lib/randutils.c: Fall back gracefully when kernel doesn't support getrandom(2).

The 3.16 kernel is supported until 2020, and various distros have kernels of the same
vintage. It's entirely possible for code built against newer headers to be run against
these kernels, so fall-back to the old “read /dev/{u,}random” method if the kernel doesn'
support getrandom()

lib/randutils.c

@@ -99,32 +99,40 @@ void random_get_bytes(void *buf, size_t nbytes)
 	unsigned char *cp = (unsigned char *)buf;
 
 #ifdef HAVE_GETRANDOM
+	errno = 0;
 	while (getrandom(buf, nbytes, 0) < 0) {
 		if (errno == EINTR)
 			continue;
 		break;
 	}
-#else
-	size_t n = nbytes;
-	int fd = random_get_fd();
-	int lose_counter = 0;
-
-	if (fd >= 0) {
-		while (n > 0) {
-			ssize_t x = read(fd, cp, n);
-			if (x <= 0) {
-				if (lose_counter++ > 16)
-					break;
-				continue;
-			}
-			n -= x;
-			cp += x;
-			lose_counter = 0;
-		}
-
-		close(fd);
-	}
+	if (errno == ENOSYS)
+	/*
+	 * We've been built against headers that support getrandom,
+	 * but the running kernel does not.
+	 * Fallback to reading from /dev/{u,}random as before
+	 */
 #endif
+	{
+		size_t n = nbytes;
+		int fd = random_get_fd();
+		int lose_counter = 0;
+
+		if (fd >= 0) {
+			while (n > 0) {
+				ssize_t x = read(fd, cp, n);
+				if (x <= 0) {
+					if (lose_counter++ > 16)
+						break;
+					continue;
+				}
+				n -= x;
+				cp += x;
+				lose_counter = 0;
+			}
+
+			close(fd);
+		}
+	}
 	/*
 	 * We do this all the time, but this is the only source of
 	 * randomness if /dev/random/urandom is out to lunch.