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Date:   Thu, 13 Jul 2023 14:14:19 +0200
From:   Christian Brauner <brauner@kernel.org>
To:     Christoph Hellwig <hch@lst.de>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Florian Weimer <fweimer@redhat.com>,
        Aleksa Sarai <cyphar@cyphar.com>,
        linux-fsdevel@vger.kernel.org, Al Viro <viro@zeniv.linux.org.uk>,
        stable@vger.kernel.org
Subject: Re: [PATCH v2] attr: block mode changes of symlinks
Message-ID: <20230713-noten-umhang-c5206ce37482@brauner>
References: <20230712-vfs-chmod-symlinks-v2-1-08cfb92b61dd@kernel.org>
 <20230713120042.GA23709@lst.de>
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On Thu, Jul 13, 2023 at 02:00:42PM +0200, Christoph Hellwig wrote:
> On Wed, Jul 12, 2023 at 08:58:49PM +0200, Christian Brauner wrote:
> > (1) Filesystems that don't implement a i_op->setattr() for symlinks.
> > 
> >     Such filesystems may or may not know that without i_op->setattr()
> >     defined, notify_change() falls back to simple_setattr() causing the
> >     inode's mode in the inode cache to be changed.
> 
> Btw, I think this fallback is pretty harmful.  At some point we should
> probably start auditing all instances and wire the ones up that should
> be using simple_setattr (probably mostly just in-memory file systems)
> and refuse attribute changes if .setattr is NULL.

Yes, I agree. For example, it is an issue or at least a potential source
for bugs for procfs files. If they don't have a i_op->setattr() handler
they still get simple_setattr() which means that they accept ATTR_MODE
changes which they were explicitly stopped from doing in 2006 in commit
6d76fa58b050 ("Don't allow chmod() on the /proc/<pid>/ files").