[v1] proc: proc_maps_open allow proc_mem_open to return NULL

[PATCH] proc: proc_maps_open allow proc_mem_open to return NULL

Posted by Jialin Wang 1 month, 4 weeks ago

The commit 65c66047259f ("proc: fix the issue of proc_mem_open returning NULL")
breaks `perf record -g -p PID` when profiling a kernel thread.

The strace of `perf record -g -p $(pgrep kswapd0)` shows:

  openat(AT_FDCWD, "/proc/65/task/65/maps", O_RDONLY) = -1 ESRCH (No such process)

This patch partially reverts the commit to fix it.

Fixes: 65c66047259f ("proc: fix the issue of proc_mem_open returning NULL")
Signed-off-by: Jialin Wang <wjl.linux@gmail.com>
---
 fs/proc/task_mmu.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 3d6d8a9f13fc..7a7ce26106ac 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -340,8 +340,8 @@ static int proc_maps_open(struct inode *inode, struct file *file,
 
 	priv->inode = inode;
 	priv->mm = proc_mem_open(inode, PTRACE_MODE_READ);
-	if (IS_ERR_OR_NULL(priv->mm)) {
-		int err = priv->mm ? PTR_ERR(priv->mm) : -ESRCH;
+	if (IS_ERR(priv->mm)) {
+		int err = PTR_ERR(priv->mm);
 
 		seq_release_private(inode, file);
 		return err;
-- 
2.50.0

Re: [PATCH] proc: proc_maps_open allow proc_mem_open to return NULL

Posted by Andrew Morton 1 month, 4 weeks ago

On Fri,  8 Aug 2025 00:54:55 +0800 Jialin Wang <wjl.linux@gmail.com> wrote:

> The commit 65c66047259f ("proc: fix the issue of proc_mem_open returning NULL")
> breaks `perf record -g -p PID` when profiling a kernel thread.
> 
> The strace of `perf record -g -p $(pgrep kswapd0)` shows:
> 
>   openat(AT_FDCWD, "/proc/65/task/65/maps", O_RDONLY) = -1 ESRCH (No such process)
> 
> This patch partially reverts the commit to fix it.

Thanks.  But "breaks" is a rather thin description of the problem!

Can you please describe the observed misbehavior fully?

> Fixes: 65c66047259f ("proc: fix the issue of proc_mem_open returning NULL")

Because we should backport this fix into 6.16.x -stable kernels.  The
-stable maintainers may wonder why we're requesting this.  Also, any
person who is having problems with their 6.16-based kernel will want
such a description so they can decide whether this fix might address
their problem.

Thanks.

[PATCH v2] proc: proc_maps_open allow proc_mem_open to return NULL

Posted by Jialin Wang 1 month, 3 weeks ago

The commit 65c66047259f ("proc: fix the issue of proc_mem_open returning
NULL") caused proc_maps_open() to return -ESRCH when proc_mem_open()
returns NULL. This breaks legitimate /proc/<pid>/maps access for kernel
threads since kernel threads have NULL mm_struct.

The regression causes perf to fail and exit when profiling a kernel thread:

  # perf record -v -g -p $(pgrep kswapd0)
  ...
  couldn't open /proc/65/task/65/maps

This patch partially reverts the commit to fix it.

Fixes: 65c66047259f ("proc: fix the issue of proc_mem_open returning NULL")
Signed-off-by: Jialin Wang <wjl.linux@gmail.com>
---
Changes in v2 (Thanks to Andrew):
- Add more detailed misbehavior description in commit message

 fs/proc/task_mmu.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 3d6d8a9f13fc..7a7ce26106ac 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -340,8 +340,8 @@ static int proc_maps_open(struct inode *inode, struct file *file,
 
 	priv->inode = inode;
 	priv->mm = proc_mem_open(inode, PTRACE_MODE_READ);
-	if (IS_ERR_OR_NULL(priv->mm)) {
-		int err = priv->mm ? PTR_ERR(priv->mm) : -ESRCH;
+	if (IS_ERR(priv->mm)) {
+		int err = PTR_ERR(priv->mm);
 
 		seq_release_private(inode, file);
 		return err;
-- 
2.50.0