proc: block mounting on top of /proc/<pid>/map_files/*

Entries under /proc/<pid>/map_files/* are ephemeral and may go away
before the process dies. As such allowing them to be used as mount
points creates the ability to leak mounts that linger until the process
dies with no ability to unmount them until then. Don't allow using them
as mountpoints.

Link: https://lore.kernel.org/r/20240806-work-procfs-v1-4-fb04e1d09f0c@kernel.orgReviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/proc/base.c

@@ -2276,8 +2276,8 @@ proc_map_files_instantiate(struct dentry *dentry,
 	inode->i_op = &proc_map_files_link_inode_operations;
 	inode->i_size = 64;
 
-	d_set_d_op(dentry, &tid_map_files_dentry_operations);
-	return d_splice_alias(inode, dentry);
+	return proc_splice_unmountable(inode, dentry,
+				       &tid_map_files_dentry_operations);
 }
 
 static struct dentry *proc_map_files_lookup(struct inode *dir,