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

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Message-ID: <b169faca-fef9-4099-920a-c34cc9a985a8@redhat.com>
Date: Thu, 14 Aug 2025 11:48:37 -0500
From: Eric Sandeen <sandeen@...hat.com>
To: Charalampos Mitrodimas <charmitro@...teo.net>,
 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>
Cc: linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2] debugfs: fix mount options not being applied

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@...hat.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@...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,