fs: add path_mounted()

Add a small helper to check whether a path refers to the root of the
mount instead of open-coding this everywhere.
Reviewed-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
Message-Id: <20230202-fs-move-mount-replace-v4-1-98f3d80d7eaa@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/namespace.c

@@ -1767,6 +1767,19 @@ bool may_mount(void)
 	return ns_capable(current->nsproxy->mnt_ns->user_ns, CAP_SYS_ADMIN);
 }
 
+/**
+ * path_mounted - check whether path is mounted
+ * @path: path to check
+ *
+ * Determine whether @path refers to the root of a mount.
+ *
+ * Return: true if @path is the root of a mount, false if not.
+ */
+static inline bool path_mounted(const struct path *path)
+{
+	return path->mnt->mnt_root == path->dentry;
+}
+
 static void warn_mandlock(void)
 {
 	pr_warn_once("=======================================================\n"
@@ -1782,7 +1795,7 @@ static int can_umount(const struct path *path, int flags)
 
 	if (!may_mount())
 		return -EPERM;
-	if (path->dentry != path->mnt->mnt_root)
+	if (!path_mounted(path))
 		return -EINVAL;
 	if (!check_mnt(mnt))
 		return -EINVAL;
@@ -2367,7 +2380,7 @@ static int do_change_type(struct path *path, int ms_flags)
 	int type;
 	int err = 0;
 
-	if (path->dentry != path->mnt->mnt_root)
+	if (!path_mounted(path))
 		return -EINVAL;
 
 	type = flags_to_propagation_type(ms_flags);
@@ -2643,7 +2656,7 @@ static int do_reconfigure_mnt(struct path *path, unsigned int mnt_flags)
 	if (!check_mnt(mnt))
 		return -EINVAL;
 
-	if (path->dentry != mnt->mnt.mnt_root)
+	if (!path_mounted(path))
 		return -EINVAL;
 
 	if (!can_change_locked_flags(mnt, mnt_flags))
@@ -2682,7 +2695,7 @@ static int do_remount(struct path *path, int ms_flags, int sb_flags,
 	if (!check_mnt(mnt))
 		return -EINVAL;
 
-	if (path->dentry != path->mnt->mnt_root)
+	if (!path_mounted(path))
 		return -EINVAL;
 
 	if (!can_change_locked_flags(mnt, mnt_flags))
@@ -2772,9 +2785,9 @@ static int do_set_group(struct path *from_path, struct path *to_path)
 
 	err = -EINVAL;
 	/* To and From paths should be mount roots */
-	if (from_path->dentry != from_path->mnt->mnt_root)
+	if (!path_mounted(from_path))
 		goto out;
-	if (to_path->dentry != to_path->mnt->mnt_root)
+	if (!path_mounted(to_path))
 		goto out;
 
 	/* Setting sharing groups is only allowed across same superblock */
@@ -2855,7 +2868,7 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
 	if (old->mnt.mnt_flags & MNT_LOCKED)
 		goto out;
 
-	if (old_path->dentry != old_path->mnt->mnt_root)
+	if (!path_mounted(old_path))
 		goto out;
 
 	if (d_is_dir(new_path->dentry) !=
@@ -2937,8 +2950,7 @@ static int do_add_mount(struct mount *newmnt, struct mountpoint *mp,
 	}
 
 	/* Refuse the same filesystem on the same mount point */
-	if (path->mnt->mnt_sb == newmnt->mnt.mnt_sb &&
-	    path->mnt->mnt_root == path->dentry)
+	if (path->mnt->mnt_sb == newmnt->mnt.mnt_sb && path_mounted(path))
 		return -EBUSY;
 
 	if (d_is_symlink(newmnt->mnt.mnt_root))
@@ -3917,11 +3929,11 @@ SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
 	if (new_mnt == root_mnt || old_mnt == root_mnt)
 		goto out4; /* loop, on the same file system  */
 	error = -EINVAL;
-	if (root.mnt->mnt_root != root.dentry)
+	if (!path_mounted(&root))
 		goto out4; /* not a mountpoint */
 	if (!mnt_has_parent(root_mnt))
 		goto out4; /* not attached */
-	if (new.mnt->mnt_root != new.dentry)
+	if (!path_mounted(&new))
 		goto out4; /* not a mountpoint */
 	if (!mnt_has_parent(new_mnt))
 		goto out4; /* not attached */
@@ -4124,7 +4136,7 @@ static int do_mount_setattr(struct path *path, struct mount_kattr *kattr)
 	struct mount *mnt = real_mount(path->mnt);
 	int err = 0;
 
-	if (path->dentry != mnt->mnt.mnt_root)
+	if (!path_mounted(path))
 		return -EINVAL;
 
 	if (kattr->mnt_userns) {