Re: [PATCH 6.1.y v2] fs: relax assertions on failure to encode file handles

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, Sep 04, 2025 at 09:21:38AM +0000, Norbert Manthey wrote:
> From: Amir Goldstein <amir73il@gmail.com>
> 
> Encoding file handles is usually performed by a filesystem >encode_fh()
> method that may fail for various reasons.
> 
> The legacy users of exportfs_encode_fh(), namely, nfsd and
> name_to_handle_at(2) syscall are ready to cope with the possibility
> of failure to encode a file handle.
> 
> There are a few other users of exportfs_encode_{fh,fid}() that
> currently have a WARN_ON() assertion when ->encode_fh() fails.
> Relax those assertions because they are wrong.
> 
> The second linked bug report states commit 16aac5ad1fa9 ("ovl: support
> encoding non-decodable file handles") in v6.6 as the regressing commit,
> but this is not accurate.
> 
> The aforementioned commit only increases the chances of the assertion
> and allows triggering the assertion with the reproducer using overlayfs,
> inotify and drop_caches.
> 
> Triggering this assertion was always possible with other filesystems and
> other reasons of ->encode_fh() failures and more particularly, it was
> also possible with the exact same reproducer using overlayfs that is
> mounted with options index=on,nfs_export=on also on kernels < v6.6.
> Therefore, I am not listing the aforementioned commit as a Fixes commit.
> 
> Backport hint: this patch will have a trivial conflict applying to
> v6.6.y, and other trivial conflicts applying to stable kernels < v6.6.
> 
> Reported-by: syzbot+ec07f6f5ce62b858579f@syzkaller.appspotmail.com
> Tested-by: syzbot+ec07f6f5ce62b858579f@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/linux-unionfs/671fd40c.050a0220.4735a.024f.GAE@google.com/
> Reported-by: Dmitry Safonov <dima@arista.com>
> Closes: https://lore.kernel.org/linux-fsdevel/CAGrbwDTLt6drB9eaUagnQVgdPBmhLfqqxAf3F+Juqy_o6oP8uw@mail.gmail.com/
> Cc: stable@vger.kernel.org
> Signed-off-by: Amir Goldstein <amir73il@gmail.com>
> Link: https://lore.kernel.org/r/20241219115301.465396-1-amir73il@gmail.com
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> [ git-llm-picked from commit 974e3fe0ac61de85015bbe5a4990cf4127b304b2 ]

Please don't make up new strings for a simple 'cherry-pick' line, as our
tools don't know how to find this commit id :(

thanks,

greg k-h