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Message-ID: <20250804172741.GZ222315@ZenIV>
Date: Mon, 4 Aug 2025 18:27:41 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Christian Brauner <brauner@...nel.org>
Cc: Thomas Weißschuh <thomas.weissschuh@...utronix.de>,
Jan Kara <jack@...e.cz>, Sargun Dhillon <sargun@...gun.me>,
Kees Cook <kees@...nel.org>, linux-fsdevel@...r.kernel.org,
linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2] fs: always return zero on success from replace_fd()
On Mon, Aug 04, 2025 at 04:52:29PM +0100, Al Viro wrote:
> On Mon, Aug 04, 2025 at 02:33:13PM +0200, Christian Brauner wrote:
>
> > + guard(spinlock)(&files->file_lock);
> > err = expand_files(files, fd);
> > if (unlikely(err < 0))
> > - goto out_unlock;
> > - return do_dup2(files, file, fd, flags);
> > + return err;
> > + err = do_dup2(files, file, fd, flags);
> > + if (err < 0)
> > + return err;
> >
> > -out_unlock:
> > - spin_unlock(&files->file_lock);
> > - return err;
> > + return 0;
> > }
>
> NAK. This is broken - do_dup2() drops ->file_lock. And that's why I
> loathe the guard() - it's too easy to get confused *and* assume that
> it will DTRT, no need to check carefully.
Note, BTW, that in actual replacing case do_dup2() has blocking
operations (closing the replaced reference) after dropping ->file_lock,
so making it locking-neutral would not be easy; doable (have it
return the old reference in the replacing case and adjust the callers
accordingly), but it's seriously not pretty (NULL/address of old file/ERR_PTR()
for return value, boilerplate in callers, etc.). Having do_dup2() called
without ->file_lock and taking it inside is not an option - we could pull
expand_files() in there, but lookup of oldfd in actual dup2(2)/dup3(2) has
to be done within the same ->file_lock scope where it is inserted into the
table.
Sure, all things equal it's better to have functions locking-neutral, but
it's not always the best approach. And while __free() allows for "we'd
passed the object to somebody else, it's not ours to consume anymore",
guard() does not.
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