[PATCH 2/2] tracefs: Only clobber mode/uid/gid on remount if asked

[Brian Norris]

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

Only clobber if the options were provided at mount time.

Note: the previous behavior was especially surprising in the presence of
automounted /sys/kernel/debug/tracing/.

  # Don't change /sys/kernel/tracing/ permissions on automount.
  umount /sys/kernel/debug/tracing/
  stat /sys/kernel/debug/tracing/.

  # Don't change /sys/kernel/tracing/ permissions.
  mount -t tracefs none /mnt/foo

  # Change /sys/kernel/tracing/ mode and uid, but not gid.
  mount -t tracefs -o uid=bar,mode=0750 none /mnt/baz

Signed-off-by: Brian Norris <briannorris@xxxxxxxxxxxx>
---
I'm open to writing an LTP test case for this, if that seems like a good
idea.

 fs/tracefs/inode.c | 31 +++++++++++++++++++++++--------
 1 file changed, 23 insertions(+), 8 deletions(-)

diff --git a/fs/tracefs/inode.c b/fs/tracefs/inode.c
index 81d26abf486f..da85b3979195 100644
--- a/fs/tracefs/inode.c
+++ b/fs/tracefs/inode.c
@@ -141,6 +141,8 @@ struct tracefs_mount_opts {
 	kuid_t uid;
 	kgid_t gid;
 	umode_t mode;
+	/* Opt_* bitfield. */
+	unsigned int opts;
 };
 
 enum {
@@ -241,6 +243,7 @@ static int tracefs_parse_options(char *data, struct tracefs_mount_opts *opts)
 	kgid_t gid;
 	char *p;
 
+	opts->opts = 0;
 	opts->mode = TRACEFS_DEFAULT_MODE;
 
 	while ((p = strsep(&data, ",")) != NULL) {
@@ -275,24 +278,36 @@ static int tracefs_parse_options(char *data, struct tracefs_mount_opts *opts)
 		 * but traditionally tracefs has ignored all mount options
 		 */
 		}
+
+		opts->opts |= BIT(token);
 	}
 
 	return 0;
 }
 
-static int tracefs_apply_options(struct super_block *sb)
+static int tracefs_apply_options(struct super_block *sb, bool remount)
 {
 	struct tracefs_fs_info *fsi = sb->s_fs_info;
 	struct inode *inode = d_inode(sb->s_root);
 	struct tracefs_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.
+	 */
+
+	if (!remount || opts->opts & BIT(Opt_mode)) {
+		inode->i_mode &= ~S_IALLUGO;
+		inode->i_mode |= opts->mode;
+	}
 
-	inode->i_uid = opts->uid;
+	if (!remount || opts->opts & BIT(Opt_uid))
+		inode->i_uid = opts->uid;
 
-	/* Set all the group ids to the mount option */
-	set_gid(sb->s_root, opts->gid);
+	if (!remount || opts->opts & BIT(Opt_gid)) {
+		/* Set all the group ids to the mount option */
+		set_gid(sb->s_root, opts->gid);
+	}
 
 	return 0;
 }
@@ -307,7 +322,7 @@ static int tracefs_remount(struct super_block *sb, int *flags, char *data)
 	if (err)
 		goto fail;
 
-	tracefs_apply_options(sb);
+	tracefs_apply_options(sb, true);
 
 fail:
 	return err;
@@ -359,7 +374,7 @@ static int trace_fill_super(struct super_block *sb, void *data, int silent)
 
 	sb->s_op = &tracefs_super_operations;
 
-	tracefs_apply_options(sb);
+	tracefs_apply_options(sb, false);
 
 	return 0;
 
-- 
2.37.2.672.g94769d06f0-goog