af_packet: avoid a false positive warning in packet_setsockopt()

Although the code is correct, the following line

	copy_from_sockptr(&req_u.req, optval, len));

triggers this warning :

memcpy: detected field-spanning write (size 28) of single field "dst" at include/linux/sockptr.h:49 (size 16)

Refactor the code to be more explicit.

Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: edumazet

net/packet/af_packet.c

@@ -3800,28 +3800,30 @@ packet_setsockopt(struct socket *sock, int level, int optname, sockptr_t optval,
 	case PACKET_TX_RING:
 	{
 		union tpacket_req_u req_u;
-		int len;
 
+		ret = -EINVAL;
 		lock_sock(sk);
 		switch (po->tp_version) {
 		case TPACKET_V1:
 		case TPACKET_V2:
-			len = sizeof(req_u.req);
+			if (optlen < sizeof(req_u.req))
+				break;
+			ret = copy_from_sockptr(&req_u.req, optval,
+						sizeof(req_u.req)) ?
+						-EINVAL : 0;
 			break;
 		case TPACKET_V3:
 		default:
-			len = sizeof(req_u.req3);
+			if (optlen < sizeof(req_u.req3))
+				break;
+			ret = copy_from_sockptr(&req_u.req3, optval,
+						sizeof(req_u.req3)) ?
+						-EINVAL : 0;
 			break;
 		}
-		if (optlen < len) {
-			ret = -EINVAL;
-		} else {
-			if (copy_from_sockptr(&req_u.req, optval, len))
-				ret = -EFAULT;
-			else
-				ret = packet_set_ring(sk, &req_u, 0,
-						    optname == PACKET_TX_RING);
-		}
+		if (!ret)
+			ret = packet_set_ring(sk, &req_u, 0,
+					      optname == PACKET_TX_RING);
 		release_sock(sk);
 		return ret;
 	}