net/sched: act_skbmod: Add SKBMOD_F_ECN option support

Currently, when doing rate limiting using the tc-police(8) action, the
easiest way is to simply drop the packets which exceed or conform the
configured bandwidth limit. Add a new option to tc-skbmod(8), so that
users may use the ECN [1] extension to explicitly inform the receiver
about the congestion instead of dropping packets "on the floor".

The 2 least significant bits of the Traffic Class field in IPv4 and IPv6
headers are used to represent different ECN states [2]:

0b00: "Non ECN-Capable Transport", Non-ECT
0b10: "ECN Capable Transport", ECT(0)
0b01: "ECN Capable Transport", ECT(1)
0b11: "Congestion Encountered", CE

As an example:

$ tc filter add dev eth0 parent 1: protocol ip prio 10 \
matchall action skbmod ecn

Doing the above marks all ECT(0) and ECT(1) packets as CE. It does NOT
affect Non-ECT or non-IP packets. In the tc-police scenario mentioned
above, users may pipe a tc-police action and a tc-skbmod "ecn" action
together to achieve ECN-based rate limiting.

For TCP connections, upon receiving a CE packet, the receiver will respond
with an ECE packet, asking the sender to reduce their congestion window.
However ECN also works with other L4 protocols e.g. DCCP and SCTP [2], and
our implementation does not touch or care about L4 headers.

The updated tc-skbmod SYNOPSIS looks like the following:

tc ... action skbmod { set SETTABLE | swap SWAPPABLE | ecn } ...

Only one of "set", "swap" or "ecn" shall be used in a single tc-skbmod
command. Trying to use more than one of them at a time is considered
undefined behavior; pipe multiple tc-skbmod commands together instead.
"set" and "swap" only affect Ethernet packets, while "ecn" only affects
IPv{4,6} packets.

It is also worth mentioning that, in theory, the same effect could be
achieved by piping a "police" action and a "bpf" action using the
bpf_skb_ecn_set_ce() helper, but this requires eBPF programming from the
user, thus impractical.

Depends on patch "net/sched: act_skbmod: Skip non-Ethernet packets".

[1] https://datatracker.ietf.org/doc/html/rfc3168
[2] https://en.wikipedia.org/wiki/Explicit_Congestion_Notification

Reviewed-by: Cong Wang <[e-mail]>
Signed-off-by: Peilin Ye <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/uapi/linux/tc_act/tc_skbmod.h

···
 #define SKBMOD_F_SMAC	0x2
 #define SKBMOD_F_ETYPE	0x4
 #define SKBMOD_F_SWAPMAC 0x8
+#define SKBMOD_F_ECN	0x10
 
 struct tc_skbmod {
 	tc_gen;

net/sched/act_skbmod.c

···
 #include <linux/kernel.h>
 #include <linux/skbuff.h>
 #include <linux/rtnetlink.h>
+#include <net/inet_ecn.h>
 #include <net/netlink.h>
 #include <net/pkt_sched.h>
 #include <net/pkt_cls.h>
···
 static unsigned int skbmod_net_id;
 static struct tc_action_ops act_skbmod_ops;
 
-#define MAX_EDIT_LEN ETH_HLEN
 static int tcf_skbmod_act(struct sk_buff *skb, const struct tc_action *a,
 			  struct tcf_result *res)
 {
 	struct tcf_skbmod *d = to_skbmod(a);
-	int action;
+	int action, max_edit_len, err;
 	struct tcf_skbmod_params *p;
 	u64 flags;
-	int err;
 
 	tcf_lastuse_update(&d->tcf_tm);
 	bstats_cpu_update(this_cpu_ptr(d->common.cpu_bstats), skb);
···
 	if (unlikely(action == TC_ACT_SHOT))
 		goto drop;
 
-	if (!skb->dev || skb->dev->type != ARPHRD_ETHER)
-		return action;
+	max_edit_len = skb_mac_header_len(skb);
+	p = rcu_dereference_bh(d->skbmod_p);
+	flags = p->flags;
 
-	/* XXX: if you are going to edit more fields beyond ethernet header
-	 * (example when you add IP header replacement or vlan swap)
-	 * then MAX_EDIT_LEN needs to change appropriately
-	*/
-	err = skb_ensure_writable(skb, MAX_EDIT_LEN);
+	/* tcf_skbmod_init() guarantees "flags" to be one of the following:
+	 *	1. a combination of SKBMOD_F_{DMAC,SMAC,ETYPE}
+	 *	2. SKBMOD_F_SWAPMAC
+	 *	3. SKBMOD_F_ECN
+	 * SKBMOD_F_ECN only works with IP packets; all other flags only work with Ethernet
+	 * packets.
+	 */
+	if (flags == SKBMOD_F_ECN) {
+		switch (skb_protocol(skb, true)) {
+		case cpu_to_be16(ETH_P_IP):
+		case cpu_to_be16(ETH_P_IPV6):
+			max_edit_len += skb_network_header_len(skb);
+			break;
+		default:
+			goto out;
+		}
+	} else if (!skb->dev || skb->dev->type != ARPHRD_ETHER) {
+		goto out;
+	}
+
+	err = skb_ensure_writable(skb, max_edit_len);
 	if (unlikely(err)) /* best policy is to drop on the floor */
 		goto drop;
 
-	p = rcu_dereference_bh(d->skbmod_p);
-	flags = p->flags;
 	if (flags & SKBMOD_F_DMAC)
 		ether_addr_copy(eth_hdr(skb)->h_dest, p->eth_dst);
 	if (flags & SKBMOD_F_SMAC)
···
 		ether_addr_copy(eth_hdr(skb)->h_source, (u8 *)tmpaddr);
 	}
 
+	if (flags & SKBMOD_F_ECN)
+		INET_ECN_set_ce(skb);
+
+out:
 	return action;
 
 drop:
···
 	index = parm->index;
 	if (parm->flags & SKBMOD_F_SWAPMAC)
 		lflags = SKBMOD_F_SWAPMAC;
+	if (parm->flags & SKBMOD_F_ECN)
+		lflags = SKBMOD_F_ECN;
 
 	err = tcf_idr_check_alloc(tn, &index, a, bind);
 	if (err < 0)