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

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Message-ID: <cf97c467-6391-44df-8ce3-570f533623b8@sandeen.net>
Date: Wed, 6 Aug 2025 11:33:11 -0500
From: Eric Sandeen <sandeen@...deen.net>
To: Charalampos Mitrodimas <charmitro@...teo.net>,
 Eric Sandeen <sandeen@...hat.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 "Rafael J. Wysocki" <rafael@...nel.org>, Danilo Krummrich <dakr@...nel.org>,
 Christian Brauner <brauner@...nel.org>, David Howells <dhowells@...hat.com>,
 linux-kernel@...r.kernel.org, linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] debugfs: fix mount options not being applied

On 8/5/25 12:22 PM, Charalampos Mitrodimas wrote:
> Eric Sandeen <sandeen@...hat.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.

Yep, I'm looking into that. mount_single used to do this, and IIRC we discussed
it before but for some reason opted not to. It seems a bit trickier than I first
expected, but I might just be dense. ;)

-Eric