fhandle: expose u64 mount id to name_to_handle_at(2)

Now that we provide a unique 64-bit mount ID interface in statx(2), we
can now provide a race-free way for name_to_handle_at(2) to provide a
file handle and corresponding mount without needing to worry about
racing with /proc/mountinfo parsing or having to open a file just to do
statx(2).

While this is not necessary if you are using AT_EMPTY_PATH and don't
care about an extra statx(2) call, users that pass full paths into
name_to_handle_at(2) need to know which mount the file handle comes from
(to make sure they don't try to open_by_handle_at a file handle from a
different filesystem) and switching to AT_EMPTY_PATH would require
allocating a file for every name_to_handle_at(2) call, turning

  err = name_to_handle_at(-EBADF, "/foo/bar/baz", &handle, &mntid,
                          AT_HANDLE_MNT_ID_UNIQUE);

into

  int fd = openat(-EBADF, "/foo/bar/baz", O_PATH | O_CLOEXEC);
  err1 = name_to_handle_at(fd, "", &handle, &unused_mntid, AT_EMPTY_PATH);
  err2 = statx(fd, "", AT_EMPTY_PATH, STATX_MNT_ID_UNIQUE, &statxbuf);
  mntid = statxbuf.stx_mnt_id;
  close(fd);

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Link: https://lore.kernel.org/r/20240828-exportfs-u64-mount-id-v3-2-10c2c4c16708@cyphar.com
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/fhandle.c

@@ -16,7 +16,8 @@
 
 static long do_sys_name_to_handle(const struct path *path,
 				  struct file_handle __user *ufh,
-				  int __user *mnt_id, int fh_flags)
+				  void __user *mnt_id, bool unique_mntid,
+				  int fh_flags)
 {
 	long retval;
 	struct file_handle f_handle;
@@ -69,9 +70,19 @@ static long do_sys_name_to_handle(const struct path *path,
 	} else
 		retval = 0;
 	/* copy the mount id */
-	if (put_user(real_mount(path->mnt)->mnt_id, mnt_id) ||
+	if (unique_mntid) {
-	    copy_to_user(ufh, handle,
+		if (put_user(real_mount(path->mnt)->mnt_id_unique,
-			 struct_size(handle, f_handle, handle_bytes)))
+			     (u64 __user *) mnt_id))
+			retval = -EFAULT;
+	} else {
+		if (put_user(real_mount(path->mnt)->mnt_id,
+			     (int __user *) mnt_id))
+			retval = -EFAULT;
+	}
+	/* copy the handle */
+	if (retval != -EFAULT &&
+		copy_to_user(ufh, handle,
+			     struct_size(handle, f_handle, handle_bytes)))
 		retval = -EFAULT;
 	kfree(handle);
 	return retval;
@@ -83,6 +94,7 @@ static long do_sys_name_to_handle(const struct path *path,
  * @name: name that should be converted to handle.
  * @handle: resulting file handle
  * @mnt_id: mount id of the file system containing the file
+ *          (u64 if AT_HANDLE_MNT_ID_UNIQUE, otherwise int)
  * @flag: flag value to indicate whether to follow symlink or not
  *        and whether a decodable file handle is required.
  *
@@ -92,7 +104,7 @@ static long do_sys_name_to_handle(const struct path *path,
  * value required.
  */
 SYSCALL_DEFINE5(name_to_handle_at, int, dfd, const char __user *, name,
-		struct file_handle __user *, handle, int __user *, mnt_id,
+		struct file_handle __user *, handle, void __user *, mnt_id,
 		int, flag)
 {
 	struct path path;
@@ -100,7 +112,8 @@ SYSCALL_DEFINE5(name_to_handle_at, int, dfd, const char __user *, name,
 	int fh_flags;
 	int err;
 
-	if (flag & ~(AT_SYMLINK_FOLLOW | AT_EMPTY_PATH | AT_HANDLE_FID))
+	if (flag & ~(AT_SYMLINK_FOLLOW | AT_EMPTY_PATH | AT_HANDLE_FID |
+		     AT_HANDLE_MNT_ID_UNIQUE))
 		return -EINVAL;
 
 	lookup_flags = (flag & AT_SYMLINK_FOLLOW) ? LOOKUP_FOLLOW : 0;
@@ -109,7 +122,9 @@ SYSCALL_DEFINE5(name_to_handle_at, int, dfd, const char __user *, name,
 		lookup_flags |= LOOKUP_EMPTY;
 	err = user_path_at(dfd, name, lookup_flags, &path);
 	if (!err) {
-		err = do_sys_name_to_handle(&path, handle, mnt_id, fh_flags);
+		err = do_sys_name_to_handle(&path, handle, mnt_id,
+					    flag & AT_HANDLE_MNT_ID_UNIQUE,
+					    fh_flags);
 		path_put(&path);
 	}
 	return err;

include/linux/syscalls.h

@@ -870,7 +870,7 @@ asmlinkage long sys_fanotify_mark(int fanotify_fd, unsigned int flags,
 #endif
 asmlinkage long sys_name_to_handle_at(int dfd, const char __user *name,
 				      struct file_handle __user *handle,
-				      int __user *mnt_id, int flag);
+				      void __user *mnt_id, int flag);
 asmlinkage long sys_open_by_handle_at(int mountdirfd,
 				      struct file_handle __user *handle,
 				      int flags);

include/uapi/linux/fcntl.h

@@ -152,6 +152,7 @@
 #define AT_HANDLE_FID		0x200	/* File handle is needed to compare
 					   object identity and may not be
 					   usable with open_by_handle_at(2). */
+#define AT_HANDLE_MNT_ID_UNIQUE	0x001	/* Return the u64 unique mount ID. */
 
 #if defined(__KERNEL__)
 #define AT_GETATTR_NOSEC	0x80000000