wireguard: selftests: simplify RNG seeding

The seed_rng() function was written to work across lots of old kernels,
back when WireGuard used a big compatibility layer. Now that things have
evolved, we can vastly simplify this, by just marking the RNG as seeded.
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/wireguard/qemu/init.c

@@ -56,26 +56,14 @@ static void print_banner(void)
 
 static void seed_rng(void)
 {
-	int fd;
-	struct {
-		int entropy_count;
-		int buffer_size;
-		unsigned char buffer[256];
-	} entropy = {
-		.entropy_count = sizeof(entropy.buffer) * 8,
-		.buffer_size = sizeof(entropy.buffer),
-		.buffer = "Adding real entropy is not actually important for these tests. Don't try this at home, kids!"
-	};
+	int bits = 256, fd;
 
-	if (mknod("/dev/urandom", S_IFCHR | 0644, makedev(1, 9)))
-		panic("mknod(/dev/urandom)");
-	fd = open("/dev/urandom", O_WRONLY);
+	pretty_message("[+] Fake seeding RNG...");
+	fd = open("/dev/random", O_WRONLY);
 	if (fd < 0)
-		panic("open(urandom)");
-	for (int i = 0; i < 256; ++i) {
-		if (ioctl(fd, RNDADDENTROPY, &entropy) < 0)
-			panic("ioctl(urandom)");
-	}
+		panic("open(random)");
+	if (ioctl(fd, RNDADDTOENTCNT, &bits) < 0)
+		panic("ioctl(RNDADDTOENTCNT)");
 	close(fd);
 }
 
@@ -270,10 +258,10 @@ static void check_leaks(void)
 
 int main(int argc, char *argv[])
 {
-	seed_rng();
 	ensure_console();
 	print_banner();
 	mount_filesystems();
+	seed_rng();
 	kmod_selftests();
 	enable_logging();
 	clear_leaks();