Commit 4356d575 authored by Aleksa Sarai's avatar Aleksa Sarai Committed by Christian Brauner
Browse files

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: default avatarJeff Layton <jlayton@kernel.org>
Signed-off-by: default avatarAleksa Sarai <cyphar@cyphar.com>
Link: https://lore.kernel.org/r/20240828-exportfs-u64-mount-id-v3-2-10c2c4c16708@cyphar.com


Reviewed-by: default avatarJan Kara <jack@suse.cz>
Reviewed-by: default avatarJosef Bacik <josef@toxicpanda.com>
Signed-off-by: default avatarChristian Brauner <brauner@kernel.org>
parent b4fef22c

fs/fhandle.c

+22 −7
Original line number Diff line number Diff line
@@ -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;
@@ -69,7 +70,17 @@ 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) {

		if (put_user(real_mount(path->mnt)->mnt_id_unique,

			     (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;
@@ -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

+1 −1
Original line number Diff line number Diff line
@@ -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

+1 −0
Original line number Diff line number Diff line
@@ -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