flow_dissector: Fix out-of-bounds warning in __skb_flow_bpf_to_target()

Fix the following out-of-bounds warning:

net/core/flow_dissector.c:835:3: warning: 'memcpy' offset [33, 48] from the object at 'flow_keys' is out of the bounds of referenced subobject 'ipv6_src' with type '__u32[4]' {aka 'unsigned int[4]'} at offset 16 [-Warray-bounds]

The problem is that the original code is trying to copy data into a
couple of struct members adjacent to each other in a single call to
memcpy().  So, the compiler legitimately complains about it. As these
are just a couple of members, fix this by copying each one of them in
separate calls to memcpy().

This helps with the ongoing efforts to globally enable -Warray-bounds
and get us closer to being able to tighten the FORTIFY_SOURCE routines
on memcpy().

Link: KSPP#109
Reported-by: kernel test robot <[email protected]>
Signed-off-by: Gustavo A. R. Silva <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

net/core/flow_dissector.c

@@ -832,8 +832,10 @@ static void __skb_flow_bpf_to_target(const struct bpf_flow_keys *flow_keys,
 		key_addrs = skb_flow_dissector_target(flow_dissector,
 						      FLOW_DISSECTOR_KEY_IPV6_ADDRS,
 						      target_container);
-		memcpy(&key_addrs->v6addrs, &flow_keys->ipv6_src,
-		       sizeof(key_addrs->v6addrs));
+		memcpy(&key_addrs->v6addrs.src, &flow_keys->ipv6_src,
+		       sizeof(key_addrs->v6addrs.src));
+		memcpy(&key_addrs->v6addrs.dst, &flow_keys->ipv6_dst,
+		       sizeof(key_addrs->v6addrs.dst));
 		key_control->addr_type = FLOW_DISSECTOR_KEY_IPV6_ADDRS;
 	}