devtmpfs regression fix: reconfigure on each mount

Prior to Linux v5.4 devtmpfs used mount_single() which treats the given
mount options as "remount" options, so it updates the configuration of
the single super_block on each mount.

Since that was changed, the mount options used for devtmpfs are ignored.
This is a regression which affect systemd - which mounts devtmpfs with
"-o mode=755,size=4m,nr_inodes=1m".

This patch restores the "remount" effect by calling reconfigure_single()

Fixes: d401727e ("devtmpfs: don't mix {ramfs,shmem}_fill_super() with mount_single()")
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/base/devtmpfs.c

@@ -65,8 +65,15 @@ static struct dentry *public_dev_mount(struct file_system_type *fs_type, int fla
 		      const char *dev_name, void *data)
 {
 	struct super_block *s = mnt->mnt_sb;
+	int err;
+
 	atomic_inc(&s->s_active);
 	down_write(&s->s_umount);
+	err = reconfigure_single(s, flags, data);
+	if (err < 0) {
+		deactivate_locked_super(s);
+		return ERR_PTR(err);
+	}
 	return dget(s->s_root);
 }
 

fs/super.c

@@ -1423,7 +1423,7 @@ struct dentry *mount_nodev(struct file_system_type *fs_type,
 }
 EXPORT_SYMBOL(mount_nodev);
 
-static int reconfigure_single(struct super_block *s,
+int reconfigure_single(struct super_block *s,
 		       int flags, void *data)
 {
 	struct fs_context *fc;

include/linux/fs_context.h

@@ -142,6 +142,8 @@ extern void put_fs_context(struct fs_context *fc);
 extern int vfs_parse_fs_param_source(struct fs_context *fc,
 				     struct fs_parameter *param);
 extern void fc_drop_locked(struct fs_context *fc);
+int reconfigure_single(struct super_block *s,
+		       int flags, void *data);
 
 /*
  * sget() wrappers to be called from the ->get_tree() op.