linux-kernel - [PATCH v3] debugfs: fix mount options not being applied

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Message-Id: <20250816-debugfs-mount-opts-v3-1-d271dad57b5b@posteo.net>
Date: Sat, 16 Aug 2025 14:14:37 +0000
From: Charalampos Mitrodimas <charmitro@...teo.net>
To: 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>, 
 Eric Sandeen <sandeen@...hat.com>
Cc: linux-kernel@...r.kernel.org, stable@...r.kernel.org, 
 Charalampos Mitrodimas <charmitro@...teo.net>
Subject: [PATCH v3] debugfs: fix mount options not being applied

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 parsed options were never
applied to the superblock when it was reused. As a result, the mount
options were ignored when debugfs was mounted.

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
Reviewed-by: Eric Sandeen <sandeen@...hat.com>
Signed-off-by: Charalampos Mitrodimas <charmitro@...teo.net>
---
Changes in v3:
- Reworded patch description to avoid confusion about OOPs
- Link to v2: https://lore.kernel.org/r/20250813-debugfs-mount-opts-v2-1-0ca79720edc6@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@...teo.net>