system: use getrandom() and fallback on urandom if syscall doesn't work

src/common/System.cpp

@@ -36,12 +36,7 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #include <fcntl.h>
 #include <signal.h>
 #ifdef __linux__
-#include <linux/version.h>
-#if LINUX_VERSION_CODE >= KERNEL_VERSION(3,17,0)
-#include <sys/syscall.h>
-#include <linux/random.h>
-#define HAS_GETRANDOM_SYSCALL 1
-#endif
+#include <sys/random.h>
 #endif
 #ifdef __native_client__
 #include <nacl/nacl_exception.h>
@@ -462,19 +457,32 @@ void GenRandomBytes(void* dest, size_t size)
 	size_t bytes_written;
 	if (nacl_secure_random(dest, size, &bytes_written) != 0 || bytes_written != size)
 		Sys::Error("nacl_secure_random failed");
-#elif defined(__linux__) && defined(HAS_GETRANDOM_SYSCALL)
-	if (syscall(SYS_getrandom, dest, size, GRND_NONBLOCK) == -1)
-		Sys::Error("Failed getrandom syscall: %s", strerror(errno));
 #elif defined(__linux__)
-	int fd = open("/dev/urandom", O_RDONLY);
-	if (fd == -1)
-		Sys::Error("Failed to open /dev/urandom: %s", strerror(errno));
-	if (read(fd, dest, size) != (ssize_t) size)
-		Sys::Error("Failed to read from /dev/urandom: %s", strerror(errno));
-	close(fd);
+	ssize_t ret = getrandom(dest, size, GRND_NONBLOCK);
+	if (ret == -1)
+	{
+		if (errno == ENOSYS)
+		{
+			Log::Warn("getrandom syscall is not supported");
+
+			int fd = open("/dev/urandom", O_RDONLY);
+			if (fd == -1)
+				Sys::Error("Failed to open /dev/urandom: %s", strerror(errno));
+			if (read(fd, dest, size) != (ssize_t) size)
+				Sys::Error("Failed to read from /dev/urandom: %s", strerror(errno));
+			close(fd);
+		}
+		else
+		{
+			Sys::Error("getrandom syscall failed: %s", strerror(errno));
+		}
+	}
+	else if (ret != size)
+	{
+		Sys::Error("getrandom syscall returned insufficient data");
+	}
 #else
 	arc4random_buf(dest, size);
-
 #endif
 }