netdev - [PATCH net-next v2 03/13] net: mctp: separate cb from direct-addressing routing

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Message-Id: <20250619-dev-forwarding-v2-3-3f81801b06c2@codeconstruct.com.au>
Date: Thu, 19 Jun 2025 16:00:38 +0800
From: Jeremy Kerr <jk@...econstruct.com.au>
To: Matt Johnston <matt@...econstruct.com.au>, 
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
 Simon Horman <horms@...nel.org>
Cc: netdev@...r.kernel.org
Subject: [PATCH net-next v2 03/13] net: mctp: separate cb from
 direct-addressing routing

Now that we have the dst->haddr populated by sendmsg (when extended
addressing is in use), we no longer need to stash the link-layer address
in the skb->cb.

Instead, only use skb->cb for incoming lladdr data.

While we're at it: remove cb->src, as was never used.

Signed-off-by: Jeremy Kerr <jk@...econstruct.com.au>
---
 include/net/mctp.h |  4 ++--
 net/mctp/af_mctp.c |  3 +--
 net/mctp/route.c   | 21 +++++----------------
 3 files changed, 8 insertions(+), 20 deletions(-)

diff --git a/include/net/mctp.h b/include/net/mctp.h
index 6c9c5c48f59a1bf45f9c9d274a3ca2b633e96c75..b3af0690f60749a9bf9f489c7118c82cfd9d577e 100644
--- a/include/net/mctp.h
+++ b/include/net/mctp.h
@@ -183,8 +183,8 @@ struct mctp_sk_key {
 struct mctp_skb_cb {
 	unsigned int	magic;
 	unsigned int	net;
-	int		ifindex; /* extended/direct addressing if set */
-	mctp_eid_t	src;
+	/* fields below provide extended addressing for ingress to recvmsg() */
+	int		ifindex;
 	unsigned char	halen;
 	unsigned char	haddr[MAX_ADDR_LEN];
 };
diff --git a/net/mctp/af_mctp.c b/net/mctp/af_mctp.c
index ca66521435b10c2299b82ed64718ddc98f1f07f3..1141a4e33aaaabef4b58a5942dbe2847f2b7fcdd 100644
--- a/net/mctp/af_mctp.c
+++ b/net/mctp/af_mctp.c
@@ -134,8 +134,7 @@ static int mctp_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
 		DECLARE_SOCKADDR(struct sockaddr_mctp_ext *,
 				 extaddr, msg->msg_name);
 
-		if (!mctp_sockaddr_ext_is_ok(extaddr) ||
-		    extaddr->smctp_halen > sizeof(cb->haddr)) {
+		if (!mctp_sockaddr_ext_is_ok(extaddr)) {
 			rc = -EINVAL;
 			goto err_free;
 		}
diff --git a/net/mctp/route.c b/net/mctp/route.c
index e11bf1c1e383cc251c5b6e2852d3756f706956c7..42d80a9f21d6a054375156e3de21be72a187c6fc 100644
--- a/net/mctp/route.c
+++ b/net/mctp/route.c
@@ -561,35 +561,28 @@ static int mctp_dst_input(struct mctp_dst *dst, struct sk_buff *skb)
 
 static int mctp_dst_output(struct mctp_dst *dst, struct sk_buff *skb)
 {
-	struct mctp_skb_cb *cb = mctp_cb(skb);
 	struct mctp_hdr *hdr = mctp_hdr(skb);
 	char daddr_buf[MAX_ADDR_LEN];
 	char *daddr = NULL;
 	int rc;
 
 	skb->protocol = htons(ETH_P_MCTP);
+	skb->pkt_type = PACKET_OUTGOING;
 
 	if (skb->len > dst->mtu) {
 		kfree_skb(skb);
 		return -EMSGSIZE;
 	}
 
-	/* If we're forwarding, we don't want to use the input path's cb,
-	 * as it holds the *source* hardware addressing information.
-	 *
-	 * We will have a PACKET_HOST skb from the dev, or PACKET_OUTGOING
-	 * from a socket; only use cb in the latter case.
-	 */
-	if (skb->pkt_type == PACKET_OUTGOING && cb->ifindex) {
-		/* direct route; use the hwaddr we stashed in sendmsg */
-		if (cb->halen != skb->dev->addr_len) {
+	/* direct route; use the hwaddr we stashed in sendmsg */
+	if (dst->halen) {
+		if (dst->halen != skb->dev->addr_len) {
 			/* sanity check, sendmsg should have already caught this */
 			kfree_skb(skb);
 			return -EMSGSIZE;
 		}
-		daddr = cb->haddr;
+		daddr = dst->haddr;
 	} else {
-		skb->pkt_type = PACKET_OUTGOING;
 		/* If lookup fails let the device handle daddr==NULL */
 		if (mctp_neigh_lookup(dst->dev, hdr->dest, daddr_buf) == 0)
 			daddr = daddr_buf;
@@ -1004,7 +997,6 @@ int mctp_local_output(struct sock *sk, struct mctp_dst *dst,
 		      struct sk_buff *skb, mctp_eid_t daddr, u8 req_tag)
 {
 	struct mctp_sock *msk = container_of(sk, struct mctp_sock, sk);
-	struct mctp_skb_cb *cb = mctp_cb(skb);
 	struct mctp_sk_key *key;
 	struct mctp_hdr *hdr;
 	unsigned long flags;
@@ -1059,9 +1051,6 @@ int mctp_local_output(struct sock *sk, struct mctp_dst *dst,
 	skb_reset_network_header(skb);
 	skb->dev = dst->dev->dev;
 
-	/* cb->net will have been set on initial ingress */
-	cb->src = saddr;
-
 	/* set up common header fields */
 	hdr = mctp_hdr(skb);
 	hdr->ver = 1;

-- 
2.39.5