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

["André Almeida" <andrealmeid@igalia.com>]

Hi Gabriel!

Em 09/04/2025 13:52, Gabriel Krisman Bertazi escreveu:
> André Almeida <andrealmeid@igalia.com> 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?