selftests/bpf: fix -Wstrict-aliasing in test_sockopt_sk.c

Let's use union with u8[4] and u32 members for sockopt buffer,
that should fix any possible aliasing issues.

test_sockopt_sk.c: In function ‘getsetsockopt’:
test_sockopt_sk.c:115:2: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
  if (*(__u32 *)buf != 0x55AA*2) {
  ^~
test_sockopt_sk.c:116:3: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
   log_err("Unexpected getsockopt(SO_SNDBUF) 0x%x != 0x55AA*2",
   ^~~~~~~

Fixes: 8a027dc0 ("selftests/bpf: add sockopt test that exercises sk helpers")
Reported-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/test_sockopt_sk.c

@@ -22,7 +22,10 @@
 static int getsetsockopt(void)
 {
 	int fd, err;
-	char buf[4] = {};
+	union {
+		char u8[4];
+		__u32 u32;
+	} buf = {};
 	socklen_t optlen;
 
 	fd = socket(AF_INET, SOCK_STREAM, 0);
@@ -33,31 +36,31 @@ static int getsetsockopt(void)
 
 	/* IP_TOS - BPF bypass */
 
-	buf[0] = 0x08;
-	err = setsockopt(fd, SOL_IP, IP_TOS, buf, 1);
+	buf.u8[0] = 0x08;
+	err = setsockopt(fd, SOL_IP, IP_TOS, &buf, 1);
 	if (err) {
 		log_err("Failed to call setsockopt(IP_TOS)");
 		goto err;
 	}
 
-	buf[0] = 0x00;
+	buf.u8[0] = 0x00;
 	optlen = 1;
-	err = getsockopt(fd, SOL_IP, IP_TOS, buf, &optlen);
+	err = getsockopt(fd, SOL_IP, IP_TOS, &buf, &optlen);
 	if (err) {
 		log_err("Failed to call getsockopt(IP_TOS)");
 		goto err;
 	}
 
-	if (buf[0] != 0x08) {
+	if (buf.u8[0] != 0x08) {
 		log_err("Unexpected getsockopt(IP_TOS) buf[0] 0x%02x != 0x08",
-			buf[0]);
+			buf.u8[0]);
 		goto err;
 	}
 
 	/* IP_TTL - EPERM */
 
-	buf[0] = 1;
-	err = setsockopt(fd, SOL_IP, IP_TTL, buf, 1);
+	buf.u8[0] = 1;
+	err = setsockopt(fd, SOL_IP, IP_TTL, &buf, 1);
 	if (!err || errno != EPERM) {
 		log_err("Unexpected success from setsockopt(IP_TTL)");
 		goto err;
@@ -65,16 +68,16 @@ static int getsetsockopt(void)
 
 	/* SOL_CUSTOM - handled by BPF */
 
-	buf[0] = 0x01;
-	err = setsockopt(fd, SOL_CUSTOM, 0, buf, 1);
+	buf.u8[0] = 0x01;
+	err = setsockopt(fd, SOL_CUSTOM, 0, &buf, 1);
 	if (err) {
 		log_err("Failed to call setsockopt");
 		goto err;
 	}
 
-	buf[0] = 0x00;
+	buf.u32 = 0x00;
 	optlen = 4;
-	err = getsockopt(fd, SOL_CUSTOM, 0, buf, &optlen);
+	err = getsockopt(fd, SOL_CUSTOM, 0, &buf, &optlen);
 	if (err) {
 		log_err("Failed to call getsockopt");
 		goto err;
@@ -84,37 +87,31 @@ static int getsetsockopt(void)
 		log_err("Unexpected optlen %d != 1", optlen);
 		goto err;
 	}
-	if (buf[0] != 0x01) {
-		log_err("Unexpected buf[0] 0x%02x != 0x01", buf[0]);
+	if (buf.u8[0] != 0x01) {
+		log_err("Unexpected buf[0] 0x%02x != 0x01", buf.u8[0]);
 		goto err;
 	}
 
 	/* SO_SNDBUF is overwritten */
 
-	buf[0] = 0x01;
-	buf[1] = 0x01;
-	buf[2] = 0x01;
-	buf[3] = 0x01;
-	err = setsockopt(fd, SOL_SOCKET, SO_SNDBUF, buf, 4);
+	buf.u32 = 0x01010101;
+	err = setsockopt(fd, SOL_SOCKET, SO_SNDBUF, &buf, 4);
 	if (err) {
 		log_err("Failed to call setsockopt(SO_SNDBUF)");
 		goto err;
 	}
 
-	buf[0] = 0x00;
-	buf[1] = 0x00;
-	buf[2] = 0x00;
-	buf[3] = 0x00;
+	buf.u32 = 0x00;
 	optlen = 4;
-	err = getsockopt(fd, SOL_SOCKET, SO_SNDBUF, buf, &optlen);
+	err = getsockopt(fd, SOL_SOCKET, SO_SNDBUF, &buf, &optlen);
 	if (err) {
 		log_err("Failed to call getsockopt(SO_SNDBUF)");
 		goto err;
 	}
 
-	if (*(__u32 *)buf != 0x55AA*2) {
+	if (buf.u32 != 0x55AA*2) {
 		log_err("Unexpected getsockopt(SO_SNDBUF) 0x%x != 0x55AA*2",
-			*(__u32 *)buf);
+			buf.u32);
 		goto err;
 	}