Re: [PATCH] debugfs: fix mount options not being applied

[Charalampos Mitrodimas <charmitro@posteo.net>]

Eric Sandeen <sandeen@redhat.com> writes:

> On 8/4/25 12:22 PM, Eric Sandeen wrote:
>> On 8/4/25 9:30 AM, Charalampos Mitrodimas wrote:
>>> Mount options (uid, gid, mode) are silently ignored when debugfs is
>>> mounted. This is a regression introduced during the conversion to the
>>> new mount API.
>>>
>>> When the mount API conversion was done, the line that sets
>>> sb->s_fs_info to the parsed options was removed. This causes
>>> debugfs_apply_options() to operate on a NULL pointer.
>>>
>>> As an example, with the bug the "mode" mount option is ignored:
>>>
>>>   $ mount -o mode=0666 -t debugfs debugfs /tmp/debugfs_test
>>>   $ mount | grep debugfs_test
>>>   debugfs on /tmp/debugfs_test type debugfs (rw,relatime)
>>>   $ ls -ld /tmp/debugfs_test
>>>   drwx------ 25 root root 0 Aug  4 14:16 /tmp/debugfs_test
>> 
>> Argh. So, this looks a lot like the issue that got fixed for tracefs in:
>> 
>> e4d32142d1de tracing: Fix tracefs mount options
>> 
>> Let me look at this; tracefs & debugfs are quite similar, so perhaps
>> keeping the fix consistent would make sense as well but I'll dig
>> into it a bit more.
>
> So, yes - a fix following the pattern of e4d32142d1de does seem to resolve
> this issue.
>
> However, I think we might be playing whack-a-mole here (fixing one fs at a time,
> when the problem is systemic) among filesystems that use get_tree_single()
> and have configurable options. For example, pstore:
>
> # umount /sys/fs/pstore 
>
> # mount -t pstore -o kmsg_bytes=65536 none /sys/fs/pstore
> # mount | grep pstore
> none on /sys/fs/pstore type pstore (rw,relatime,seclabel)
>
> # mount -o remount,kmsg_bytes=65536 /sys/fs/pstore
> # mount | grep pstore
> none on /sys/fs/pstore type pstore (rw,relatime,seclabel,kmsg_bytes=65536)
> #
>
> I think gadgetfs most likely has the same problem but I'm not yet sure
> how to test that.
>
> I have no real objection to merging your patch, though I like the
> consistency of following e4d32142d1de a bit more. But I think we should
> find a graceful solution so that any filesystem using get_tree_single
> can avoid this pitfall, if possible.

Hi, thanks for the review, and yes you're right.

Maybe a potential systemic fix would be to make get_tree_single() always
call fc->ops->reconfigure() after vfs_get_super() when reusing an
existing superblock, fixing all affected filesystems at once.

>
> -Eric