libertas: 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 txpd around members tx_dest_addr_high
and tx_dest_addr_low 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 queue_id.

"pahole" shows no size nor member offset changes to struct txpd.
"objdump -d" shows no object code changes.
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211118184104.1283637-1-keescook@chromium.org

drivers/net/wireless/marvell/libertas/host.h

@@ -308,10 +308,12 @@ struct txpd {
 	__le32 tx_packet_location;
 	/* Tx packet length */
 	__le16 tx_packet_length;
-	/* First 2 byte of destination MAC address */
-	u8 tx_dest_addr_high[2];
-	/* Last 4 byte of destination MAC address */
-	u8 tx_dest_addr_low[4];
+	struct_group(tx_dest_addr,
+		/* First 2 byte of destination MAC address */
+		u8 tx_dest_addr_high[2];
+		/* Last 4 byte of destination MAC address */
+		u8 tx_dest_addr_low[4];
+	);
 	/* Pkt Priority */
 	u8 priority;
 	/* Pkt Trasnit Power control */

drivers/net/wireless/marvell/libertas/tx.c

@@ -113,6 +113,7 @@ netdev_tx_t lbs_hard_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	p802x_hdr = skb->data;
 	pkt_len = skb->len;
 
+	BUILD_BUG_ON(sizeof(txpd->tx_dest_addr) != ETH_ALEN);
 	if (priv->wdev->iftype == NL80211_IFTYPE_MONITOR) {
 		struct tx_radiotap_hdr *rtap_hdr = (void *)skb->data;
 
@@ -124,10 +125,10 @@ netdev_tx_t lbs_hard_start_xmit(struct sk_buff *skb, struct net_device *dev)
 		pkt_len -= sizeof(*rtap_hdr);
 
 		/* copy destination address from 802.11 header */
-		memcpy(txpd->tx_dest_addr_high, p802x_hdr + 4, ETH_ALEN);
+		memcpy(&txpd->tx_dest_addr, p802x_hdr + 4, ETH_ALEN);
 	} else {
 		/* copy destination address from 802.3 header */
-		memcpy(txpd->tx_dest_addr_high, p802x_hdr, ETH_ALEN);
+		memcpy(&txpd->tx_dest_addr, p802x_hdr, ETH_ALEN);
 	}
 
 	txpd->tx_packet_length = cpu_to_le16(pkt_len);