linux-kernel - Re: [PATCH v2] fs: always return zero on success from replace

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Message-ID: <20250804155229.GY222315@ZenIV>
Date: Mon, 4 Aug 2025 16:52:29 +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 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.