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

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Date:   Fri, 24 Nov 2023 17:02:02 +0100
From:   Christian Brauner <brauner@...nel.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Jann Horn <jannh@...gle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH] fs/pipe: Fix lockdep false-positive in watchqueue
 pipe_write()

On Fri, Nov 24, 2023 at 03:53:47PM +0000, David Howells wrote:
> Jann Horn <jannh@...gle.com> wrote:
> 
> > +	/*
> > +	 * Reject writing to watch queue pipes before the point where we lock
> > +	 * the pipe.
> > +	 * Otherwise, lockdep would be unhappy if the caller already has another
> > +	 * pipe locked.
> > +	 * If we had to support locking a normal pipe and a notification pipe at
> > +	 * the same time, we could set up lockdep annotations for that, but
> > +	 * since we don't actually need that, it's simpler to just bail here.
> > +	 */
> > +	if (pipe_has_watch_queue(pipe))
> > +		return -EXDEV;
> > +
> 
> Linus wanted it to be possible for the user to write to a notificaiton pipe.

Since it has been disabled from the start and nothing has ever actually
materialized we just don't need to care. If we start caring this needs
more work anyway iirc.