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>

diff --git a/fs/fhandle.c b/fs/fhandle.c
index 6e8cea16790ef28256e46e8c4e9986e9812ea80e..8cb665629f4a895b13ed3a2c99a85d18fbc12cce 100644 (file)
--- a/fs/fhandle.c
+++ b/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) ||
-           copy_to_user(ufh, handle,
-                        struct_size(handle, f_handle, handle_bytes)))
+       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;
        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;

diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 4bcf6754738d4da35ba05cdd2d902028e48550a5..5758104921e66e2e3b815bacb1953ac892599a46 100644 (file)
--- a/include/linux/syscalls.h
+++ b/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);

diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index 38a6d66d9e8871b27b464c009344bde6a1e9de0e..87e2dec79fea4ef21e694f3c069ec229926b6d08 100644 (file)
--- a/include/uapi/linux/fcntl.h
+++ b/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