This is a note to let you know that I've just added the patch titled
file: always lock position for FMODE_ATOMIC_POS
to the 5.10-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
file-always-lock-position-for-fmode_atomic_pos.patch
and it can be found in the queue-5.10 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 20ea1e7d13c1b544fe67c4a8dc3943bb1ab33e6f Mon Sep 17 00:00:00 2001
From: Christian Brauner <brauner@xxxxxxxxxx>
Date: Mon, 24 Jul 2023 17:00:49 +0200
Subject: file: always lock position for FMODE_ATOMIC_POS
From: Christian Brauner <brauner@xxxxxxxxxx>
commit 20ea1e7d13c1b544fe67c4a8dc3943bb1ab33e6f upstream.
The pidfd_getfd() system call allows a caller with ptrace_may_access()
abilities on another process to steal a file descriptor from this
process. This system call is used by debuggers, container runtimes,
system call supervisors, networking proxies etc. So while it is a
special interest system call it is used in common tools.
That ability ends up breaking our long-time optimization in fdget_pos(),
which "knew" that if we had exclusive access to the file descriptor
nobody else could access it, and we didn't need the lock for the file
position.
That check for file_count(file) was always fairly subtle - it depended
on __fdget() not incrementing the file count for single-threaded
processes and thus included that as part of the rule - but it did mean
that we didn't need to take the lock in all those traditional unix
process contexts.
So it's sad to see this go, and I'd love to have some way to re-instate
the optimization. At the same time, the lock obviously isn't ever
contended in the case we optimized, so all we were optimizing away is
the atomics and the cacheline dirtying. Let's see if anybody even
notices that the optimization is gone.
Link: https://lore.kernel.org/linux-fsdevel/20230724-vfs-fdget_pos-v1-1-a4abfd7103f3@xxxxxxxxxx/
Fixes: 8649c322f75c ("pid: Implement pidfd_getfd syscall")
Cc: stable@xxxxxxxxxx
Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
fs/file.c | 6 ++----
1 file changed, 2 insertions(+), 4 deletions(-)
--- a/fs/file.c
+++ b/fs/file.c
@@ -1013,10 +1013,8 @@ unsigned long __fdget_pos(unsigned int f
struct file *file = (struct file *)(v & ~3);
if (file && (file->f_mode & FMODE_ATOMIC_POS)) {
- if (file_count(file) > 1) {
- v |= FDPUT_POS_UNLOCK;
- mutex_lock(&file->f_pos_lock);
- }
+ v |= FDPUT_POS_UNLOCK;
+ mutex_lock(&file->f_pos_lock);
}
return v;
}
Patches currently in stable-queue which might be from brauner@xxxxxxxxxx are
queue-5.10/file-always-lock-position-for-fmode_atomic_pos.patch
