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futexes: Remove rw parameter from get_futex_key()

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Currently, futexes have two problem:

A) The current futex code doesn't handle private file mappings properly.

get_futex_key() uses PageAnon() to distinguish file and
anon, which can cause the following bad scenario:

  1) thread-A call futex(private-mapping, FUTEX_WAIT), it
     sleeps on file mapping object.
  2) thread-B writes a variable and it makes it cow.
  3) thread-B calls futex(private-mapping, FUTEX_WAKE), it
     wakes up blocked thread on the anonymous page. (but it's nothing)

B) Current futex code doesn't handle zero page properly.

Read mode get_user_pages() can return zero page, but current
futex code doesn't handle it at all. Then, zero page makes
infinite loop internally.

The solution is to use write mode get_user_page() always for
page lookup. It prevents the lookup of both file page of private
mappings and zero page.

Performance concerns:

Probaly very little, because glibc always initialize variables
for futex before to call futex(). It means glibc users never see
the overhead of this patch.

Compatibility concerns:

This patch has few compatibility issues. After this patch,
FUTEX_WAIT require writable access to futex variables (read-only
mappings makes EFAULT). But practically it's not a problem,
glibc always initalizes variables for futexes explicitly - nobody
uses read-only mappings.

Reported-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Darren Hart <dvhltc@us.ibm.com>
Cc: <stable@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Nick Piggin <npiggin@suse.de>
Cc: Ulrich Drepper <drepper@gmail.com>
LKML-Reference: <20100105162633.45A2.A69D9226@jp.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
KOSAKI Motohiro 2010-01-05 16:32:43 +09:00 committed by Ingo Molnar
parent 7284ce6c9f
commit 7485d0d375
1 changed files with 12 additions and 15 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

27
kernel/futex.c
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@ -203,8 +203,6 @@ static void drop_futex_key_refs(union futex_key *key)
* @uaddr: virtual address of the futex
* @fshared: 0 for a PROCESS_PRIVATE futex, 1 for PROCESS_SHARED
* @key: address where result is stored.
* @rw: mapping needs to be read/write (values: VERIFY_READ,
* VERIFY_WRITE)
*
* Returns a negative error code or 0
* The key words are stored in *key on success.
@ -216,7 +214,7 @@ static void drop_futex_key_refs(union futex_key *key)
* lock_page() might sleep, the caller should not hold a spinlock.
*/
static int
get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key, int rw)
get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key)
{
unsigned long address = (unsigned long)uaddr;
struct mm_struct *mm = current->mm;
@ -239,7 +237,7 @@ get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key, int rw)
* but access_ok() should be faster than find_vma()
*/
if (!fshared) {
if (unlikely(!access_ok(rw, uaddr, sizeof(u32))))
if (unlikely(!access_ok(VERIFY_WRITE, uaddr, sizeof(u32))))
return -EFAULT;
key->private.mm = mm;
key->private.address = address;
@ -248,7 +246,7 @@ get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key, int rw)
}
again:
err = get_user_pages_fast(address, 1, rw == VERIFY_WRITE, &page);
err = get_user_pages_fast(address, 1, 1, &page);
if (err < 0)
return err;
@ -867,7 +865,7 @@ static int futex_wake(u32 __user *uaddr, int fshared, int nr_wake, u32 bitset)
if (!bitset)
return -EINVAL;
ret = get_futex_key(uaddr, fshared, &key, VERIFY_READ);
ret = get_futex_key(uaddr, fshared, &key);
if (unlikely(ret != 0))
goto out;
@ -913,10 +911,10 @@ futex_wake_op(u32 __user *uaddr1, int fshared, u32 __user *uaddr2,
int ret, op_ret;
retry:
ret = get_futex_key(uaddr1, fshared, &key1, VERIFY_READ);
ret = get_futex_key(uaddr1, fshared, &key1);
if (unlikely(ret != 0))
goto out;
ret = get_futex_key(uaddr2, fshared, &key2, VERIFY_WRITE);
ret = get_futex_key(uaddr2, fshared, &key2);
if (unlikely(ret != 0))
goto out_put_key1;
@ -1175,11 +1173,10 @@ retry:
pi_state = NULL;
}
ret = get_futex_key(uaddr1, fshared, &key1, VERIFY_READ);
ret = get_futex_key(uaddr1, fshared, &key1);
if (unlikely(ret != 0))
goto out;
ret = get_futex_key(uaddr2, fshared, &key2,
requeue_pi ? VERIFY_WRITE : VERIFY_READ);
ret = get_futex_key(uaddr2, fshared, &key2);
if (unlikely(ret != 0))
goto out_put_key1;
@ -1738,7 +1735,7 @@ static int futex_wait_setup(u32 __user *uaddr, u32 val, int fshared,
*/
retry:
q->key = FUTEX_KEY_INIT;
ret = get_futex_key(uaddr, fshared, &q->key, VERIFY_READ);
ret = get_futex_key(uaddr, fshared, &q->key);
if (unlikely(ret != 0))
return ret;
@ -1904,7 +1901,7 @@ static int futex_lock_pi(u32 __user *uaddr, int fshared,
q.requeue_pi_key = NULL;
retry:
q.key = FUTEX_KEY_INIT;
ret = get_futex_key(uaddr, fshared, &q.key, VERIFY_WRITE);
ret = get_futex_key(uaddr, fshared, &q.key);
if (unlikely(ret != 0))
goto out;
@ -2023,7 +2020,7 @@ retry:
if ((uval & FUTEX_TID_MASK) != task_pid_vnr(current))
return -EPERM;
ret = get_futex_key(uaddr, fshared, &key, VERIFY_WRITE);
ret = get_futex_key(uaddr, fshared, &key);
if (unlikely(ret != 0))
goto out;
@ -2215,7 +2212,7 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, int fshared,
rt_waiter.task = NULL;
key2 = FUTEX_KEY_INIT;
ret = get_futex_key(uaddr2, fshared, &key2, VERIFY_WRITE);
ret = get_futex_key(uaddr2, fshared, &key2);
if (unlikely(ret != 0))
goto out;