[PATCH v2] debugfs: fix mount options not being applied

[PATCH v2] debugfs: fix mount options not being applied

[Charalampos Mitrodimas]

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.

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@vger.kernel.org
Signed-off-by: Charalampos Mitrodimas <charmitro@posteo.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,
-- 
Charalampos Mitrodimas <charmitro@posteo.net>

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

[Eric Sandeen]

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?

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@redhat.com>

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@vger.kernel.org
> Signed-off-by: Charalampos Mitrodimas <charmitro@posteo.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,

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

[Charalampos Mitrodimas]

Eric Sandeen <sandeen@redhat.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@redhat.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@vger.kernel.org
>> Signed-off-by: Charalampos Mitrodimas <charmitro@posteo.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,