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

[Charalampos Mitrodimas <charmitro@posteo.net>]

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>