Re: [PATCH 0/3] ovl: Enable support for casefold filesystems

[André Almeida]

Hi Gabriel!

Em 09/04/2025 13:52, Gabriel Krisman Bertazi escreveu:
> André Almeida <andrealmeid@xxxxxxxxxx> writes:
>
> > Hi all,
> >
> > We would like to support the usage of casefold filesystems with
> > overlayfs. This patchset do some of the work needed for that, but I'm
> > sure there are more places that need to be tweaked so please share your
> > feedback for this work.
>
> I didn't look the patches yet, but this is going to be quite tricky.
> For a start, consider the semantics when mixing volumes with different
> case settings for lower/upper/work directories.  And that could be any
> setting, such as whether the directory has +F, the encoding version and
> the encoding flags (strict mode).  Any mismatch will look bonkers and
> you want to forbid the mount.
>
> Consider upperdir is case-sensitive but lowerdir is not.  In this case,
> I suspect the case-exact name would be hidden by the upper, but the
> inexact-case would still resolve from the lower when it shouldn't, and
> can't be raised again.  If we have the other way around, the upper
> will hide more than one file from the lower and it is a matter of luck
> which file we are getting.
>
> In addition, if we have a case-insensitive on top of a case-sensitive,
> there is no way we can do the case-insensitive lookup on the lower
> without doing a sequential search across the entire directory.  Then it
> is again a matter of luck which file we are getting.
>
> The issue can appear even on the same volume, since case-insensitiveness
> is actually per-directory and can be flipped when a directory is empty.
> If something like the below is possible, we are in the same situation
> again:
>
> mkdir lower/ci
> chattr +F lower/ci
> touch lower/ci/BLA
> mount -o overlay none upperdir=upper,lowerdir=lower,workdir=work merged
> rm -r merged/ci/BLA    // creates a whiteout in upper
>                         // merged looks empty and should be allowed to drop +F
> chattr -F merged/ci
>
> So we'd also need to always forbid clearing the +F attribute and perhaps
> forbid it from ever being set on the merged directory.  We also want to
> require the encoding version and flags to match.

Thank you for the prompt response. I agree with you, for the next 
version I will implement such restrictions. I think it will be better to 
start with very restrict possibilities and then slowly dropping then, if 
they prove to be not problematic.

> > * Implementation
> >
> > The most obvious place that required change was the strncmp() inside of
> > ovl_cache_entry_find(), that I managed to convert to use d_same_name(),
> > that will then call the generic_ci_d_compare function if it's set for
> > the dentry. There are more strncmp() around ovl, but I would rather hear
> > feedback about this approach first than already implementing this around
> > the code.
>
> I'd suggest marking it as an RFC since it is not a functional
> implementation yet, IIUC.

Ops, you are right, I forgot to tag it as such.

> > > * Testing
> > sudo mount -t tmpfs -o casefold tmpfs mnt/
> > cd mnt/
> > mkdir dir
> > chattr +F dir
> > cd dir/
> > mkdir upper lower
> > mkdir lower/A lower/b lower/c
> > mkdir upper/a upper/b upper/d
> > mkdir merged work
> > sudo mount -t overlay overlay -olowerdir=lower,upperdir=upper,workdir=work, merged
> > ls /tmp/mnt/dir/merged/
> > a  b  c  d
> >
> > And ovl is respecting the equivalent names. `a` points to a merged dir
> > between `A` and `a`, but giving that upperdir has a lowercase `a`, this
> > is the name displayed here.
>
> Did you try fstests generic/556?  It might require some work to make it
> run over ovl, but it will exercise several cases that are quite
> hard to spot.

I haven't tried, I'm not sure which directory should I point to fstests 
to use for the test, the merged one?