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

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Message-Id: <20250804-debugfs-mount-opts-v1-1-bc05947a80b5@posteo.net>
Date: Mon, 04 Aug 2025 14:30:04 +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, 
 Charalampos Mitrodimas <charmitro@...teo.net>
Subject: [PATCH] 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 line that sets
sb->s_fs_info to the parsed options was removed. This causes
debugfs_apply_options() to operate on a NULL pointer.

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

Fix this by restoring the missing sb->s_fs_info assignment in
debugfs_fill_super() and by calling debugfs_reconfigure() in
debugfs_get_tree() to apply options when reusing an existing
superblock.

Fixes: a20971c18752 ("vfs: Convert debugfs to use the new mount API")
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=220406
Signed-off-by: Charalampos Mitrodimas <charmitro@...teo.net>
---
 fs/debugfs/inode.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
index a0357b0cf362d8ac47ff810e162402d6a8ae2cb9..ffe6402c77126b2a23beaa85160dfe578f59599c 100644
--- a/fs/debugfs/inode.c
+++ b/fs/debugfs/inode.c
@@ -275,6 +275,7 @@ static int debugfs_fill_super(struct super_block *sb, struct fs_context *fc)
 	set_default_d_op(sb, &debugfs_dops);
 	sb->s_d_flags |= DCACHE_DONTCACHE;

+	sb->s_fs_info = fc->s_fs_info;
 	debugfs_apply_options(sb);

 	return 0;
@@ -282,10 +283,15 @@ 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)
+		err = debugfs_reconfigure(fc);
+	return err;
 }

 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>