net/mlx5e: Use struct_group() for memcpy() region

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.

Use struct_group() in struct vlan_ethhdr around members h_dest and
h_source, so they can be referenced together. This will allow memcpy()
and sizeof() to more easily reason about sizes, improve readability,
and avoid future warnings about writing beyond the end of h_dest.

"pahole" shows no size nor member offset changes to struct vlan_ethhdr.
"objdump -d" shows no object code changes.

Fixes: 34802a42 ("net/mlx5e: Do not modify the TX SKB")
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>

drivers/net/ethernet/mellanox/mlx5/core/en_tx.c

@@ -208,7 +208,7 @@ static inline void mlx5e_insert_vlan(void *start, struct sk_buff *skb, u16 ihs)
 	int cpy1_sz = 2 * ETH_ALEN;
 	int cpy2_sz = ihs - cpy1_sz;
 
-	memcpy(vhdr, skb->data, cpy1_sz);
+	memcpy(&vhdr->addrs, skb->data, cpy1_sz);
 	vhdr->h_vlan_proto = skb->vlan_proto;
 	vhdr->h_vlan_TCI = cpu_to_be16(skb_vlan_tag_get(skb));
 	memcpy(&vhdr->h_vlan_encapsulated_proto, skb->data + cpy1_sz, cpy2_sz);

include/linux/if_vlan.h

@@ -46,8 +46,10 @@ struct vlan_hdr {
  *	@h_vlan_encapsulated_proto: packet type ID or len
  */
 struct vlan_ethhdr {
-	unsigned char	h_dest[ETH_ALEN];
-	unsigned char	h_source[ETH_ALEN];
+	struct_group(addrs,
+		unsigned char	h_dest[ETH_ALEN];
+		unsigned char	h_source[ETH_ALEN];
+	);
 	__be16		h_vlan_proto;
 	__be16		h_vlan_TCI;
 	__be16		h_vlan_encapsulated_proto;