[PATCH 2/2] userfaultfd: selftests: modify selftest to use /dev/userfaultfd

[Axel Rasmussen <axelrasmussen@google.com>]

Prefer this new interface, but if using it fails for any reason just
fall back to using userfaultfd(2) as before.

Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
---
 tools/testing/selftests/vm/userfaultfd.c | 23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/vm/userfaultfd.c b/tools/testing/selftests/vm/userfaultfd.c
index 92a4516f8f0d..a50c430f036c 100644
--- a/tools/testing/selftests/vm/userfaultfd.c
+++ b/tools/testing/selftests/vm/userfaultfd.c
@@ -383,13 +383,32 @@ static void assert_expected_ioctls_present(uint64_t mode, uint64_t ioctls)
 	}
 }
 
+static void __userfaultfd_open_dev(void)
+{
+	int fd;
+
+	uffd = -1;
+	fd = open("/dev/userfaultfd", O_RDWR | O_CLOEXEC);
+	if (fd < 0)
+		return;
+
+	uffd = ioctl(fd, USERFAULTFD_IOC_NEW,
+		     O_CLOEXEC | O_NONBLOCK | UFFD_USER_MODE_ONLY);
+	close(fd);
+}
+
 static void userfaultfd_open(uint64_t *features)
 {
 	struct uffdio_api uffdio_api;
 
-	uffd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK | UFFD_USER_MODE_ONLY);
+	__userfaultfd_open_dev();
+	if (uffd < 0) {
+		printf("/dev/userfaultfd failed, fallback to userfaultfd(2)\n");
+		uffd = syscall(__NR_userfaultfd,
+			       O_CLOEXEC | O_NONBLOCK | UFFD_USER_MODE_ONLY);
+	}
 	if (uffd < 0)
-		err("userfaultfd syscall not available in this kernel");
+		err("userfaultfd syscall failed");
 	uffd_flags = fcntl(uffd, F_GETFD, NULL);
 
 	uffdio_api.api = UFFD_API;
-- 
2.35.1.1178.g4f1659d476-goog