[PATCH v3] debugfs: Only clobber mode/uid/gid on remount if asked

[Brian Norris <briannorris@chromium.org>]

Users may have explicitly configured their debugfs permissions; we
shouldn't overwrite those just because a second mount appeared.

Only clobber if the options were provided at mount time.

Existing behavior:

  ## Pre-existing status: debugfs is 0755.
  # chmod 755 /sys/kernel/debug/
  # stat -c '%A' /sys/kernel/debug/
  drwxr-xr-x

  ## New mount sets kernel-default permissions:
  # mount -t debugfs none /mnt/foo
  # stat -c '%A' /mnt/foo
  drwx------

  ## Unexpected: the original mount changed permissions:
  # stat -c '%A' /sys/kernel/debug
  drwx------

New behavior:

  ## Pre-existing status: debugfs is 0755.
  # chmod 755 /sys/kernel/debug/
  # stat -c '%A' /sys/kernel/debug/
  drwxr-xr-x

  ## New mount inherits existing permissions:
  # mount -t debugfs none /mnt/foo
  # stat -c '%A' /mnt/foo
  drwxr-xr-x

  ## Expected: old mount is unchanged:
  # stat -c '%A' /sys/kernel/debug
  drwxr-xr-x

Full test cases are being submitted to LTP.

Signed-off-by: Brian Norris <briannorris@chromium.org>
---
Temporary LTP branch here:
https://github.com/computersforpeace/ltp/commits/debugfs

I'll put in an LTP pull request once the kernel tree actually contains
my patches.

Companion patch:
  tracefs: Only clobber mode/uid/gid on remount if asked
  https://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace.git/commit/?h=trace-v6.0-rc4&id=47311db8e8f33011d90dee76b39c8886120cdda4

Changes in v3:
* Factor out _debugfs_apply_options() to hide boolean arg
* Drop unused return value

Changes in v2:
* Update commit description
* Include LTP patch in notes
* Split from tracefs patch

 fs/debugfs/inode.c | 37 ++++++++++++++++++++++++++++++-------
 1 file changed, 30 insertions(+), 7 deletions(-)

diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
index 3dcf0b8b4e93..83749cd67e0d 100644
--- a/fs/debugfs/inode.c
+++ b/fs/debugfs/inode.c
@@ -82,6 +82,8 @@ struct debugfs_mount_opts {
 	kuid_t uid;
 	kgid_t gid;
 	umode_t mode;
+	/* Opt_* bitfield. */
+	unsigned int opts;
 };
 
 enum {
@@ -111,6 +113,7 @@ static int debugfs_parse_options(char *data, struct debugfs_mount_opts *opts)
 	kgid_t gid;
 	char *p;
 
+	opts->opts = 0;
 	opts->mode = DEBUGFS_DEFAULT_MODE;
 
 	while ((p = strsep(&data, ",")) != NULL) {
@@ -145,24 +148,44 @@ static int debugfs_parse_options(char *data, struct debugfs_mount_opts *opts)
 		 * but traditionally debugfs has ignored all mount options
 		 */
 		}
+
+		opts->opts |= BIT(token);
 	}
 
 	return 0;
 }
 
-static int debugfs_apply_options(struct super_block *sb)
+static void _debugfs_apply_options(struct super_block *sb, bool remount)
 {
 	struct debugfs_fs_info *fsi = sb->s_fs_info;
 	struct inode *inode = d_inode(sb->s_root);
 	struct debugfs_mount_opts *opts = &fsi->mount_opts;
 
-	inode->i_mode &= ~S_IALLUGO;
-	inode->i_mode |= opts->mode;
+	/*
+	 * On remount, only reset mode/uid/gid if they were provided as mount
+	 * options.
+	 */
 
-	inode->i_uid = opts->uid;
-	inode->i_gid = opts->gid;
+	if (!remount || opts->opts & BIT(Opt_mode)) {
+		inode->i_mode &= ~S_IALLUGO;
+		inode->i_mode |= opts->mode;
+	}
 
-	return 0;
+	if (!remount || opts->opts & BIT(Opt_uid))
+		inode->i_uid = opts->uid;
+
+	if (!remount || opts->opts & BIT(Opt_gid))
+		inode->i_gid = opts->gid;
+}
+
+static void debugfs_apply_options(struct super_block *sb)
+{
+	_debugfs_apply_options(sb, false);
+}
+
+static void debugfs_apply_options_remount(struct super_block *sb)
+{
+	_debugfs_apply_options(sb, true);
 }
 
 static int debugfs_remount(struct super_block *sb, int *flags, char *data)
@@ -175,7 +198,7 @@ static int debugfs_remount(struct super_block *sb, int *flags, char *data)
 	if (err)
 		goto fail;
 
-	debugfs_apply_options(sb);
+	debugfs_apply_options_remount(sb);
 
 fail:
 	return err;
-- 
2.37.2.789.g6183377224-goog