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bpf: Introduce task_vma bpf_iter

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Introduce task_vma bpf_iter to print memory information of a process. It
can be used to print customized information similar to /proc/<pid>/maps.

Current /proc/<pid>/maps and /proc/<pid>/smaps provide information of
vma's of a process. However, these information are not flexible enough to
cover all use cases. For example, if a vma cover mixed 2MB pages and 4kB
pages (x86_64), there is no easy way to tell which address ranges are
backed by 2MB pages. task_vma solves the problem by enabling the user to
generate customize information based on the vma (and vma->vm_mm,
vma->vm_file, etc.).

To access the vma safely in the BPF program, task_vma iterator holds
target mmap_lock while calling the BPF program. If the mmap_lock is
contended, task_vma unlocks mmap_lock between iterations to unblock the
writer(s). This lock contention avoidance mechanism is similar to the one
used in show_smaps_rollup().

Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210212183107.50963-2-songliubraving@fb.com
This commit is contained in:
Song Liu 2021-02-12 10:31:05 -08:00 committed by Alexei Starovoitov
parent a79e88dd2c
commit 3a7b35b899
1 changed files with 266 additions and 1 deletions
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267
kernel/bpf/task_iter.c
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@ -286,9 +286,248 @@ static const struct seq_operations task_file_seq_ops = {
.show = task_file_seq_show,
};
struct bpf_iter_seq_task_vma_info {
/* The first field must be struct bpf_iter_seq_task_common.
* this is assumed by {init, fini}_seq_pidns() callback functions.
*/
struct bpf_iter_seq_task_common common;
struct task_struct *task;
struct vm_area_struct *vma;
u32 tid;
unsigned long prev_vm_start;
unsigned long prev_vm_end;
};
enum bpf_task_vma_iter_find_op {
task_vma_iter_first_vma, /* use mm->mmap */
task_vma_iter_next_vma, /* use curr_vma->vm_next */
task_vma_iter_find_vma, /* use find_vma() to find next vma */
};
static struct vm_area_struct *
task_vma_seq_get_next(struct bpf_iter_seq_task_vma_info *info)
{
struct pid_namespace *ns = info->common.ns;
enum bpf_task_vma_iter_find_op op;
struct vm_area_struct *curr_vma;
struct task_struct *curr_task;
u32 curr_tid = info->tid;
/* If this function returns a non-NULL vma, it holds a reference to
* the task_struct, and holds read lock on vma->mm->mmap_lock.
* If this function returns NULL, it does not hold any reference or
* lock.
*/
if (info->task) {
curr_task = info->task;
curr_vma = info->vma;
/* In case of lock contention, drop mmap_lock to unblock
* the writer.
*
* After relock, call find(mm, prev_vm_end - 1) to find
* new vma to process.
*
* +------+------+-----------+
* | VMA1 | VMA2 | VMA3 |
* +------+------+-----------+
* | | | |
* 4k 8k 16k 400k
*
* For example, curr_vma == VMA2. Before unlock, we set
*
* prev_vm_start = 8k
* prev_vm_end = 16k
*
* There are a few cases:
*
* 1) VMA2 is freed, but VMA3 exists.
*
* find_vma() will return VMA3, just process VMA3.
*
* 2) VMA2 still exists.
*
* find_vma() will return VMA2, process VMA2->next.
*
* 3) no more vma in this mm.
*
* Process the next task.
*
* 4) find_vma() returns a different vma, VMA2'.
*
* 4.1) If VMA2 covers same range as VMA2', skip VMA2',
* because we already covered the range;
* 4.2) VMA2 and VMA2' covers different ranges, process
* VMA2'.
*/
if (mmap_lock_is_contended(curr_task->mm)) {
info->prev_vm_start = curr_vma->vm_start;
info->prev_vm_end = curr_vma->vm_end;
op = task_vma_iter_find_vma;
mmap_read_unlock(curr_task->mm);
if (mmap_read_lock_killable(curr_task->mm))
goto finish;
} else {
op = task_vma_iter_next_vma;
}
} else {
again:
curr_task = task_seq_get_next(ns, &curr_tid, true);
if (!curr_task) {
info->tid = curr_tid + 1;
goto finish;
}
if (curr_tid != info->tid) {
info->tid = curr_tid;
/* new task, process the first vma */
op = task_vma_iter_first_vma;
} else {
/* Found the same tid, which means the user space
* finished data in previous buffer and read more.
* We dropped mmap_lock before returning to user
* space, so it is necessary to use find_vma() to
* find the next vma to process.
*/
op = task_vma_iter_find_vma;
}
if (!curr_task->mm)
goto next_task;
if (mmap_read_lock_killable(curr_task->mm))
goto finish;
}
switch (op) {
case task_vma_iter_first_vma:
curr_vma = curr_task->mm->mmap;
break;
case task_vma_iter_next_vma:
curr_vma = curr_vma->vm_next;
break;
case task_vma_iter_find_vma:
/* We dropped mmap_lock so it is necessary to use find_vma
* to find the next vma. This is similar to the mechanism
* in show_smaps_rollup().
*/
curr_vma = find_vma(curr_task->mm, info->prev_vm_end - 1);
/* case 1) and 4.2) above just use curr_vma */
/* check for case 2) or case 4.1) above */
if (curr_vma &&
curr_vma->vm_start == info->prev_vm_start &&
curr_vma->vm_end == info->prev_vm_end)
curr_vma = curr_vma->vm_next;
break;
}
if (!curr_vma) {
/* case 3) above, or case 2) 4.1) with vma->next == NULL */
mmap_read_unlock(curr_task->mm);
goto next_task;
}
info->task = curr_task;
info->vma = curr_vma;
return curr_vma;
next_task:
put_task_struct(curr_task);
info->task = NULL;
curr_tid++;
goto again;
finish:
if (curr_task)
put_task_struct(curr_task);
info->task = NULL;
info->vma = NULL;
return NULL;
}
static void *task_vma_seq_start(struct seq_file *seq, loff_t *pos)
{
struct bpf_iter_seq_task_vma_info *info = seq->private;
struct vm_area_struct *vma;
vma = task_vma_seq_get_next(info);
if (vma && *pos == 0)
++*pos;
return vma;
}
static void *task_vma_seq_next(struct seq_file *seq, void *v, loff_t *pos)
{
struct bpf_iter_seq_task_vma_info *info = seq->private;
++*pos;
return task_vma_seq_get_next(info);
}
struct bpf_iter__task_vma {
__bpf_md_ptr(struct bpf_iter_meta *, meta);
__bpf_md_ptr(struct task_struct *, task);
__bpf_md_ptr(struct vm_area_struct *, vma);
};
DEFINE_BPF_ITER_FUNC(task_vma, struct bpf_iter_meta *meta,
struct task_struct *task, struct vm_area_struct *vma)
static int __task_vma_seq_show(struct seq_file *seq, bool in_stop)
{
struct bpf_iter_seq_task_vma_info *info = seq->private;
struct bpf_iter__task_vma ctx;
struct bpf_iter_meta meta;
struct bpf_prog *prog;
meta.seq = seq;
prog = bpf_iter_get_info(&meta, in_stop);
if (!prog)
return 0;
ctx.meta = &meta;
ctx.task = info->task;
ctx.vma = info->vma;
return bpf_iter_run_prog(prog, &ctx);
}
static int task_vma_seq_show(struct seq_file *seq, void *v)
{
return __task_vma_seq_show(seq, false);
}
static void task_vma_seq_stop(struct seq_file *seq, void *v)
{
struct bpf_iter_seq_task_vma_info *info = seq->private;
if (!v) {
(void)__task_vma_seq_show(seq, true);
} else {
/* info->vma has not been seen by the BPF program. If the
* user space reads more, task_vma_seq_get_next should
* return this vma again. Set prev_vm_start to ~0UL,
* so that we don't skip the vma returned by the next
* find_vma() (case task_vma_iter_find_vma in
* task_vma_seq_get_next()).
*/
info->prev_vm_start = ~0UL;
info->prev_vm_end = info->vma->vm_end;
mmap_read_unlock(info->task->mm);
put_task_struct(info->task);
info->task = NULL;
}
}
static const struct seq_operations task_vma_seq_ops = {
.start = task_vma_seq_start,
.next = task_vma_seq_next,
.stop = task_vma_seq_stop,
.show = task_vma_seq_show,
};
BTF_ID_LIST(btf_task_file_ids)
BTF_ID(struct, task_struct)
BTF_ID(struct, file)
BTF_ID(struct, vm_area_struct)
static const struct bpf_iter_seq_info task_seq_info = {
.seq_ops = &task_seq_ops,
@ -328,6 +567,26 @@ static struct bpf_iter_reg task_file_reg_info = {
.seq_info = &task_file_seq_info,
};
static const struct bpf_iter_seq_info task_vma_seq_info = {
.seq_ops = &task_vma_seq_ops,
.init_seq_private = init_seq_pidns,
.fini_seq_private = fini_seq_pidns,
.seq_priv_size = sizeof(struct bpf_iter_seq_task_vma_info),
};
static struct bpf_iter_reg task_vma_reg_info = {
.target = "task_vma",
.feature = BPF_ITER_RESCHED,
.ctx_arg_info_size = 2,
.ctx_arg_info = {
{ offsetof(struct bpf_iter__task_vma, task),
PTR_TO_BTF_ID_OR_NULL },
{ offsetof(struct bpf_iter__task_vma, vma),
PTR_TO_BTF_ID_OR_NULL },
},
.seq_info = &task_vma_seq_info,
};
static int __init task_iter_init(void)
{
int ret;
@ -339,6 +598,12 @@ static int __init task_iter_init(void)
task_file_reg_info.ctx_arg_info[0].btf_id = btf_task_file_ids[0];
task_file_reg_info.ctx_arg_info[1].btf_id = btf_task_file_ids[1];
return bpf_iter_reg_target(&task_file_reg_info);
ret = bpf_iter_reg_target(&task_file_reg_info);
if (ret)
return ret;
task_vma_reg_info.ctx_arg_info[0].btf_id = btf_task_file_ids[0];
task_vma_reg_info.ctx_arg_info[1].btf_id = btf_task_file_ids[2];
return bpf_iter_reg_target(&task_vma_reg_info);
}
late_initcall(task_iter_init);