mount-util: restore compat for kernels without MOUNT_ATTR_NOSYMFOLLOW (< 5.14)

Follow-up for 6753bd8a2f

Replaces #41341

README

@@ -60,7 +60,7 @@ REQUIREMENTS:
 
         Linux kernel ≥ 5.11 for epoll_pwait2()
                      ≥ 5.12 for idmapped mount (mount_setattr())
-                     ≥ 5.14 for cgroup.kill and quotactl_fd()
+                     ≥ 5.14 for cgroup.kill, quotactl_fd(), and MOUNT_ATTR_NOSYMFOLLOW
 
         ⚠️ Kernel versions below 5.14 ("recommended baseline") have significant gaps
            in functionality and are not recommended for use with this version
@@ -77,7 +77,7 @@ REQUIREMENTS:
                      ≥ 6.10 for fcntl(F_DUPFD_QUERY), unprivileged linkat(AT_EMPTY_PATH),
                                 and block device 'partscan' sysfs attribute
                      ≥ 6.12 for AT_HANDLE_MNT_ID_UNIQUE
-                     ≥ 6.13 for PIDFD_GET_INFO and {set,remove}xattrat() and
+                     ≥ 6.13 for PIDFD_GET_INFO, {set,remove}xattrat(), and
                                 FSCONFIG_SET_FD support for overlayfs layers
                      ≥ 6.16 for coredump pattern '%F' (pidfd) specifier and SO_PASSRIGHTS
 

src/shared/mount-util.c

@@ -1988,10 +1988,19 @@ int fsmount_credentials_fs(int *ret_fsfd) {
         if (fsconfig(fs_fd, FSCONFIG_CMD_CREATE, NULL, NULL, 0) < 0)
                 return -errno;
 
-        int mfd = fsmount(fs_fd, FSMOUNT_CLOEXEC,
-                          ms_flags_to_mount_attr(credentials_fs_mount_flags(/* ro= */ false)));
+        unsigned mount_attrs = ms_flags_to_mount_attr(credentials_fs_mount_flags(/* ro = */ false));
+
+        int mfd = RET_NERRNO(fsmount(fs_fd, FSMOUNT_CLOEXEC, mount_attrs));
+        if (mfd == -EINVAL) {
+                /* MS_NOSYMFOLLOW was added in kernel 5.10, but the new mount API counterpart was missing
+                 * until 5.14 (c.f. https://github.com/torvalds/linux/commit/dd8b477f9a3d8edb136207acb3652e1a34a661b7).
+                 *
+                 * TODO: drop this once our baseline is raised to 5.14 */
+                assert(FLAGS_SET(mount_attrs, MOUNT_ATTR_NOSYMFOLLOW));
+                mfd = RET_NERRNO(fsmount(fs_fd, FSMOUNT_CLOEXEC, mount_attrs & ~MOUNT_ATTR_NOSYMFOLLOW));
+        }
         if (mfd < 0)
-                return -errno;
+                return mfd;
 
         if (ret_fsfd)
                 *ret_fsfd = TAKE_FD(fs_fd);