linux-kernel - Re: [PATCH v2] debugfs: fix mount options not being applied

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Message-ID: <87a53zuzgr.fsf@posteo.net>
Date: Sat, 16 Aug 2025 14:13:10 +0000
From: Charalampos Mitrodimas <charmitro@...teo.net>
To: 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,  stable@...r.kernel.org
Subject: Re: [PATCH v2] debugfs: fix mount options not being applied

Eric Sandeen <sandeen@...hat.com> writes:

> On 8/13/25 6:55 PM, 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.
>
> The change looks fine to me but I'm a little confused by this paragraph.
> Is there something in the current code that will lead to an OOPs of
> the kernel on a NULL ptr?

Ah, sorry for the confusion, I worded that poorly. There's no OOPs in
the code.

Sending a v3 for it.

>
> I just don't want to generate CVEs if we don't have to ;)
>
> As for the code change itself,
>
> Reviewed-by: Eric Sandeen <sandeen@...hat.com>

Thanks!

>
> Thanks!
>
>> Fix this by following the same pattern as the tracefs fix in commit
>> e4d32142d1de ("tracing: Fix tracefs mount options"). Call
>> debugfs_reconfigure() in debugfs_get_tree() to apply the mount options
>> to the superblock after it has been created or reused.
>> 
>> 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
>> 
>> With the fix applied, it works as expected:
>> 
>>   $ mount -o mode=0666 -t debugfs debugfs /tmp/debugfs_test
>>   $ mount | grep debugfs_test
>>   debugfs on /tmp/debugfs_test type debugfs (rw,relatime,mode=666)
>>   $ ls -ld /tmp/debugfs_test
>>   drw-rw-rw- 37 root root 0 Aug  2 17:28 /tmp/debugfs_test
>> 
>> Fixes: a20971c18752 ("vfs: Convert debugfs to use the new mount API")
>> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=220406
>> Cc: stable@...r.kernel.org
>> Signed-off-by: Charalampos Mitrodimas <charmitro@...teo.net>
>> ---
>> Changes in v2:
>> - Follow the same pattern as e4d32142d1de ("tracing: Fix tracefs mount options")
>> - Add Cc: stable tag
>> - Link to v1: https://lore.kernel.org/r/20250804-debugfs-mount-opts-v1-1-bc05947a80b5@posteo.net
>> ---
>>  fs/debugfs/inode.c | 11 ++++++++++-
>>  1 file changed, 10 insertions(+), 1 deletion(-)
>> 
>> diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
>> index a0357b0cf362d8ac47ff810e162402d6a8ae2cb9..c12d649df6a5435050f606c2828a9a7cc61922e4 100644
>> --- a/fs/debugfs/inode.c
>> +++ b/fs/debugfs/inode.c
>> @@ -183,6 +183,9 @@ static int debugfs_reconfigure(struct fs_context *fc)
>>  	struct debugfs_fs_info *sb_opts = sb->s_fs_info;
>>  	struct debugfs_fs_info *new_opts = fc->s_fs_info;
>>  
>> +	if (!new_opts)
>> +		return 0;
>> +
>>  	sync_filesystem(sb);
>>  
>>  	/* structure copy of new mount options to sb */
>> @@ -282,10 +285,16 @@ static int debugfs_fill_super(struct super_block *sb, struct fs_context *fc)
>>  
>>  static int debugfs_get_tree(struct fs_context *fc)
>>  {
>> +	int err;
>> +
>>  	if (!(debugfs_allow & DEBUGFS_ALLOW_API))
>>  		return -EPERM;
>>  
>> -	return get_tree_single(fc, debugfs_fill_super);
>> +	err = get_tree_single(fc, debugfs_fill_super);
>> +	if (err)
>> +		return err;
>> +
>> +	return debugfs_reconfigure(fc);
>>  }
>>  
>>  static void debugfs_free_fc(struct fs_context *fc)
>> 
>> ---
>> base-commit: 3c4a063b1f8ab71352df1421d9668521acb63cd9
>> change-id: 20250804-debugfs-mount-opts-2a68d7741f05
>> 
>> Best regards,