[PATCH RFC 08/16] fs: support getname_maybe_null() in move_mount()

[Christian Brauner <brauner@kernel.org>]

Allow move_mount() to work with NULL path arguments.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/namespace.c     | 93 +++++++++++++++++++++++++++++++++++-------------------
 include/linux/fs.h |  1 +
 2 files changed, 61 insertions(+), 33 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 66b9cea1cf66..612f73481d35 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2465,6 +2465,7 @@ int count_mounts(struct mnt_namespace *ns, struct mount *mnt)
 enum mnt_tree_flags_t {
 	MNT_TREE_MOVE = BIT(0),
 	MNT_TREE_BENEATH = BIT(1),
+	MNT_TREE_PROPAGATION = BIT(2),
 };
 
 /**
@@ -3434,8 +3435,8 @@ static int can_move_mount_beneath(const struct path *from,
 	return 0;
 }
 
-static int do_move_mount(struct path *old_path, struct path *new_path,
-			 bool beneath)
+static int do_move_mount(struct path *old_path,
+			 struct path *new_path, enum mnt_tree_flags_t flags)
 {
 	struct mnt_namespace *ns;
 	struct mount *p;
@@ -3443,8 +3444,7 @@ static int do_move_mount(struct path *old_path, struct path *new_path,
 	struct mount *parent;
 	struct mountpoint *mp, *old_mp;
 	int err;
-	bool attached;
-	enum mnt_tree_flags_t flags = 0;
+	bool attached, beneath = flags & MNT_TREE_BENEATH;
 
 	mp = do_lock_mount(new_path, beneath);
 	if (IS_ERR(mp))
@@ -3545,7 +3545,7 @@ static int do_move_mount_old(struct path *path, const char *old_name)
 	if (err)
 		return err;
 
-	err = do_move_mount(&old_path, path, false);
+	err = do_move_mount(&old_path, path, 0);
 	path_put(&old_path);
 	return err;
 }
@@ -4386,6 +4386,21 @@ SYSCALL_DEFINE3(fsmount, int, fs_fd, unsigned int, flags,
 	return ret;
 }
 
+static inline int vfs_move_mount(struct path *from_path, struct path *to_path,
+				 enum mnt_tree_flags_t mflags)
+{
+	int ret;
+
+	ret = security_move_mount(from_path, to_path);
+	if (ret)
+		return ret;
+
+	if (mflags & MNT_TREE_PROPAGATION)
+		return do_set_group(from_path, to_path);
+
+	return do_move_mount(from_path, to_path, mflags);
+}
+
 /*
  * Move a mount from one place to another.  In combination with
  * fsopen()/fsmount() this is used to install a new mount and in combination
@@ -4399,8 +4414,12 @@ SYSCALL_DEFINE5(move_mount,
 		int, to_dfd, const char __user *, to_pathname,
 		unsigned int, flags)
 {
-	struct path from_path, to_path;
-	unsigned int lflags;
+	struct path to_path __free(path_put) = {};
+	struct path from_path __free(path_put) = {};
+	struct filename *to_name __free(putname) = NULL;
+	struct filename *from_name __free(putname) = NULL;
+	unsigned int lflags, uflags;
+	enum mnt_tree_flags_t mflags = 0;
 	int ret = 0;
 
 	if (!may_mount())
@@ -4413,43 +4432,51 @@ SYSCALL_DEFINE5(move_mount,
 	    (MOVE_MOUNT_BENEATH | MOVE_MOUNT_SET_GROUP))
 		return -EINVAL;
 
-	/* If someone gives a pathname, they aren't permitted to move
-	 * from an fd that requires unmount as we can't get at the flag
-	 * to clear it afterwards.
-	 */
+	if (flags & MOVE_MOUNT_SET_GROUP)	mflags |= MNT_TREE_PROPAGATION;
+	if (flags & MOVE_MOUNT_BENEATH)		mflags |= MNT_TREE_BENEATH;
+
 	lflags = 0;
 	if (flags & MOVE_MOUNT_F_SYMLINKS)	lflags |= LOOKUP_FOLLOW;
 	if (flags & MOVE_MOUNT_F_AUTOMOUNTS)	lflags |= LOOKUP_AUTOMOUNT;
-	if (flags & MOVE_MOUNT_F_EMPTY_PATH)	lflags |= LOOKUP_EMPTY;
-
-	ret = user_path_at(from_dfd, from_pathname, lflags, &from_path);
-	if (ret < 0)
-		return ret;
+	if (flags & MOVE_MOUNT_F_EMPTY_PATH)	uflags = AT_EMPTY_PATH;
+	from_name = getname_maybe_null(from_pathname, uflags);
+	if (IS_ERR(from_name))
+		return PTR_ERR(from_name);
 
 	lflags = 0;
 	if (flags & MOVE_MOUNT_T_SYMLINKS)	lflags |= LOOKUP_FOLLOW;
 	if (flags & MOVE_MOUNT_T_AUTOMOUNTS)	lflags |= LOOKUP_AUTOMOUNT;
-	if (flags & MOVE_MOUNT_T_EMPTY_PATH)	lflags |= LOOKUP_EMPTY;
+	if (flags & MOVE_MOUNT_T_EMPTY_PATH)	uflags = AT_EMPTY_PATH;
+	to_name = getname_maybe_null(to_pathname, uflags);
+	if (IS_ERR(to_name))
+		return PTR_ERR(to_name);
+
+	if (!to_name && to_dfd >= 0) {
+		CLASS(fd_raw, f_to)(to_dfd);
+		if (fd_empty(f_to))
+			return -EBADF;
+
+		to_path = fd_file(f_to)->f_path;
+		path_get(&to_path);
+	} else {
+		ret = filename_lookup(to_dfd, to_name, lflags, &to_path, NULL);
+		if (ret)
+			return ret;
+	}
 
-	ret = user_path_at(to_dfd, to_pathname, lflags, &to_path);
-	if (ret < 0)
-		goto out_from;
+	if (!from_name && from_dfd >= 0) {
+		CLASS(fd_raw, f_from)(from_dfd);
+		if (fd_empty(f_from))
+			return -EBADF;
 
-	ret = security_move_mount(&from_path, &to_path);
-	if (ret < 0)
-		goto out_to;
+		return vfs_move_mount(&fd_file(f_from)->f_path, &to_path, mflags);
+	}
 
-	if (flags & MOVE_MOUNT_SET_GROUP)
-		ret = do_set_group(&from_path, &to_path);
-	else
-		ret = do_move_mount(&from_path, &to_path,
-				    (flags & MOVE_MOUNT_BENEATH));
+	ret = filename_lookup(from_dfd, from_name, lflags, &from_path, NULL);
+	if (ret)
+		return ret;
 
-out_to:
-	path_put(&to_path);
-out_from:
-	path_put(&from_path);
-	return ret;
+	return vfs_move_mount(&from_path, &to_path, mflags);
 }
 
 /*
diff --git a/include/linux/fs.h b/include/linux/fs.h
index e71d58c7f59c..7e9df867538d 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2855,6 +2855,7 @@ static inline struct filename *getname_maybe_null(const char __user *name, int f
 	return __getname_maybe_null(name);
 }
 extern void putname(struct filename *name);
+DEFINE_FREE(putname, struct filename *, if (!IS_ERR_OR_NULL(_T)) putname(_T))
 
 extern int finish_open(struct file *file, struct dentry *dentry,
 			int (*open)(struct inode *, struct file *));

-- 
2.47.2