cxgb4: 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 fw_eth_tx_pkt_vm_wr around members ethmacdst,
ethmacsrc, ethtype, and vlantci, 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 ethmacdst.

"pahole" shows no size nor member offset changes to struct
fw_eth_tx_pkt_vm_wr. "objdump -d" shows no object code changes.
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/chelsio/cxgb4/sge.c

@@ -1842,8 +1842,10 @@ static netdev_tx_t cxgb4_vf_eth_xmit(struct sk_buff *skb,
 	 * (including the VLAN tag) into the header so we reject anything
 	 * smaller than that ...
 	 */
-	fw_hdr_copy_len = sizeof(wr->ethmacdst) + sizeof(wr->ethmacsrc) +
-			  sizeof(wr->ethtype) + sizeof(wr->vlantci);
+	BUILD_BUG_ON(sizeof(wr->firmware) !=
+		     (sizeof(wr->ethmacdst) + sizeof(wr->ethmacsrc) +
+		      sizeof(wr->ethtype) + sizeof(wr->vlantci)));
+	fw_hdr_copy_len = sizeof(wr->firmware);
 	ret = cxgb4_validate_skb(skb, dev, fw_hdr_copy_len);
 	if (ret)
 		goto out_free;
@@ -1924,7 +1926,7 @@ static netdev_tx_t cxgb4_vf_eth_xmit(struct sk_buff *skb,
 	wr->equiq_to_len16 = cpu_to_be32(wr_mid);
 	wr->r3[0] = cpu_to_be32(0);
 	wr->r3[1] = cpu_to_be32(0);
-	skb_copy_from_linear_data(skb, (void *)wr->ethmacdst, fw_hdr_copy_len);
+	skb_copy_from_linear_data(skb, &wr->firmware, fw_hdr_copy_len);
 	end = (u64 *)wr + flits;
 
 	/* If this is a Large Send Offload packet we'll put in an LSO CPL

drivers/net/ethernet/chelsio/cxgb4/t4fw_api.h

@@ -794,10 +794,12 @@ struct fw_eth_tx_pkt_vm_wr {
 	__be32 op_immdlen;
 	__be32 equiq_to_len16;
 	__be32 r3[2];
-	u8 ethmacdst[6];
-	u8 ethmacsrc[6];
+	struct_group(firmware,
+		u8 ethmacdst[ETH_ALEN];
+		u8 ethmacsrc[ETH_ALEN];
 		__be16 ethtype;
 		__be16 vlantci;
+	);
 };
 
 #define FW_CMD_MAX_TIMEOUT 10000

drivers/net/ethernet/chelsio/cxgb4vf/sge.c

@@ -1167,10 +1167,7 @@ netdev_tx_t t4vf_eth_xmit(struct sk_buff *skb, struct net_device *dev)
 	struct cpl_tx_pkt_core *cpl;
 	const struct skb_shared_info *ssi;
 	dma_addr_t addr[MAX_SKB_FRAGS + 1];
-	const size_t fw_hdr_copy_len = (sizeof(wr->ethmacdst) +
-					sizeof(wr->ethmacsrc) +
-					sizeof(wr->ethtype) +
-					sizeof(wr->vlantci));
+	const size_t fw_hdr_copy_len = sizeof(wr->firmware);
 
 	/*
 	 * The chip minimum packet length is 10 octets but the firmware
@@ -1267,7 +1264,7 @@ netdev_tx_t t4vf_eth_xmit(struct sk_buff *skb, struct net_device *dev)
 	wr->equiq_to_len16 = cpu_to_be32(wr_mid);
 	wr->r3[0] = cpu_to_be32(0);
 	wr->r3[1] = cpu_to_be32(0);
-	skb_copy_from_linear_data(skb, (void *)wr->ethmacdst, fw_hdr_copy_len);
+	skb_copy_from_linear_data(skb, &wr->firmware, fw_hdr_copy_len);
 	end = (u64 *)wr + flits;
 
 	/*