Re: [PATCH 6.1.y 1/1] fs: relax assertions on failure to encode file handles

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Sep 01, 2025 at 09:54:03PM +0200, Greg Kroah-Hartman wrote:
> On Mon, Sep 01, 2025 at 03:35:59PM +0000, Norbert Manthey wrote:
> > From: Amir Goldstein <amir73il@gmail.com>
> > 
> > commit 974e3fe0ac61de85015bbe5a4990cf4127b304b2 upstream.
> > 
> > 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>
> > Signed-off-by: Amir Goldstein <amir73il@gmail.com>
> > Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> 
> I never signed off on the original commit, so why was this added?
> 
> > 
> > (fuzzy picked from commit f47c834a9131ae64bee3c462f4e610c67b0a000f)
> > Applied with LLM-adjusted hunks for 1 functions from us.amazon.nova
> > - Changed the function call from `exportfs_encode_fid` to `exportfs_encode_inode_fh` to match the destination code.

Wait, that was just fuzz matching, the real body didn't even change.

> > - Removed the warning message as per the patch.

I do not understand this change, what exactly was this?

> Please put this in the proper place, and in the proper format, if you
> want to add "notes" to the backport.
> 
> But really, it took a LLM to determine an abi change?  That feels like
> total overkill as you then had to actually manually check it as well.
> But hey, it's your cpu cycles to burn, not mine...

Again, total overkill, 1 minute doing a simple git merge resolution
would have done the same thing, right?

confused as to why this took a whole new tool?  We have good merge
resolution tools for git these days, what's wrong with using one of the
many ones out there?

thanks,

greg k-h