kdat: Relax uffd checks (v2)

v2: When uffd is present, the reported features may still be 0,
    so we need one more bool for uffd syscall itself.
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/cr-check.c

@@ -1053,9 +1053,14 @@ static int check_uffd(void)
 		UFFD_FEATURE_EVENT_UNMAP |
 		UFFD_FEATURE_EVENT_REMOVE;
 
-	if (kerndat_uffd(true))
+	if (kerndat_uffd())
 		return -1;
 
+	if (!kdat.has_uffd) {
+		pr_err("UFFD is not supported\n");
+		return -1;
+	}
+
 	if ((kdat.uffd_features & features) != features) {
 		pr_err("Userfaultfd missing essential features\n");
 		return -1;

criu/cr-service.c

@@ -900,7 +900,7 @@ static int handle_feature_check(int sk, CriuReq * msg)
 
 		if ((msg->features->has_lazy_pages == 1) &&
 		    (msg->features->lazy_pages == true)) {
-			ret = kerndat_uffd(true);
+			ret = kerndat_uffd();
 
 			/*
 			 * Not checking for specific UFFD features yet.
@@ -909,7 +909,7 @@ static int handle_feature_check(int sk, CriuReq * msg)
 			 * be extended in the future for a more detailed
 			 * UFFD feature check.
 			 */
-			if (ret)
+			if (ret || !kdat.has_uffd)
 				feat.lazy_pages = false;
 			else
 				feat.lazy_pages = true;

criu/include/kerndat.h

@@ -49,6 +49,7 @@ struct kerndat_s {
 	bool has_tcp_half_closed;
 	bool stack_guard_gap_hidden;
 	int lsm;
+	bool has_uffd;
 	unsigned long uffd_features;
 };
 
@@ -71,6 +72,6 @@ enum {
 extern int kerndat_fs_virtualized(unsigned int which, u32 kdev);
 
 extern int kerndat_tcp_repair();
-extern int kerndat_uffd(bool need_uffd);
+extern int kerndat_uffd(void);
 
 #endif /* __CR_KERNDAT_H__ */

criu/kerndat.c

@@ -715,7 +715,7 @@ unl:
 	}
 }
 
-int kerndat_uffd(bool need_uffd)
+int kerndat_uffd(void)
 {
 	struct uffdio_api uffdio_api;
 	int uffd;
@@ -727,14 +727,16 @@ int kerndat_uffd(bool need_uffd)
 	 * on this system. Additionally checking for ENOSYS
 	 * makes sure it is actually not implemented.
 	 */
-	if (uffd == -1 && errno == ENOSYS) {
-		if (!need_uffd)
+	if (uffd == -1) {
+		if (errno == ENOSYS)
 			return 0;
 
 		pr_err("Lazy pages are not available\n");
 		return -1;
 	}
 
+	kdat.has_uffd = true;
+
 	uffdio_api.api = UFFD_API;
 	uffdio_api.features = 0;
 	if (ioctl(uffd, UFFDIO_API, &uffdio_api)) {
@@ -792,7 +794,7 @@ int kerndat_init(void)
 	if (!ret)
 		ret = kerndat_detect_stack_guard_gap();
 	if (!ret)
-		ret = kerndat_uffd(opts.lazy_pages);
+		ret = kerndat_uffd();
 
 	kerndat_lsm();
 	kerndat_mmap_min_addr();

criu/uffd.c

@@ -1115,7 +1115,7 @@ int cr_lazy_pages(bool daemon)
 	int lazy_sk;
 	int ret;
 
-	if (kerndat_uffd(true))
+	if (kerndat_uffd() || !kdat.has_uffd)
 		return -1;
 
 	if (prepare_dummy_pstree())