if (unlikely(should_fail_futex(true)))
return -EFAULT;
/*
* Find the top_waiter and determine if there are additional waiters.
* If the caller intends to requeue more than 1 waiter to pifutex,
* force futex_lock_pi_atomic() to set the FUTEX_WAITERS bit now,
* as we have means to handle the possible fault. If not, don't set
* the bit unecessarily as it will force the subsequent unlock to enter
* the kernel.
*/
top_waiter = futex_top_waiter(hb1, key1);
/* There are no waiters, nothing for us to do. */
if (!top_waiter)
return 0;
/* Ensure we requeue to the expected futex. */
if (!match_futex(top_waiter->requeue_pi_key, key2))
return -EINVAL;
/*
* Try to take the lock for top_waiter. Set the FUTEX_WAITERS bit in
* the contended case or if set_waiters is 1. The pi_state is returned
* in ps in contended cases.
*/
vpid = task_pid_vnr(top_waiter->task);
ret = futex_lock_pi_atomic(pifutex, hb2, key2, ps, top_waiter->task,
exiting, set_waiters);
if (ret == 1) {
requeue_pi_wake_futex(top_waiter, key2, hb2);
return vpid;
}
return ret;
}
/**
* futex_requeue() - Requeue waiters from uaddr1 to uaddr2
* @uaddr1: source futex user address
* @flags: futex flags (FLAGS_SHARED, etc.)
* @uaddr2: target futex user address
* @nr_wake: number of waiters to wake (must be 1 for requeue_pi)
* @nr_requeue: number of waiters to requeue (0-INT_MAX)
* @cmpval: @uaddr1 expected value (or %NULL)
* @requeue_pi: if we are attempting to requeue from a non-pi futex to a
* pi futex (pi to pi requeue is not supported)
*
* Requeue waiters on uaddr1 to uaddr2. In the requeue_pi case, try to acquire
* uaddr2 atomically on behalf of the top waiter.
*
* Return:
* - >=0 - on success, the number of tasks requeued or woken;
* - <0 - on error
*/
static int futex_requeue(u32 __user *uaddr1, unsigned int flags,
u32 __user *uaddr2, int nr_wake, int nr_requeue,
u32 *cmpval, int requeue_pi)
{
union futex_key key1 = FUTEX_KEY_INIT, key2 = FUTEX_KEY_INIT;
int task_count = 0, ret;
struct futex_pi_state *pi_state = NULL;
struct futex_hash_bucket *hb1, *hb2;
struct futex_q *this, *next;
DEFINE_WAKE_Q(wake_q);
if (nr_wake < 0 || nr_requeue < 0)
return -EINVAL;
/*
* When PI not supported: return -ENOSYS if requeue_pi is true,
* consequently the compiler knows requeue_pi is always false past
* this point which will optimize away all the conditional code
* further down.
*/
if (!IS_ENABLED(CONFIG_FUTEX_PI) && requeue_pi)
return -ENOSYS;
if (requeue_pi) {
/*
* Requeue PI only works on two distinct uaddrs. This
* check is only valid for private futexes. See below.
*/
if (uaddr1 == uaddr2)
return -EINVAL;
/*
* requeue_pi requires a pi_state, try to allocate it now
* without any locks in case it fails.
*/
if (refill_pi_state_cache())
return -ENOMEM;
/*
* requeue_pi must wake as many tasks as it can, up to nr_wake
* + nr_requeue, since it acquires the rt_mutex prior to
* returning to userspace, so as to not leave the rt_mutex with
* waiters and no owner. However, second and third wake-ups
* cannot be predicted as they involve race conditions with the
* first wake and a fault while looking up the pi_state. Both
* pthread_cond_signal() and pthread_cond_broadcast() should
* use nr_wake=1.
*/
if (nr_wake != 1)
return -EINVAL;
}
retry:
ret = get_futex_key(uaddr1, flags & FLAGS_SHARED, &key1, FUTEX_READ);
if (unlikely(ret != 0))
return ret;
ret = get_futex_key(uaddr2, flags & FLAGS_SHARED, &key2,
requeue_pi ? FUTEX_WRITE : FUTEX_READ);
if (unlikely(ret != 0))
return ret;
/*
* The check above which compares uaddrs is not sufficient for
* shared futexes. We need to compare the keys:
*/
if (requeue_pi && match_futex(&key1, &key2))
return -EINVAL;
hb1 = hash_futex(&key1);
hb2 = hash_futex(&key2);
retry_private:
hb_waiters_inc(hb2);
double_lock_hb(hb1, hb2);
if (likely(cmpval != NULL)) {
u32 curval;
ret = get_futex_value_locked(&curval, uaddr1);
if (unlikely(ret)) {
double_unlock_hb(hb1, hb2);
hb_waiters_dec(hb2);
ret = get_user(curval, uaddr1);
if (ret)
return ret;
if (!(flags & FLAGS_SHARED))
goto retry_private;
goto retry;
}
if (curval != *cmpval) {
ret = -EAGAIN;
goto out_unlock;
}
}
if (requeue_pi && (task_count - nr_wake < nr_requeue)) {
struct task_struct *exiting = NULL;
/*
* Attempt to acquire uaddr2 and wake the top waiter. If we
* intend to requeue waiters, force setting the FUTEX_WAITERS
* bit. We force this here where we are able to easily handle
* faults rather in the requeue loop below.
*/
ret = futex_proxy_trylock_atomic(uaddr2, hb1, hb2, &key1,
&key2, &pi_state,
&exiting, nr_requeue);
/*
* At this point the top_waiter has either taken uaddr2 or is
* waiting on it. If the former, then the pi_state will not
* exist yet, look it up one more time to ensure we have a
* reference to it. If the lock was taken, ret contains the
* vpid of the top waiter task.
* If the lock was not taken, we have pi_state and an initial
* refcount on it. In case of an error we have nothing.
*/
if (ret > 0) {
WARN_ON(pi_state);
task_count++;
/*
* If we acquired the lock, then the user space value
* of uaddr2 should be vpid. It cannot be changed by
* the top waiter as it is blocked on hb2 lock if it
* tries to do so. If something fiddled with it behind
* our back the pi state lookup might unearth it. So
* we rather use the known value than rereading and
* handing potential crap to lookup_pi_state.
*
* If that call succeeds then we have pi_state and an
* initial refcount on it.
*/
ret = lookup_pi_state(uaddr2, ret, hb2, &key2,
&pi_state, &exiting);
}
switch (ret) {
case 0:
/* We hold a reference on the pi state. */
break;
/* If the above failed, then pi_state is NULL */
case -EFAULT:
double_unlock_hb(hb1, hb2);
hb_waiters_dec(hb2);
ret = fault_in_user_writeable(uaddr2);
if (!ret)
goto retry;
return ret;
case -EBUSY:
case -EAGAIN:
/*
* Two reasons for this:
* - EBUSY: Owner is exiting and we just wait for the
* exit to complete.
* - EAGAIN: The user space value changed.
*/
double_unlock_hb(hb1, hb2);
hb_waiters_dec(hb2);
/*
* Handle the case where the owner is in the middle of
* exiting. Wait for the exit to complete otherwise
* this task might loop forever, aka. live lock.
*/
wait_for_owner_exiting(ret, exiting);
cond_resched();
goto retry;
default:
goto out_unlock;
}
}
plist_for_each_entry_safe(this, next, &hb1->chain, list) {
if (task_count - nr_wake >= nr_requeue)
break;
if (!match_futex(&this->key, &key1))
continue;
/*
* FUTEX_WAIT_REQEUE_PI and FUTEX_CMP_REQUEUE_PI should always
* be paired with each other and no other futex ops.
*
* We should never be requeueing a futex_q with a pi_state,
* which is awaiting a futex_unlock_pi().
*/
if ((requeue_pi && !this->rt_waiter) ||
(!requeue_pi && this->rt_waiter) ||
this->pi_state) {
ret = -EINVAL;
break;
}
/*
* Wake nr_wake waiters. For requeue_pi, if we acquired the
* lock, we already woke the top_waiter. If not, it will be
* woken by futex_unlock_pi().
*/
if (++task_count <= nr_wake && !requeue_pi) {
mark_wake_futex(&wake_q, this);
continue;
}
/* Ensure we requeue to the expected futex for requeue_pi. */
if (requeue_pi && !match_futex(this->requeue_pi_key, &key2)) {
ret = -EINVAL;
break;
}
/*
* Requeue nr_requeue waiters and possibly one more in the case
* of requeue_pi if we couldn't acquire the lock atomically.
*/
if (requeue_pi) {
/*
* Prepare the waiter to take the rt_mutex. Take a
* refcount on the pi_state and store the pointer in
* the futex_q object of the waiter.
*/
get_pi_state(pi_state);
this->pi_state = pi_state;
ret = rt_mutex_start_proxy_lock(&pi_state->pi_mutex,
this->rt_waiter,
this->task);
if (ret == 1) {
/*
* We got the lock. We do neither drop the
* refcount on pi_state nor clear
* this->pi_state because the waiter needs the
* pi_state for cleaning up the user space
* value. It will drop the refcount after
* doing so.
*/
requeue_pi_wake_futex(this, &key2, hb2);
continue;
} else if (ret) {
/*
* rt_mutex_start_proxy_lock() detected a
* potential deadlock when we tried to queue
* that waiter. Drop the pi_state reference
* which we took above and remove the pointer
* to the state from the waiters futex_q
* object.
*/
this->pi_state = NULL;
put_pi_state(pi_state);
/*
* We stop queueing more waiters and let user
* space deal with the mess.
*/
break;
}
}
requeue_futex(this, hb1, hb2, &key2);
}
/*
* We took an extra initial reference to the pi_state either
* in futex_proxy_trylock_atomic() or in lookup_pi_state(). We
* need to drop it here again.
*/
put_pi_state(pi_state);
out_unlock:
double_unlock_hb(hb1, hb2);
wake_up_q(&wake_q);
hb_waiters_dec(hb2);
return ret ? ret : task_count;
}
/* The key must be already stored in q->key. */
static inline struct futex_hash_bucket *queue_lock(struct futex_q *q)
__acquires(&hb->lock)
{
struct futex_hash_bucket *hb;
hb = hash_futex(&q->key);
/*
* Increment the counter before taking the lock so that
* a potential waker won't miss a to-be-slept task that is
* waiting for the spinlock. This is safe as all queue_lock()
* users end up calling queue_me(). Similarly, for housekeeping,
* decrement the counter at queue_unlock() when some error has
* occurred and we don't end up adding the task to the list.
*/
hb_waiters_inc(hb); /* implies smp_mb(); (A) */
q->lock_ptr = &hb->lock;
spin_lock(&hb->lock);
return hb;
}
static inline void
queue_unlock(struct futex_hash_bucket *hb)
__releases(&hb->lock)
{
spin_unlock(&hb->lock);
hb_waiters_dec(hb);
}
static inline void __queue_me(struct futex_q *q, struct futex_hash_bucket *hb)
{
int prio;
/*
* The priority used to register this element is
* - either the real thread-priority for the real-time threads
* (i.e. threads with a priority lower than MAX_RT_PRIO)
* - or MAX_RT_PRIO for non-RT threads.
* Thus, all RT-threads are woken first in priority order, and
* the others are woken last, in FIFO order.
*/
prio = min(current->normal_prio, MAX_RT_PRIO);
plist_node_init(&q->list, prio);
plist_add(&q->list, &hb->chain);
q->task = current;
}
/**
* queue_me() - Enqueue the futex_q on the futex_hash_bucket
* @q: The futex_q to enqueue
* @hb: The destination hash bucket
*
* The hb->lock must be held by the caller, and is released here. A call to
* queue_me() is typically paired with exactly one call to unqueue_me(). The
* exceptions involve the PI related operations, which may use unqueue_me_pi()
* or nothing if the unqueue is done as part of the wake process and the unqueue
* state is implicit in the state of woken task (see futex_wait_requeue_pi() for
* an example).
*/
static inline void queue_me(struct futex_q *q, struct futex_hash_bucket *hb)
__releases(&hb->lock)
{
__queue_me(q, hb);
spin_unlock(&hb->lock);
}
/**
* unqueue_me() - Remove the futex_q from its futex_hash_bucket
* @q: The futex_q to unqueue
*
* The q->lock_ptr must not be held by the caller. A call to unqueue_me() must
* be paired with exactly one earlier call to queue_me().
*
* Return:
* - 1 - if the futex_q was still queued (and we removed unqueued it);
* - 0 - if the futex_q was already removed by the waking thread
*/
static int unqueue_me(struct futex_q *q)
{
spinlock_t *lock_ptr;
int ret = 0;
/* In the common case we don't take the spinlock, which is nice. */
retry:
/*
* q->lock_ptr can change between this read and the following spin_lock.
* Use READ_ONCE to forbid the compiler from reloading q->lock_ptr and
* optimizing lock_ptr out of the logic below.
*/
lock_ptr = READ_ONCE(q->lock_ptr);
if (lock_ptr != NULL) {
spin_lock(lock_ptr);
/*
* q->lock_ptr can change between reading it and
* spin_lock(), causing us to take the wrong lock. This
* corrects the race condition.
*
* Reasoning goes like this: if we have the wrong lock,
* q->lock_ptr must have changed (maybe several times)
* between reading it and the spin_lock(). It can
* change again after the spin_lock() but only if it was
* already changed before the spin_lock(). It cannot,
* however, change back to the original value. Therefore
* we can detect whether we acquired the correct lock.
*/
if (unlikely(lock_ptr != q->lock_ptr)) {
spin_unlock(lock_ptr);
goto retry;
}
__unqueue_futex(q);
BUG_ON(q->pi_state);
spin_unlock(lock_ptr);
ret = 1;
}
return ret;
}
/*
* PI futexes can not be requeued and must remove themself from the
* hash bucket. The hash bucket lock (i.e. lock_ptr) is held.
*/
static void unqueue_me_pi(struct futex_q *q)
{
__unqueue_futex(q);
BUG_ON(!q->pi_state);
put_pi_state(q->pi_state);
q->pi_state = NULL;
}
static int __fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
struct task_struct *argowner)
{
struct futex_pi_state *pi_state = q->pi_state;
struct task_struct *oldowner, *newowner;
u32 uval, curval, newval, newtid;
int err = 0;
oldowner = pi_state->owner;
/*
* We are here because either:
*
* - we stole the lock and pi_state->owner needs updating to reflect
* that (@argowner == current),
*
* or:
*
* - someone stole our lock and we need to fix things to point to the
* new owner (@argowner == NULL).
*
* Either way, we have to replace the TID in the user space variable.
* This must be atomic as we have to preserve the owner died bit here.
*
* Note: We write the user space value _before_ changing the pi_state
* because we can fault here. Imagine swapped out pages or a fork
* that marked all the anonymous memory readonly for cow.
*
* Modifying pi_state _before_ the user space value would leave the
* pi_state in an inconsistent state when we fault here, because we
* need to drop the locks to handle the fault. This might be observed
* in the PID check in lookup_pi_state.
*/
retry:
if (!argowner) {
if (oldowner != current) {
/*
* We raced against a concurrent self; things are
* already fixed up. Nothing to do.
*/
return 0;
}
if (__rt_mutex_futex_trylock(&pi_state->pi_mutex)) {
/* We got the lock. pi_state is correct. Tell caller. */
return 1;
}
/*
* The trylock just failed, so either there is an owner or
* there is a higher priority waiter than this one.
*/
newowner = rt_mutex_owner(&pi_state->pi_mutex);
/*
* If the higher priority waiter has not yet taken over the
* rtmutex then newowner is NULL. We can't return here with
* that state because it's inconsistent vs. the user space
* state. So drop the locks and try again. It's a valid
* situation and not any different from the other retry
* conditions.
*/
if (unlikely(!newowner)) {
err = -EAGAIN;
goto handle_err;
}
} else {
WARN_ON_ONCE(argowner != current);
if (oldowner == current) {
/*
* We raced against a concurrent self; things are
* already fixed up. Nothing to do.
*/
return 1;
}
newowner = argowner;
}
newtid = task_pid_vnr(newowner) | FUTEX_WAITERS;
/* Owner died? */
if (!pi_state->owner)
newtid |= FUTEX_OWNER_DIED;
err = get_futex_value_locked(&uval, uaddr);
if (err)
goto handle_err;
for (;;) {
newval = (uval & FUTEX_OWNER_DIED) | newtid;
err = cmpxchg_futex_value_locked(&curval, uaddr, uval, newval);
if (err)
goto handle_err;
if (curval == uval)
break;
uval = curval;
}
/*
* We fixed up user space. Now we need to fix the pi_state
* itself.
*/
pi_state_update_owner(pi_state, newowner);
return argowner == current;
/*
* In order to reschedule or handle a page fault, we need to drop the
* locks here. In the case of a fault, this gives the other task
* (either the highest priority waiter itself or the task which stole
* the rtmutex) the chance to try the fixup of the pi_state. So once we
* are back from handling the fault we need to check the pi_state after
* reacquiring the locks and before trying to do another fixup. When
* the fixup has been done already we simply return.
*
* Note: we hold both hb->lock and pi_mutex->wait_lock. We can safely
* drop hb->lock since the caller owns the hb -> futex_q relation.
* Dropping the pi_mutex->wait_lock requires the state revalidate.
*/
handle_err:
raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
spin_unlock(q->lock_ptr);
switch (err) {
case -EFAULT:
err = fault_in_user_writeable(uaddr);
break;
case -EAGAIN:
cond_resched();
err = 0;
break;
default:
WARN_ON_ONCE(1);
break;
}
spin_lock(q->lock_ptr);
raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
/*
* Check if someone else fixed it for us:
*/
if (pi_state->owner != oldowner)
return argowner == current;
/* Retry if err was -EAGAIN or the fault in succeeded */
if (!err)
goto retry;
/*
* fault_in_user_writeable() failed so user state is immutable. At
* best we can make the kernel state consistent but user state will
* be most likely hosed and any subsequent unlock operation will be
* rejected due to PI futex rule [10].
*
* Ensure that the rtmutex owner is also the pi_state owner despite
* the user space value claiming something different. There is no
* point in unlocking the rtmutex if current is the owner as it
* would need to wait until the next waiter has taken the rtmutex
* to guarantee consistent state. Keep it simple. Userspace asked
* for this wreckaged state.
*
* The rtmutex has an owner - either current or some other
* task. See the EAGAIN loop above.
*/
pi_state_update_owner(pi_state, rt_mutex_owner(&pi_state->pi_mutex));
return err;
}
static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
struct task_struct *argowner)
{
struct futex_pi_state *pi_state = q->pi_state;
int ret;
lockdep_assert_held(q->lock_ptr);
raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
ret = __fixup_pi_state_owner(uaddr, q, argowner);
raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
return ret;
}
static long futex_wait_restart(struct restart_block *restart);
/**
* fixup_owner() - Post lock pi_state and corner case management
* @uaddr: user address of the futex
* @q: futex_q (contains pi_state and access to the rt_mutex)
* @locked: if the attempt to take the rt_mutex succeeded (1) or not (0)
*
* After attempting to lock an rt_mutex, this function is called to cleanup
* the pi_state owner as well as handle race conditions that may allow us to
* acquire the lock. Must be called with the hb lock held.
*
* Return:
* - 1 - success, lock taken;
* - 0 - success, lock not taken;
* - <0 - on error (-EFAULT)
*/
static int fixup_owner(u32 __user *uaddr, struct futex_q *q, int locked)
{
if (locked) {
/*
* Got the lock. We might not be the anticipated owner if we
* did a lock-steal - fix up the PI-state in that case:
*
* Speculative pi_state->owner read (we don't hold wait_lock);
* since we own the lock pi_state->owner == current is the
* stable state, anything else needs more attention.
*/
if (q->pi_state->owner != current)
return fixup_pi_state_owner(uaddr, q, current);
return 1;
}
/*
* If we didn't get the lock; check if anybody stole it from us. In
* that case, we need to fix up the uval to point to them instead of
* us, otherwise bad things happen. [10]
*
* Another speculative read; pi_state->owner == current is unstable
* but needs our attention.
*/
if (q->pi_state->owner == current)
return fixup_pi_state_owner(uaddr, q, NULL);
/*
* Paranoia check. If we did not take the lock, then we should not be
* the owner of the rt_mutex. Warn and establish consistent state.
*/
if (WARN_ON_ONCE(rt_mutex_owner(&q->pi_state->pi_mutex) == current))
return fixup_pi_state_owner(uaddr, q, current);
return 0;
}
/**
* futex_wait_queue_me() - queue_me() and wait for wakeup, timeout, or signal
* @hb: the futex hash bucket, must be locked by the caller
* @q: the futex_q to queue up on
* @timeout: the prepared hrtimer_sleeper, or null for no timeout
*/
static void futex_wait_queue_me(struct futex_hash_bucket *hb, struct futex_q *q,
struct hrtimer_sleeper *timeout)
{
/*
* The task state is guaranteed to be set before another task can
* wake it. set_current_state() is implemented using smp_store_mb() and
* queue_me() calls spin_unlock() upon completion, both serializing
* access to the hash list and forcing another memory barrier.
*/
set_current_state(TASK_INTERRUPTIBLE);
queue_me(q, hb);
/* Arm the timer */
if (timeout)
hrtimer_sleeper_start_expires(timeout, HRTIMER_MODE_ABS);
/*
* If we have been removed from the hash list, then another task
* has tried to wake us, and we can skip the call to schedule().
*/
if (likely(!plist_node_empty(&q->list))) {
/*
* If the timer has already expired, current will already be
* flagged for rescheduling. Only call schedule if there
* is no timeout, or if it has yet to expire.
*/
if (!timeout || timeout->task)
freezable_schedule();
}
__set_current_state(TASK_RUNNING);
}
/**
* futex_wait_setup() - Prepare to wait on a futex
* @uaddr: the futex userspace address
* @val: the expected value
* @flags: futex flags (FLAGS_SHARED, etc.)
* @q: the associated futex_q
* @hb: storage for hash_bucket pointer to be returned to caller
*
* Setup the futex_q and locate the hash_bucket. Get the futex value and
* compare it with the expected value. Handle atomic faults internally.
* Return with the hb lock held and a q.key reference on success, and unlocked
* with no q.key reference on failure.
*
* Return:
* - 0 - uaddr contains val and hb has been locked;
* - <1 - -EFAULT or -EWOULDBLOCK (uaddr does not contain val) and hb is unlocked
*/
static int futex_wait_setup(u32 __user *uaddr, u32 val, unsigned int flags,
struct futex_q *q, struct futex_hash_bucket **hb)
{
u32 uval;
int ret;
/*
* Access the page AFTER the hash-bucket is locked.
* Order is important:
*
* Userspace waiter: val = var; if (cond(val)) futex_wait(&var, val);
* Userspace waker: if (cond(var)) { var = new; futex_wake(&var); }
*
* The basic logical guarantee of a futex is that it blocks ONLY
* if cond(var) is known to be true at the time of blocking, for
* any cond. If we locked the hash-bucket after testing *uaddr, that
* would open a race condition where we could block indefinitely with
* cond(var) false, which would violate the guarantee.
*
* On the other hand, we insert q and release the hash-bucket only
* after testing *uaddr. This guarantees that futex_wait() will NOT
* absorb a wakeup if *uaddr does not match the desired values
* while the syscall executes.
*/
retry:
ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &q->key, FUTEX_READ);
if (unlikely(ret != 0))
return ret;
retry_private:
*hb = queue_lock(q);
ret = get_futex_value_locked(&uval, uaddr);
if (ret) {
queue_unlock(*hb);
ret = get_user(uval, uaddr);
if (ret)
return ret;
if (!(flags & FLAGS_SHARED))
goto retry_private;
goto retry;
}
if (uval != val) {
queue_unlock(*hb);
ret = -EWOULDBLOCK;
}
return ret;
}
static int futex_wait(u32 __user *uaddr, unsigned int flags, u32 val,
ktime_t *abs_time, u32 bitset)
{
struct hrtimer_sleeper timeout, *to;
struct restart_block *restart;
struct futex_hash_bucket *hb;
struct futex_q q = futex_q_init;
int ret;
if (!bitset)
return -EINVAL;
q.bitset = bitset;
to = futex_setup_timer(abs_time, &timeout, flags,
current->timer_slack_ns);
retry:
/*
* Prepare to wait on uaddr. On success, holds hb lock and increments
* q.key refs.
*/
ret = futex_wait_setup(uaddr, val, flags, &q, &hb);
if (ret)
goto out;
/* queue_me and wait for wakeup, timeout, or a signal. */
futex_wait_queue_me(hb, &q, to);
/* If we were woken (and unqueued), we succeeded, whatever. */
ret = 0;
/* unqueue_me() drops q.key ref */
if (!unqueue_me(&q))
goto out;
ret = -ETIMEDOUT;
if (to && !to->task)
goto out;
/*
* We expect signal_pending(current), but we might be the
* victim of a spurious wakeup as well.
*/
if (!signal_pending(current))
goto retry;
ret = -ERESTARTSYS;
if (!abs_time)
goto out;
restart = &current->restart_block;
restart->futex.uaddr = uaddr;
restart->futex.val = val;
restart->futex.time = *abs_time;
restart->futex.bitset = bitset;
restart->futex.flags = flags | FLAGS_HAS_TIMEOUT;
ret = set_restart_fn(restart, futex_wait_restart);
out:
if (to) {
hrtimer_cancel(&to->timer);
destroy_hrtimer_on_stack(&to->timer);
}
return ret;
}
static long futex_wait_restart(struct restart_block *restart)
{
u32 __user *uaddr = restart->futex.uaddr;
ktime_t t, *tp = NULL;
if (restart->futex.flags & FLAGS_HAS_TIMEOUT) {
t = restart->futex.time;
tp = &t;
}
restart->fn = do_no_restart_syscall;
return (long)futex_wait(uaddr, restart->futex.flags,
restart->futex.val, tp, restart->futex.bitset);
}
/*
* Userspace tried a 0 -> TID atomic transition of the futex value
* and failed. The kernel side here does the whole locking operation:
* if there are waiters then it will block as a consequence of relying
* on rt-mutexes, it does PI, etc. (Due to races the kernel might see
* a 0 value of the futex too.).
*
* Also serves as futex trylock_pi()'ing, and due semantics.
*/
static int futex_lock_pi(u32 __user *uaddr, unsigned int flags,
ktime_t *time, int trylock)
{
struct hrtimer_sleeper timeout, *to;
struct task_struct *exiting = NULL;
struct rt_mutex_waiter rt_waiter;
struct futex_hash_bucket *hb;
struct futex_q q = futex_q_init;
int res, ret;
if (!IS_ENABLED(CONFIG_FUTEX_PI))
return -ENOSYS;
if (refill_pi_state_cache())
return -ENOMEM;
to = futex_setup_timer(time, &timeout, FLAGS_CLOCKRT, 0);
retry:
ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &q.key, FUTEX_WRITE);
if (unlikely(ret != 0))
goto out;
retry_private:
hb = queue_lock(&q);
ret = futex_lock_pi_atomic(uaddr, hb, &q.key, &q.pi_state, current,
&exiting, 0);
if (unlikely(ret)) {
/*
* Atomic work succeeded and we got the lock,
* or failed. Either way, we do _not_ block.
*/
switch (ret) {
case 1:
/* We got the lock. */
ret = 0;
goto out_unlock_put_key;
case -EFAULT:
goto uaddr_faulted;
case -EBUSY:
case -EAGAIN:
/*
* Two reasons for this:
* - EBUSY: Task is exiting and we just wait for the
* exit to complete.
* - EAGAIN: The user space value changed.
*/
queue_unlock(hb);
/*
* Handle the case where the owner is in the middle of
* exiting. Wait for the exit to complete otherwise
* this task might loop forever, aka. live lock.
*/
wait_for_owner_exiting(ret, exiting);
cond_resched();
goto retry;
default:
goto out_unlock_put_key;
}
}
WARN_ON(!q.pi_state);
/*
* Only actually queue now that the atomic ops are done:
*/
__queue_me(&q, hb);
if (trylock) {
ret = rt_mutex_futex_trylock(&q.pi_state->pi_mutex);
/* Fixup the trylock return value: */
ret = ret ? 0 : -EWOULDBLOCK;
goto no_block;
}
rt_mutex_init_waiter(&rt_waiter);
/*
* On PREEMPT_RT_FULL, when hb->lock becomes an rt_mutex, we must not
* hold it while doing rt_mutex_start_proxy(), because then it will
* include hb->lock in the blocking chain, even through we'll not in
* fact hold it while blocking. This will lead it to report -EDEADLK
* and BUG when futex_unlock_pi() interleaves with this.
*
* Therefore acquire wait_lock while holding hb->lock, but drop the
* latter before calling __rt_mutex_start_proxy_lock(). This
* interleaves with futex_unlock_pi() -- which does a similar lock
* handoff -- such that the latter can observe the futex_q::pi_state
* before __rt_mutex_start_proxy_lock() is done.
*/
raw_spin_lock_irq(&q.pi_state->pi_mutex.wait_lock);
spin_unlock(q.lock_ptr);
/*
* __rt_mutex_start_proxy_lock() unconditionally enqueues the @rt_waiter
* such that futex_unlock_pi() is guaranteed to observe the waiter when
* it sees the futex_q::pi_state.
*/
ret = __rt_mutex_start_proxy_lock(&q.pi_state->pi_mutex, &rt_waiter, current);
raw_spin_unlock_irq(&q.pi_state->pi_mutex.wait_lock);
if (ret) {
if (ret == 1)
ret = 0;
goto cleanup;
}
if (unlikely(to))
hrtimer_sleeper_start_expires(to, HRTIMER_MODE_ABS);
ret = rt_mutex_wait_proxy_lock(&q.pi_state->pi_mutex, to, &rt_waiter);
cleanup:
spin_lock(q.lock_ptr);
/*
* If we failed to acquire the lock (deadlock/signal/timeout), we must
* first acquire the hb->lock before removing the lock from the
* rt_mutex waitqueue, such that we can keep the hb and rt_mutex wait
* lists consistent.
*
* In particular; it is important that futex_unlock_pi() can not
* observe this inconsistency.
*/
if (ret && !rt_mutex_cleanup_proxy_lock(&q.pi_state->pi_mutex, &rt_waiter))
ret = 0;
no_block:
/*
* Fixup the pi_state owner and possibly acquire the lock if we
* haven't already.
*/
res = fixup_owner(uaddr, &q, !ret);
/*
* If fixup_owner() returned an error, proprogate that. If it acquired
* the lock, clear our -ETIMEDOUT or -EINTR.
*/
if (res)
ret = (res < 0) ? res : 0;
unqueue_me_pi(&q);
spin_unlock(q.lock_ptr);
goto out;
out_unlock_put_key:
queue_unlock(hb);
out:
if (to) {
hrtimer_cancel(&to->timer);
destroy_hrtimer_on_stack(&to->timer);
}
return ret != -EINTR ? ret : -ERESTARTNOINTR;
uaddr_faulted:
queue_unlock(hb);
ret = fault_in_user_writeable(uaddr);
if (ret)
goto out;
if (!(flags & FLAGS_SHARED))
goto retry_private;
goto retry;
}
/*
* Userspace attempted a TID -> 0 atomic transition, and failed.
* This is the in-kernel slowpath: we look up the PI state (if any),
* and do the rt-mutex unlock.
*/
static int futex_unlock_pi(u32 __user *uaddr, unsigned int flags)
{
u32 curval, uval, vpid = task_pid_vnr(current);
union futex_key key = FUTEX_KEY_INIT;
struct futex_hash_bucket *hb;
struct futex_q *top_waiter;
int ret;
if (!IS_ENABLED(CONFIG_FUTEX_PI))
return -ENOSYS;
retry:
if (get_user(uval, uaddr))
return -EFAULT;
/*
* We release only a lock we actually own:
*/
if ((uval & FUTEX_TID_MASK) != vpid)
return -EPERM;
ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &key, FUTEX_WRITE);
if (ret)
return ret;
hb = hash_futex(&key);
spin_lock(&hb->lock);
/*
* Check waiters first. We do not trust user space values at
* all and we at least want to know if user space fiddled
* with the futex value instead of blindly unlocking.
*/
top_waiter = futex_top_waiter(hb, &key);
if (top_waiter) {
struct futex_pi_state *pi_state = top_waiter->pi_state;
ret = -EINVAL;
if (!pi_state)
goto out_unlock;
/*
* If current does not own the pi_state then the futex is
* inconsistent and user space fiddled with the futex value.
*/
if (pi_state->owner != current)
goto out_unlock;
get_pi_state(pi_state);
/*
* By taking wait_lock while still holding hb->lock, we ensure
* there is no point where we hold neither; and therefore
* wake_futex_pi() must observe a state consistent with what we
* observed.
*
* In particular; this forces __rt_mutex_start_proxy() to
* complete such that we're guaranteed to observe the
* rt_waiter. Also see the WARN in wake_futex_pi().
*/
raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
spin_unlock(&hb->lock);
/* drops pi_state->pi_mutex.wait_lock */
ret = wake_futex_pi(uaddr, uval, pi_state);
put_pi_state(pi_state);
/*
* Success, we're done! No tricky corner cases.
*/
if (!ret)
return ret;
/*
* The atomic access to the futex value generated a
* pagefault, so retry the user-access and the wakeup:
*/
if (ret == -EFAULT)
goto pi_faulted;
/*
* A unconditional UNLOCK_PI op raced against a waiter
* setting the FUTEX_WAITERS bit. Try again.
*/
if (ret == -EAGAIN)
goto pi_retry;
/*
* wake_futex_pi has detected invalid state. Tell user
* space.
*/
return ret;
}
/*
* We have no kernel internal state, i.e. no waiters in the
* kernel. Waiters which are about to queue themselves are stuck
* on hb->lock. So we can safely ignore them. We do neither
* preserve the WAITERS bit not the OWNER_DIED one. We are the
* owner.
*/
if ((ret = cmpxchg_futex_value_locked(&curval, uaddr, uval, 0))) {
spin_unlock(&hb->lock);
switch (ret) {
case -EFAULT:
goto pi_faulted;
case -EAGAIN:
goto pi_retry;
default:
WARN_ON_ONCE(1);
return ret;
}
}
/*
* If uval has changed, let user space handle it.
*/
ret = (curval == uval) ? 0 : -EAGAIN;
out_unlock:
spin_unlock(&hb->lock);
return ret;
pi_retry:
cond_resched();
goto retry;
pi_faulted:
ret = fault_in_user_writeable(uaddr);
if (!ret)
goto retry;
return ret;
}
/**
* handle_early_requeue_pi_wakeup() - Detect early wakeup on the initial futex
* @hb: the hash_bucket futex_q was original enqueued on
* @q: the futex_q woken while waiting to be requeued
* @key2: the futex_key of the requeue target futex
* @timeout: the timeout associated with the wait (NULL if none)
*
* Detect if the task was woken on the initial futex as opposed to the requeue
* target futex. If so, determine if it was a timeout or a signal that caused
* the wakeup and return the appropriate error code to the caller. Must be
* called with the hb lock held.
*
* Return:
* - 0 = no early wakeup detected;
* - <0 = -ETIMEDOUT or -ERESTARTNOINTR
*/
static inline
int handle_early_requeue_pi_wakeup(struct futex_hash_bucket *hb,
struct futex_q *q, union futex_key *key2,
struct hrtimer_sleeper *timeout)
{
int ret = 0;
/*
* With the hb lock held, we avoid races while we process the wakeup.
* We only need to hold hb (and not hb2) to ensure atomicity as the
* wakeup code can't change q.key from uaddr to uaddr2 if we hold hb.
* It can't be requeued from uaddr2 to something else since we don't
* support a PI aware source futex for requeue.
*/
if (!match_futex(&q->key, key2)) {
WARN_ON(q->lock_ptr && (&hb->lock != q->lock_ptr));
/*
* We were woken prior to requeue by a timeout or a signal.
* Unqueue the futex_q and determine which it was.
*/
plist_del(&q->list, &hb->chain);
hb_waiters_dec(hb);
/* Handle spurious wakeups gracefully */
ret = -EWOULDBLOCK;
if (timeout && !timeout->task)
ret = -ETIMEDOUT;
else if (signal_pending(current))
ret = -ERESTARTNOINTR;
}
return ret;
}
/**
* futex_wait_requeue_pi() - Wait on uaddr and take uaddr2
* @uaddr: the futex we initially wait on (non-pi)
* @flags: futex flags (FLAGS_SHARED, FLAGS_CLOCKRT, etc.), they must be
* the same type, no requeueing from private to shared, etc.
* @val: the expected value of uaddr
* @abs_time: absolute timeout
* @bitset: 32 bit wakeup bitset set by userspace, defaults to all
* @uaddr2: the pi futex we will take prior to returning to user-space
*
* The caller will wait on uaddr and will be requeued by futex_requeue() to
* uaddr2 which must be PI aware and unique from uaddr. Normal wakeup will wake
* on uaddr2 and complete the acquisition of the rt_mutex prior to returning to
* userspace. This ensures the rt_mutex maintains an owner when it has waiters;
* without one, the pi logic would not know which task to boost/deboost, if
* there was a need to.
*
* We call schedule in futex_wait_queue_me() when we enqueue and return there
* via the following--
* 1) wakeup on uaddr2 after an atomic lock acquisition by futex_requeue()
* 2) wakeup on uaddr2 after a requeue
* 3) signal
* 4) timeout
*
* If 3, cleanup and return -ERESTARTNOINTR.
*
* If 2, we may then block on trying to take the rt_mutex and return via:
* 5) successful lock
* 6) signal
* 7) timeout
* 8) other lock acquisition failure
*
* If 6, return -EWOULDBLOCK (restarting the syscall would do the same).
*
* If 4 or 7, we cleanup and return with -ETIMEDOUT.
*
* Return:
* - 0 - On success;
* - <0 - On error
*/
static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
u32 val, ktime_t *abs_time, u32 bitset,
u32 __user *uaddr2)
{
struct hrtimer_sleeper timeout, *to;
struct rt_mutex_waiter rt_waiter;
struct futex_hash_bucket *hb;
union futex_key key2 = FUTEX_KEY_INIT;
struct futex_q q = futex_q_init;
int res, ret;
if (!IS_ENABLED(CONFIG_FUTEX_PI))
return -ENOSYS;
if (uaddr == uaddr2)
return -EINVAL;
if (!bitset)
return -EINVAL;
to = futex_setup_timer(abs_time, &timeout, flags,
current->timer_slack_ns);
/*
* The waiter is allocated on our stack, manipulated by the requeue
* code while we sleep on uaddr.
*/
rt_mutex_init_waiter(&rt_waiter);
ret = get_futex_key(uaddr2, flags & FLAGS_SHARED, &key2, FUTEX_WRITE);
if (unlikely(ret != 0))
goto out;
q.bitset = bitset;
q.rt_waiter = &rt_waiter;
q.requeue_pi_key = &key2;
/*
* Prepare to wait on uaddr. On success, increments q.key (key1) ref
* count.
*/
ret = futex_wait_setup(uaddr, val, flags, &q, &hb);
if (ret)
goto out;
/*
* The check above which compares uaddrs is not sufficient for
* shared futexes. We need to compare the keys:
*/
if (match_futex(&q.key, &key2)) {
queue_unlock(hb);
ret = -EINVAL;
goto out;
}
/* Queue the futex_q, drop the hb lock, wait for wakeup. */
futex_wait_queue_me(hb, &q, to);
spin_lock(&hb->lock);
ret = handle_early_requeue_pi_wakeup(hb, &q, &key2, to);
spin_unlock(&hb->lock);
if (ret)
goto out;
/*
* In order for us to be here, we know our q.key == key2, and since
* we took the hb->lock above, we also know that futex_requeue() has
* completed and we no longer have to concern ourselves with a wakeup
* race with the atomic proxy lock acquisition by the requeue code. The
* futex_requeue dropped our key1 reference and incremented our key2
* reference count.
*/
/*
* Check if the requeue code acquired the second futex for us and do
* any pertinent fixup.
*/
if (!q.rt_waiter) {
if (q.pi_state && (q.pi_state->owner != current)) {
spin_lock(q.lock_ptr);
ret = fixup_owner(uaddr2, &q, true);
/*
* Drop the reference to the pi state which
* the requeue_pi() code acquired for us.
*/
put_pi_state(q.pi_state);
spin_unlock(q.lock_ptr);
/*
* Adjust the return value. It's either -EFAULT or
* success (1) but the caller expects 0 for success.
*/
ret = ret < 0 ? ret : 0;
}
} else {
struct rt_mutex *pi_mutex;
/*
* We have been woken up by futex_unlock_pi(), a timeout, or a
* signal. futex_unlock_pi() will not destroy the lock_ptr nor
* the pi_state.
*/
WARN_ON(!q.pi_state);
pi_mutex = &q.pi_state->pi_mutex;
ret = rt_mutex_wait_proxy_lock(pi_mutex, to, &rt_waiter);
spin_lock(q.lock_ptr);
if (ret && !rt_mutex_cleanup_proxy_lock(pi_mutex, &rt_waiter))
ret = 0;
debug_rt_mutex_free_waiter(&rt_waiter);
/*
* Fixup the pi_state owner and possibly acquire the lock if we
* haven't already.
*/
res = fixup_owner(uaddr2, &q, !ret);
/*
* If fixup_owner() returned an error, proprogate that. If it
* acquired the lock, clear -ETIMEDOUT or -EINTR.
*/
if (res)
ret = (res < 0) ? res : 0;
unqueue_me_pi(&q);
spin_unlock(q.lock_ptr);
}
if (ret == -EINTR) {
/*
* We've already been requeued, but cannot restart by calling
* futex_lock_pi() directly. We could restart this syscall, but
* it would detect that the user space "val" changed and return
* -EWOULDBLOCK. Save the overhead of the restart and return
* -EWOULDBLOCK directly.
*/
ret = -EWOULDBLOCK;
}
out:
if (to) {
hrtimer_cancel(&to->timer);
destroy_hrtimer_on_stack(&to->timer);
}
return ret;
}
/*
* Support for robust futexes: the kernel cleans up held futexes at
* thread exit time.
*
* Implementation: user-space maintains a per-thread list of locks it
* is holding. Upon do_exit(), the kernel carefully walks this list,
* and marks all locks that are owned by this thread with the
* FUTEX_OWNER_DIED bit, and wakes up a waiter (if any). The list is
* always manipulated with the lock held, so the list is private and
* per-thread. Userspace also maintains a per-thread 'list_op_pending'
* field, to allow the kernel to clean up if the thread dies after
* acquiring the lock, but just before it could have added itself to
* the list. There can only be one such pending lock.
*/
/**
* sys_set_robust_list() - Set the robust-futex list head of a task
* @head: pointer to the list-head
* @len: length of the list-head, as userspace expects
*/
SYSCALL_DEFINE2(set_robust_list, struct robust_list_head __user *, head,
size_t, len)
{
if (!futex_cmpxchg_enabled)
return -ENOSYS;
/*
* The kernel knows only one size for now:
*/
if (unlikely(len != sizeof(*head)))
return -EINVAL;
current->robust_list = head;
return 0;
}
/**
* sys_get_robust_list() - Get the robust-futex list head of a task
* @pid: pid of the process [zero for current task]
* @head_ptr: pointer to a list-head pointer, the kernel fills it in
* @len_ptr: pointer to a length field, the kernel fills in the header size
*/
SYSCALL_DEFINE3(get_robust_list, int, pid,
struct robust_list_head __user * __user *, head_ptr,
size_t __user *, len_ptr)
{
struct robust_list_head __user *head;
unsigned long ret;
struct task_struct *p;
if (!futex_cmpxchg_enabled)
return -ENOSYS;
rcu_read_lock();
ret = -ESRCH;
if (!pid)
p = current;
else {
p = find_task_by_vpid(pid);
if (!p)
goto err_unlock;
}
ret = -EPERM;
if (!ptrace_may_access(p, PTRACE_MODE_READ_REALCREDS))
goto err_unlock;
head = p->robust_list;
rcu_read_unlock();
if (put_user(sizeof(*head), len_ptr))
return -EFAULT;
return put_user(head, head_ptr);
err_unlock:
rcu_read_unlock();
return ret;
}
/* Constants for the pending_op argument of handle_futex_death */
#define HANDLE_DEATH_PENDING true
#define HANDLE_DEATH_LIST false
/*
* Process a futex-list entry, check whether it's owned by the
* dying task, and do notification if so:
*/
static int handle_futex_death(u32 __user *uaddr, struct task_struct *curr,
bool pi, bool pending_op)
{
u32 uval, nval, mval;
int err;
/* Futex address must be 32bit aligned */
if ((((unsigned long)uaddr) % sizeof(*uaddr)) != 0)
return -1;
retry:
if (get_user(uval, uaddr))
return -1;
/*
* Special case for regular (non PI) futexes. The unlock path in
* user space has two race scenarios:
*
* 1. The unlock path releases the user space futex value and
* before it can execute the futex() syscall to wake up
* waiters it is killed.
*
* 2. A woken up waiter is killed before it can acquire the
* futex in user space.
*
* In both cases the TID validation below prevents a wakeup of
* potential waiters which can cause these waiters to block
* forever.
*
* In both cases the following conditions are met:
*
* 1) task->robust_list->list_op_pending != NULL
* @pending_op == true
* 2) User space futex value == 0
* 3) Regular futex: @pi == false
*
* If these conditions are met, it is safe to attempt waking up a
* potential waiter without touching the user space futex value and
* trying to set the OWNER_DIED bit. The user space futex value is
* uncontended and the rest of the user space mutex state is
* consistent, so a woken waiter will just take over the
* uncontended futex. Setting the OWNER_DIED bit would create
* inconsistent state and malfunction of the user space owner died
* handling.
*/
if (pending_op && !pi && !uval) {
futex_wake(uaddr, 1, 1, FUTEX_BITSET_MATCH_ANY);
return 0;
}
if ((uval & FUTEX_TID_MASK) != task_pid_vnr(curr))
return 0;
/*
* Ok, this dying thread is truly holding a futex
* of interest. Set the OWNER_DIED bit atomically
* via cmpxchg, and if the value had FUTEX_WAITERS
* set, wake up a waiter (if any). (We have to do a
* futex_wake() even if OWNER_DIED is already set -
* to handle the rare but possible case of recursive
* thread-death.) The rest of the cleanup is done in
* userspace.
*/
mval = (uval & FUTEX_WAITERS) | FUTEX_OWNER_DIED;
/*
* We are not holding a lock here, but we want to have
* the pagefault_disable/enable() protection because
* we want to handle the fault gracefully. If the
* access fails we try to fault in the futex with R/W
* verification via get_user_pages. get_user() above
* does not guarantee R/W access. If that fails we
* give up and leave the futex locked.
*/
if ((err = cmpxchg_futex_value_locked(&nval, uaddr, uval, mval))) {
switch (err) {
case -EFAULT:
if (fault_in_user_writeable(uaddr))
return -1;
goto retry;
case -EAGAIN:
cond_resched();
goto retry;
default:
WARN_ON_ONCE(1);
return err;
}
}
if (nval != uval)
goto retry;
/*
* Wake robust non-PI futexes here. The wakeup of
* PI futexes happens in exit_pi_state():
*/
if (!pi && (uval & FUTEX_WAITERS))
futex_wake(uaddr, 1, 1, FUTEX_BITSET_MATCH_ANY);
return 0;
}
/*
* Fetch a robust-list pointer. Bit 0 signals PI futexes:
*/
static inline int fetch_robust_entry(struct robust_list __user **entry,
struct robust_list __user * __user *head,
unsigned int *pi)
{
unsigned long uentry;
if (get_user(uentry, (unsigned long __user *)head))
return -EFAULT;
*entry = (void __user *)(uentry & ~1UL);
*pi = uentry & 1;
return 0;
}
/*
* Walk curr->robust_list (very carefully, it's a userspace list!)
* and mark any locks found there dead, and notify any waiters.
*
* We silently return on any sign of list-walking problem.
*/
static void exit_robust_list(struct task_struct *curr)
{
struct robust_list_head __user *head = curr->robust_list;
struct robust_list __user *entry, *next_entry, *pending;
unsigned int limit = ROBUST_LIST_LIMIT, pi, pip;
unsigned int next_pi;
unsigned long futex_offset;
int rc;
if (!futex_cmpxchg_enabled)
return;
/*
* Fetch the list head (which was registered earlier, via
* sys_set_robust_list()):
*/
if (fetch_robust_entry(&entry, &head->list.next, &pi))
return;
/*
* Fetch the relative futex offset:
*/
if (get_user(futex_offset, &head->futex_offset))
return;
/*
* Fetch any possibly pending lock-add first, and handle it
* if it exists:
*/
if (fetch_robust_entry(&pending, &head->list_op_pending, &pip))
return;
next_entry = NULL; /* avoid warning with gcc */
while (entry != &head->list) {
/*
* Fetch the next entry in the list before calling
* handle_futex_death:
*/
rc = fetch_robust_entry(&next_entry, &entry->next, &next_pi);
/*
* A pending lock might already be on the list, so
* don't process it twice:
*/
if (entry != pending) {
if (handle_futex_death((void __user *)entry + futex_offset,
curr, pi, HANDLE_DEATH_LIST))
return;
}
if (rc)
return;
entry = next_entry;
pi = next_pi;
/*
* Avoid excessively long or circular lists:
*/
if (!--limit)
break;
cond_resched();
}
if (pending) {
handle_futex_death((void __user *)pending + futex_offset,
curr, pip, HANDLE_DEATH_PENDING);
}
}
static void futex_cleanup(struct task_struct *tsk)
{
if (unlikely(tsk->robust_list)) {
exit_robust_list(tsk);
tsk->robust_list = NULL;
}
#ifdef CONFIG_COMPAT
if (unlikely(tsk->compat_robust_list)) {
compat_exit_robust_list(tsk);
tsk->compat_robust_list = NULL;
}
#endif
if (unlikely(!list_empty(&tsk->pi_state_list)))
exit_pi_state_list(tsk);
}
/**
* futex_exit_recursive - Set the tasks futex state to FUTEX_STATE_DEAD
* @tsk: task to set the state on
*
* Set the futex exit state of the task lockless. The futex waiter code
* observes that state when a task is exiting and loops until the task has
* actually finished the futex cleanup. The worst case for this is that the
* waiter runs through the wait loop until the state becomes visible.
*
* This is called from the recursive fault handling path in do_exit().
*
* This is best effort. Either the futex exit code has run already or
* not. If the OWNER_DIED bit has been set on the futex then the waiter can
* take it over. If not, the problem is pushed back to user space. If the
* futex exit code did not run yet, then an already queued waiter might
* block forever, but there is nothing which can be done about that.
*/
void futex_exit_recursive(struct task_struct *tsk)
{
/* If the state is FUTEX_STATE_EXITING then futex_exit_mutex is held */
if (tsk->futex_state == FUTEX_STATE_EXITING)
mutex_unlock(&tsk->futex_exit_mutex);
tsk->futex_state = FUTEX_STATE_DEAD;
}
static void futex_cleanup_begin(struct task_struct *tsk)
{
/*
* Prevent various race issues against a concurrent incoming waiter
* including live locks by forcing the waiter to block on
* tsk->futex_exit_mutex when it observes FUTEX_STATE_EXITING in
* attach_to_pi_owner().
*/
mutex_lock(&tsk->futex_exit_mutex);
/*
* Switch the state to FUTEX_STATE_EXITING under tsk->pi_lock.
*
* This ensures that all subsequent checks of tsk->futex_state in
* attach_to_pi_owner() must observe FUTEX_STATE_EXITING with
* tsk->pi_lock held.
*
* It guarantees also that a pi_state which was queued right before
* the state change under tsk->pi_lock by a concurrent waiter must
* be observed in exit_pi_state_list().
*/
raw_spin_lock_irq(&tsk->pi_lock);
tsk->futex_state = FUTEX_STATE_EXITING;
raw_spin_unlock_irq(&tsk->pi_lock);
}
static void futex_cleanup_end(struct task_struct *tsk, int state)
{
/*
* Lockless store. The only side effect is that an observer might
* take another loop until it becomes visible.
*/
tsk->futex_state = state;
/*
* Drop the exit protection. This unblocks waiters which observed
* FUTEX_STATE_EXITING to reevaluate the state.
*/
mutex_unlock(&tsk->futex_exit_mutex);
}
void futex_exec_release(struct task_struct *tsk)
{
/*
* The state handling is done for consistency, but in the case of
* exec() there is no way to prevent futher damage as the PID stays
* the same. But for the unlikely and arguably buggy case that a
* futex is held on exec(), this provides at least as much state
* consistency protection which is possible.
*/
futex_cleanup_begin(tsk);
futex_cleanup(tsk);
/*
* Reset the state to FUTEX_STATE_OK. The task is alive and about
* exec a new binary.
*/
futex_cleanup_end(tsk, FUTEX_STATE_OK);
}
void futex_exit_release(struct task_struct *tsk)
{
futex_cleanup_begin(tsk);
futex_cleanup(tsk);
futex_cleanup_end(tsk, FUTEX_STATE_DEAD);
}
long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
u32 __user *uaddr2, u32 val2, u32 val3)
{
int cmd = op & FUTEX_CMD_MASK;
unsigned int flags = 0;
if (!(op & FUTEX_PRIVATE_FLAG))
flags |= FLAGS_SHARED;
if (op & FUTEX_CLOCK_REALTIME) {
flags |= FLAGS_CLOCKRT;
if (cmd != FUTEX_WAIT_BITSET && cmd != FUTEX_WAIT_REQUEUE_PI)
return -ENOSYS;
}
switch (cmd) {
case FUTEX_LOCK_PI:
case FUTEX_UNLOCK_PI:
case FUTEX_TRYLOCK_PI:
case FUTEX_WAIT_REQUEUE_PI:
case FUTEX_CMP_REQUEUE_PI:
if (!futex_cmpxchg_enabled)
return -ENOSYS;
}
switch (cmd) {
case FUTEX_WAIT:
val3 = FUTEX_BITSET_MATCH_ANY;
fallthrough;
case FUTEX_WAIT_BITSET:
return futex_wait(uaddr, flags, val, timeout, val3);
case FUTEX_WAKE:
val3 = FUTEX_BITSET_MATCH_ANY;
fallthrough;
case FUTEX_WAKE_BITSET:
return futex_wake(uaddr, flags, val, val3);
case FUTEX_REQUEUE:
return futex_requeue(uaddr, flags, uaddr2, val, val2, NULL, 0);
case FUTEX_CMP_REQUEUE:
return futex_requeue(uaddr, flags, uaddr2, val, val2, &val3, 0);
case FUTEX_WAKE_OP:
return futex_wake_op(uaddr, flags, uaddr2, val, val2, val3);
case FUTEX_LOCK_PI:
return futex_lock_pi(uaddr, flags, timeout, 0);
case FUTEX_UNLOCK_PI:
return futex_unlock_pi(uaddr, flags);
case FUTEX_TRYLOCK_PI:
return futex_lock_pi(uaddr, flags, NULL, 1);
case FUTEX_WAIT_REQUEUE_PI:
val3 = FUTEX_BITSET_MATCH_ANY;
return futex_wait_requeue_pi(uaddr, flags, val, timeout, val3,
uaddr2);
case FUTEX_CMP_REQUEUE_PI:
return futex_requeue(uaddr, flags, uaddr2, val, val2, &val3, 1);
}
return -ENOSYS;
}
static __always_inline bool futex_cmd_has_timeout(u32 cmd)
{
switch (cmd) {
case FUTEX_WAIT:
case FUTEX_LOCK_PI:
case FUTEX_WAIT_BITSET:
case FUTEX_WAIT_REQUEUE_PI:
return true;
}
return false;
}
static __always_inline int
futex_init_timeout(u32 cmd, u32 op, struct timespec64 *ts, ktime_t *t)
{
if (!timespec64_valid(ts))
return -EINVAL;
*t = timespec64_to_ktime(*ts);
if (cmd == FUTEX_WAIT)
*t = ktime_add_safe(ktime_get(), *t);
else if (cmd != FUTEX_LOCK_PI && !(op & FUTEX_CLOCK_REALTIME))
*t = timens_ktime_to_host(CLOCK_MONOTONIC, *t);
return 0;
}
SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
const struct __kernel_timespec __user *, utime,
u32 __user *, uaddr2, u32, val3)
{
int ret, cmd = op & FUTEX_CMD_MASK;
ktime_t t, *tp = NULL;
struct timespec64 ts;
if (utime && futex_cmd_has_timeout(cmd)) {
if (unlikely(should_fail_futex(!(op & FUTEX_PRIVATE_FLAG))))
return -EFAULT;
if (get_timespec64(&ts, utime))
return -EFAULT;
ret = futex_init_timeout(cmd, op, &ts, &t);
if (ret)
return ret;
tp = &t;
}
return do_futex(uaddr, op, val, tp, uaddr2, (unsigned long)utime, val3);
}
#ifdef CONFIG_COMPAT
/*
* Fetch a robust-list pointer. Bit 0 signals PI futexes:
*/
static inline int
compat_fetch_robust_entry(compat_uptr_t *uentry, struct robust_list __user **entry,
compat_uptr_t __user *head, unsigned int *pi)
{
if (get_user(*uentry, head))
return -EFAULT;
*entry = compat_ptr((*uentry) & ~1);
*pi = (unsigned int)(*uentry) & 1;
return 0;
}
static void __user *futex_uaddr(struct robust_list __user *entry,
compat_long_t futex_offset)
{
compat_uptr_t base = ptr_to_compat(entry);
void __user *uaddr = compat_ptr(base + futex_offset);
return uaddr;
}
/*
* Walk curr->robust_list (very carefully, it's a userspace list!)
* and mark any locks found there dead, and notify any waiters.
*
* We silently return on any sign of list-walking problem.
*/
static void compat_exit_robust_list(struct task_struct *curr)
{
struct compat_robust_list_head __user *head = curr->compat_robust_list;
struct robust_list __user *entry, *next_entry, *pending;
unsigned int limit = ROBUST_LIST_LIMIT, pi, pip;
unsigned int next_pi;
compat_uptr_t uentry, next_uentry, upending;
compat_long_t futex_offset;
int rc;
if (!futex_cmpxchg_enabled)
return;
/*
* Fetch the list head (which was registered earlier, via
* sys_set_robust_list()):
*/
if (compat_fetch_robust_entry(&uentry, &entry, &head->list.next, &pi))
return;
/*
* Fetch the relative futex offset:
*/
if (get_user(futex_offset, &head->futex_offset))
return;
/*
* Fetch any possibly pending lock-add first, and handle it
* if it exists:
*/
if (compat_fetch_robust_entry(&upending, &pending,
&head->list_op_pending, &pip))
return;
next_entry = NULL; /* avoid warning with gcc */
while (entry != (struct robust_list __user *) &head->list) {
/*
* Fetch the next entry in the list before calling
* handle_futex_death:
*/
rc = compat_fetch_robust_entry(&next_uentry, &next_entry,
(compat_uptr_t __user *)&entry->next, &next_pi);
/*
* A pending lock might already be on the list, so
* dont process it twice:
*/
if (entry != pending) {
void __user *uaddr = futex_uaddr(entry, futex_offset);
if (handle_futex_death(uaddr, curr, pi,
HANDLE_DEATH_LIST))
return;
}
if (rc)
return;
uentry = next_uentry;
entry = next_entry;
pi = next_pi;
/*
* Avoid excessively long or circular lists:
*/
if (!--limit)
break;
cond_resched();
}
if (pending) {
void __user *uaddr = futex_uaddr(pending, futex_offset);
handle_futex_death(uaddr, curr, pip, HANDLE_DEATH_PENDING);
}
}
COMPAT_SYSCALL_DEFINE2(set_robust_list,
struct compat_robust_list_head __user *, head,
compat_size_t, len)
{
if (!futex_cmpxchg_enabled)
return -ENOSYS;
if (unlikely(len != sizeof(*head)))
return -EINVAL;
current->compat_robust_list = head;
return 0;
}
COMPAT_SYSCALL_DEFINE3(get_robust_list, int, pid,
compat_uptr_t __user *, head_ptr,
compat_size_t __user *, len_ptr)
{
struct compat_robust_list_head __user *head;
unsigned long ret;
struct task_struct *p;
if (!futex_cmpxchg_enabled)
return -ENOSYS;
rcu_read_lock();
ret = -ESRCH;
if (!pid)
p = current;
else {
p = find_task_by_vpid(pid);
if (!p)
goto err_unlock;
}
ret = -EPERM;
if (!ptrace_may_access(p, PTRACE_MODE_READ_REALCREDS))
goto err_unlock;
head = p->compat_robust_list;
rcu_read_unlock();
if (put_user(sizeof(*head), len_ptr))
return -EFAULT;
return put_user(ptr_to_compat(head), head_ptr);
err_unlock:
rcu_read_unlock();
return ret;
}
#endif /* CONFIG_COMPAT */
#ifdef CONFIG_COMPAT_32BIT_TIME
SYSCALL_DEFINE6(futex_time32, u32 __user *, uaddr, int, op, u32, val,
const struct old_timespec32 __user *, utime, u32 __user *, uaddr2,
u32, val3)
{
int ret, cmd = op & FUTEX_CMD_MASK;
ktime_t t, *tp = NULL;
struct timespec64 ts;
if (utime && futex_cmd_has_timeout(cmd)) {
if (get_old_timespec32(&ts, utime))
return -EFAULT;
ret = futex_init_timeout(cmd, op, &ts, &t);
if (ret)
return ret;
tp = &t;
}
return do_futex(uaddr, op, val, tp, uaddr2, (unsigned long)utime, val3);
}
#endif /* CONFIG_COMPAT_32BIT_TIME */
static void __init futex_detect_cmpxchg(void)
{
#ifndef CONFIG_HAVE_FUTEX_CMPXCHG
u32 curval;
/*
* This will fail and we want it. Some arch implementations do
* runtime detection of the futex_atomic_cmpxchg_inatomic()
* functionality. We want to know that before we call in any
* of the complex code paths. Also we want to prevent
* registration of robust lists in that case. NULL is
* guaranteed to fault and we get -EFAULT on functional
* implementation, the non-functional ones will return
* -ENOSYS.
*/
if (cmpxchg_futex_value_locked(&curval, NULL, 0, 0) == -EFAULT)
futex_cmpxchg_enabled = 1;
#endif
}
static int __init futex_init(void)
{
unsigned int futex_shift;
unsigned long i;
#if CONFIG_BASE_SMALL
futex_hashsize = 16;
#else
futex_hashsize = roundup_pow_of_two(256 * num_possible_cpus());
#endif
futex_queues = alloc_large_system_hash("futex", sizeof(*futex_queues),
futex_hashsize, 0,
futex_hashsize < 256 ? HASH_SMALL : 0,
&futex_shift, NULL,
futex_hashsize, futex_hashsize);
futex_hashsize = 1UL << futex_shift;
futex_detect_cmpxchg();
for (i = 0; i < futex_hashsize; i++) {
atomic_set(&futex_queues[i].waiters, 0);
plist_head_init(&futex_queues[i].chain);
spin_lock_init(&futex_queues[i].lock);
}
return 0;
}
core_initcall(futex_init);
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