hv_netvsc: Avoid field-overflowing memcpy()

In preparation for FORTIFY_SOURCE performing compile-time and run-time
field bounds checking for memcpy(), memmove(), and memset(), avoid
intentionally writing across neighboring fields.

Add flexible array to represent start of buf_info, improving readability
and avoid future warning where memcpy() thinks it is writing past the
end of the structure.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/hyperv/hyperv_net.h

@@ -1163,6 +1163,7 @@ struct rndis_set_request {
 	u32 info_buflen;
 	u32 info_buf_offset;
 	u32 dev_vc_handle;
+	u8  info_buf[];
 };
 
 /* Response to NdisSetRequest */

drivers/net/hyperv/rndis_filter.c

@@ -1051,10 +1051,8 @@ static int rndis_filter_set_packet_filter(struct rndis_device *dev,
 	set = &request->request_msg.msg.set_req;
 	set->oid = RNDIS_OID_GEN_CURRENT_PACKET_FILTER;
 	set->info_buflen = sizeof(u32);
-	set->info_buf_offset = sizeof(struct rndis_set_request);
-
-	memcpy((void *)(unsigned long)set + sizeof(struct rndis_set_request),
-	       &new_filter, sizeof(u32));
+	set->info_buf_offset = offsetof(typeof(*set), info_buf);
+	memcpy(set->info_buf, &new_filter, sizeof(u32));
 
 	ret = rndis_filter_send_request(dev, request);
 	if (ret == 0) {