linux-kernel - Re: [PATCH] fs/pipe: Fix lockdep false-positive in watchqueue pipe

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Date:   Fri, 24 Nov 2023 16:19:00 +0100
From:   Christian Brauner <brauner@...nel.org>
To:     Jann Horn <jannh@...gle.com>
Cc:     Christian Brauner <brauner@...nel.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzkaller-bugs@...glegroups.com,
        Alexander Viro <viro@...iv.linux.org.uk>,
        David Howells <dhowells@...hat.com>
Subject: Re: [PATCH] fs/pipe: Fix lockdep false-positive in watchqueue pipe_write()

On Fri, 24 Nov 2023 16:08:22 +0100, Jann Horn wrote:
> When you try to splice between a normal pipe and a notification pipe,
> get_pipe_info(..., true) fails, so splice() falls back to treating the
> notification pipe like a normal pipe - so we end up in
> iter_file_splice_write(), which first locks the input pipe, then calls
> vfs_iter_write(), which locks the output pipe.
> 
> Lockdep complains about that, because we're taking a pipe lock while
> already holding another pipe lock.
> 
> [...]

Yeah, that looks to be a improvement in general, since you can't upgrade
a regular pipe to a O_NOTIFICATION pipe. IOW, peforming that check with
pipe_lock() isn't necessary.

(The check for watch queue in pipe_set_size() called from pipe_fcntl()
 also wouldn't need to be done with __pipe_lock() held fwiw.)

---

Applied to the vfs.misc branch of the vfs/vfs.git tree.
Patches in the vfs.misc branch should appear in linux-next soon.

Please report any outstanding bugs that were missed during review in a
new review to the original patch series allowing us to drop it.

It's encouraged to provide Acked-bys and Reviewed-bys even though the
patch has now been applied. If possible patch trailers will be updated.

Note that commit hashes shown below are subject to change due to rebase,
trailer updates or similar. If in doubt, please check the listed branch.

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
branch: vfs.misc

[1/1] fs/pipe: Fix lockdep false-positive in watchqueue pipe_write()
      https://git.kernel.org/vfs/vfs/c/efb8f498327c