linux-kernel - Re: [PATCH 0/6] overlayfs + casefolding

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Message-ID: <CAOQ4uxg8p2Kg0BKrU4NSUzLVVLWcW=vLaw4kJkVR1Q-LyRbRXA@mail.gmail.com>
Date: Tue, 20 May 2025 10:05:14 +0200
From: Amir Goldstein <amir73il@...il.com>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: linux-fsdevel@...r.kernel.org, linux-bcachefs@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-unionfs@...r.kernel.org, 
	Miklos Szeredi <miklos@...redi.hu>, Alexander Viro <viro@...iv.linux.org.uk>, 
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH 0/6] overlayfs + casefolding

On Tue, May 20, 2025 at 7:16 AM Kent Overstreet
<kent.overstreet@...ux.dev> wrote:
>
> This series allows overlayfs and casefolding to safely be used on the
> same filesystem by providing exclusion to ensure that overlayfs never
> has to deal with casefolded directories.
>
> Currently, overlayfs can't be used _at all_ if a filesystem even
> supports casefolding, which is really nasty for users.
>
> Components:
>
> - filesystem has to track, for each directory, "does any _descendent_
>   have casefolding enabled"
>
> - new inode flag to pass this to VFS layer
>
> - new dcache methods for providing refs for overlayfs, and filesystem
>   methods for safely clearing this flag
>
> - new superblock flag for indicating to overlayfs & dcache "filesystem
>   supports casefolding, it's safe to use provided new dcache methods are
>   used"
>

I don't think that this is really needed.

Too bad you did not ask before going through the trouble of this implementation.

I think it is enough for overlayfs to know the THIS directory has no
casefolding.

in ovl_lookup() that returns a merged directory, ovl_dentry_weird() would
result in -EIO if any of the real directories have casefolding and we can add
another sanotify in ovl_lookup_single() that the 'base' dentry is not weird()
to cover the case of casefolder changed on an underlying reference directory.

Obviously, if any of the overlayfs layer root dirs have casefolding enabled the
mount would fail.

w.r.t enabling casefolding underneath overlayfs, overlayfs documentation says:

"Changes to underlying filesystems
---------------------------------

Changes to the underlying filesystems while part of a mounted overlay
filesystem are not allowed.  If the underlying filesystem is changed,
the behavior of the overlay is undefined, though it will not result in
a crash or deadlock."

So why is enabling casefolding on underlying layers so special that we
should have specific protection for that?

>From what I remember in ext4, enabling casefolding is only allowed
on empty directories.

Is this also the case for bcachefs?

If that is the case, then the situation is even simpler -
If filesystem can singal to vfs/ovl that directory is empty (i.e. S_EMPTYDIR)
then overlayfs can ignore this dir altogether when composing
the merged directory.

But again, I don't think there is a good reason to treat this case
of changing the underlying layer specially.

Please explain if I missed anything.

Thanks,
Amir.