Re: [PATCH v6 9/9] ovl: Support mounting case-insensitive enabled layers

[Gabriel Krisman Bertazi <gabriel@krisman.be>]

"NeilBrown" <neil@brown.name> writes:

> On Thu, 28 Aug 2025, Amir Goldstein wrote:
>> On Tue, Aug 26, 2025 at 9:01 PM André Almeida <andrealmeid@igalia.com> wrote:
>> >
>> >
>> >
>> > Em 26/08/2025 04:31, Amir Goldstein escreveu:
>> > > On Mon, Aug 25, 2025 at 3:31 PM André Almeida <andrealmeid@igalia.com> wrote:
>> > >>
>> > >> Hi Amir,
>> > >>
>> > >> Em 22/08/2025 16:17, Amir Goldstein escreveu:
>> > >>
>> > >> [...]
>> > >>
>> > >>     /*
>> > >>>>>> -        * Allow filesystems that are case-folding capable but deny composing
>> > >>>>>> -        * ovl stack from case-folded directories.
>> > >>>>>> +        * Exceptionally for layers with casefold, we accept that they have
>> > >>>>>> +        * their own hash and compare operations
>> > >>>>>>             */
>> > >>>>>> -       if (sb_has_encoding(dentry->d_sb))
>> > >>>>>> -               return IS_CASEFOLDED(d_inode(dentry));
>> > >>>>>> +       if (ofs->casefold)
>> > >>>>>> +               return false;
>> > >>>>>
>> > >>>>> I think this is better as:
>> > >>>>>            if (sb_has_encoding(dentry->d_sb))
>> > >>>>>                    return false;
>> > >>>>>
>> > >>>
>> > >>> And this still fails the test "Casefold enabled" for me.
>> > >>>
>> > >>> Maybe you are confused because this does not look like
>> > >>> a test failure. It looks like this:
>> > >>>
>> > >>> generic/999 5s ...  [19:10:21][  150.667994] overlayfs: failed lookup
>> > >>> in lower (ovl-lower/casefold, name='subdir', err=-116): parent wrong
>> > >>> casefold
>> > >>> [  150.669741] overlayfs: failed lookup in lower (ovl-lower/casefold,
>> > >>> name='subdir', err=-116): parent wrong casefold
>> > >>> [  150.760644] overlayfs: failed lookup in lower (/ovl-lower,
>> > >>> name='casefold', err=-66): child wrong casefold
>> > >>>    [19:10:24] [not run]
>> > >>> generic/999 -- overlayfs does not support casefold enabled layers
>> > >>> Ran: generic/999
>> > >>> Not run: generic/999
>> > >>> Passed all 1 tests
>> > >>>
>> > >>
>> > >> This is how the test output looks before my changes[1] to the test:
>> > >>
>> > >> $ ./run.sh
>> > >> FSTYP         -- ext4
>> > >> PLATFORM      -- Linux/x86_64 archlinux 6.17.0-rc1+ #1174 SMP
>> > >> PREEMPT_DYNAMIC Mon Aug 25 10:18:09 -03 2025
>> > >> MKFS_OPTIONS  -- -F /dev/vdc
>> > >> MOUNT_OPTIONS -- -o acl,user_xattr /dev/vdc /tmp/dir2
>> > >>
>> > >> generic/999 1s ... [not run] overlayfs does not support casefold enabled
>> > >> layers
>> > >> Ran: generic/999
>> > >> Not run: generic/999
>> > >> Passed all 1 tests
>> > >>
>> > >>
>> > >> And this is how it looks after my changes[1] to the test:
>> > >>
>> > >> $ ./run.sh
>> > >> FSTYP         -- ext4
>> > >> PLATFORM      -- Linux/x86_64 archlinux 6.17.0-rc1+ #1174 SMP
>> > >> PREEMPT_DYNAMIC Mon Aug 25 10:18:09 -03 2025
>> > >> MKFS_OPTIONS  -- -F /dev/vdc
>> > >> MOUNT_OPTIONS -- -o acl,user_xattr /dev/vdc /tmp/dir2
>> > >>
>> > >> generic/999        1s
>> > >> Ran: generic/999
>> > >> Passed all 1 tests
>> > >>
>> > >> So, as far as I can tell, the casefold enabled is not being skipped
>> > >> after the fix to the test.
>> > >
>> > > Is this how it looks with your v6 or after fixing the bug:
>> > > https://lore.kernel.org/linux-unionfs/68a8c4d7.050a0220.37038e.005c.GAE@google.com/
>> > >
>> > > Because for me this skipping started after fixing this bug
>> > > Maybe we fixed the bug incorrectly, but I did not see what the problem
>> > > was from a quick look.
>> > >
>> > > Can you test with my branch:
>> > > https://github.com/amir73il/linux/commits/ovl_casefold/
>> > >
>> >
>> > Right, our branches have a different base, mine is older and based on
>> > the tag vfs/vfs-6.18.mount.
>> >
>> > I have now tested with your branch, and indeed the test fails with
>> > "overlayfs does not support casefold enabled". I did some debugging and
>> > the missing commit from my branch that is making this difference here is
>> > e8bd877fb76bb9f3 ("ovl: fix possible double unlink"). After reverting it
>> > on top of your branch, the test works. I'm not sure yet why this
>> > prevents the mount, but this is the call trace when the error happens:
>> 
>> Wow, that is an interesting development race...
>> 
>> >
>> > TID/PID 860/860 (mount/mount):
>> >
>> >                      entry_SYSCALL_64_after_hwframe+0x77
>> >                      do_syscall_64+0xa2
>> >                      x64_sys_call+0x1bc3
>> >                      __x64_sys_fsconfig+0x46c
>> >                      vfs_cmd_create+0x60
>> >                      vfs_get_tree+0x2e
>> >                      ovl_get_tree+0x19
>> >                      get_tree_nodev+0x70
>> >                      ovl_fill_super+0x53b
>> > !    0us [-EINVAL]  ovl_parent_lock
>> >
>> > And for the ovl_parent_lock() arguments, *parent="work", *child="#7". So
>> > right now I'm trying to figure out why the dentry for #7 is not hashed.
>> >
>> 
>> The reason is this:
>> 
>> static struct dentry *ext4_lookup(...
>> {
>> ...
>>         if (IS_ENABLED(CONFIG_UNICODE) && !inode && IS_CASEFOLDED(dir)) {
>>                 /* Eventually we want to call d_add_ci(dentry, NULL)
>>                  * for negative dentries in the encoding case as
>>                  * well.  For now, prevent the negative dentry
>>                  * from being cached.
>>                  */
>>                 return NULL;
>>         }
>> 
>>         return d_splice_alias(inode, dentry);
>> }
>> 
>> Neil,
>> 
>> Apparently, the assumption that
>> ovl_lookup_temp() => ovl_lookup_upper() => lookup_one()
>> returns a hashed dentry is not always true.
>> 
>> It may be always true for all the filesystems that are currently
>> supported as an overlayfs
>> upper layer fs (?), but it does not look like you can count on this
>> for the wider vfs effort
>> and we should try to come up with a solution for ovl_parent_lock()
>> that will allow enabling
>> casefolding on overlayfs layers.
>> 
>> This patch seems to work. WDYT?
>> 
>> Thanks,
>> Amir.
>> 
>> commit 5dfcd10378038637648f3f422e3d5097eb6faa5f
>> Author: Amir Goldstein <amir73il@gmail.com>
>> Date:   Wed Aug 27 19:55:26 2025 +0200
>> 
>>     ovl: adapt ovl_parent_lock() to casefolded directories
>> 
>>     e8bd877fb76bb9f3 ("ovl: fix possible double unlink") added a sanity
>>     check of !d_unhashed(child) to try to verify that child dentry was not
>>     unlinked while parent dir was unlocked.
>> 
>>     This "was not unlink" check has a false positive result in the case of
>>     casefolded parent dir, because in that case, ovl_create_temp() returns
>>     an unhashed dentry.
>> 
>>     Change the "was not unlinked" check to use cant_mount(child).
>>     cant_mount(child) means that child was unlinked while we have been
>>     holding a reference to child, so it could not have become negative.
>> 
>>     This fixes the error in ovl_parent_lock() in ovl_check_rename_whiteout()
>>     after ovl_create_temp() and allows mount of overlayfs with casefolding
>>     enabled layers.
>> 
>>     Reported-by: André Almeida <andrealmeid@igalia.com>
>>     Link: https://lore.kernel.org/r/18704e8c-c734-43f3-bc7c-b8be345e1bf5@igalia.com/
>>     Signed-off-by: Amir Goldstein <amir73il@gmail.com>
>> 
>> diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
>> index bec4a39d1b97c..bffbb59776720 100644
>> --- a/fs/overlayfs/util.c
>> +++ b/fs/overlayfs/util.c
>> @@ -1551,9 +1551,23 @@ void ovl_copyattr(struct inode *inode)
>> 
>>  int ovl_parent_lock(struct dentry *parent, struct dentry *child)
>>  {
>> +       bool is_unlinked;
>> +
>>         inode_lock_nested(parent->d_inode, I_MUTEX_PARENT);
>> -       if (!child ||
>> -           (!d_unhashed(child) && child->d_parent == parent))
>> +       if (!child)
>> +               return 0;
>> +
>> +       /*
>> +        * After re-acquiring parent dir lock, verify that child was not moved
>> +        * to another parent and that it was not unlinked. cant_mount() means
>> +        * that child was unlinked while parent was unlocked. Since we are
>> +        * holding a reference to child, it could not have become negative.
>> +        * d_unhashed(child) is not a strong enough indication for unlinked,
>> +        * because with casefolded parent dir, ovl_create_temp() returns an
>> +        * unhashed dentry.
>> +        */
>> +       is_unlinked = cant_mount(child) || WARN_ON_ONCE(d_is_negative(child));
>> +       if (!is_unlinked && child->d_parent == parent)
>>                 return 0;
>> 
>>         inode_unlock(parent->d_inode);
>> 
>
> I don't feel comfortable with that.  Letting ovl_parent_lock() succeed
> on an unhashed dentry doesn't work for my longer term plans for locking.
> I would really rather we got that dentry hashed.
>
> What is happening is :
>   - lookup on non-existent name -> unhashed dentry
>   - vfs_create on that dentry - still unhashed
>   - rename of that unhashed dentry -> confusion in ovl_parent_lock()
>
> If this were being done from user-space there would be another lookup
> after the create and before the rename, and that would result in a
> hashed dentry.
>
> Could ovl_create_real() do a lookup for the name if the dentry isn't
> hashed?  That should result in a dentry that can safely be passed to
> ovl_parent_lock().

Might be a good time to mention I have a branch enabling negative
dentries in casefolded directories.  It didn't have any major issues last
time I posted, but it didn't get much interest.  It should be enough to
resolve the unhashed dentries after a lookup due to casefolding.

I'd need to revisit and retest, but it is a way out of it.

-- 
Gabriel Krisman Bertazi