criu: uffd runtime detection

Now that userfaultfd/lazy-pages support is enable all the time, this
adds runtime detection of userfaultfd. On a system without the
userfaultfd syscall following is printed:

uffd daemon:

(00.000004) Error (uffd.c:176): lazy-pages: Runtime detection of userfaultfd failed on this system.
(00.000024) Error (uffd.c:177): lazy-pages: Processes cannot be lazy-restored on this system.

or criu restore

(00.457047)   6858: Error (uffd.c:176): lazy-pages: Runtime detection of userfaultfd failed on this system.
(00.457049)   6858: Error (uffd.c:177): lazy-pages: Processes cannot be lazy-restored on this system.
Signed-off-by: Adrian Reber <areber@redhat.com>
Acked-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/uffd.c

@@ -160,10 +160,33 @@ out:
 	return ret;
 }
 
+/* Runtime detection if userfaultfd can be used */
+
+static int check_for_uffd()
+{
+	int uffd;
+
+	uffd = syscall(SYS_userfaultfd, 0);
+	/*
+	 * uffd == -1 is probably enough to not use lazy-restore
+	 * on this system. Additionally checking for ENOSYS
+	 * makes sure it is actually not implemented.
+	 */
+	if ((uffd == -1) && (errno == ENOSYS)) {
+		pr_err("Runtime detection of userfaultfd failed on this system.\n");
+		pr_err("Processes cannot be lazy-restored on this system.\n");
+		return -1;
+	}
+	close(uffd);
+	return 0;
+}
+
 /* This function is used by 'criu restore --lazy-pages' */
 int setup_uffd(struct task_restore_args *task_args, int pid)
 {
 	struct uffdio_api uffdio_api;
+	if (check_for_uffd())
+		return -1;
 	/*
 	 * Open userfaulfd FD which is passed to the restorer blob and
 	 * to a second process handling the userfaultfd page faults.
@@ -810,6 +833,9 @@ int cr_lazy_pages()
 	int epollfd;
 	int ret;
 
+	if (check_for_uffd())
+		return -1;
+
 	if (!opts.addr) {
 		pr_info("Please specify a file name for the unix domain socket\n");
 		pr_info("used to communicate between the lazy-pages server\n");