[PATCH RESEND net-next 1/2] net/sched: act_skbmod: Add SKBMOD_F_ECN option support

From: Peilin Ye
Date: Tue Jul 27 2021 - 21:33:25 EST

From: Peilin Ye <peilin.ye@xxxxxxxxxxxxx>

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 <cong.wang@xxxxxxxxxxxxx>
Signed-off-by: Peilin Ye <peilin.ye@xxxxxxxxxxxxx>
Hi all,

These 2 patches should apply to net-next now, since "net/sched:
act_skbmod: Skip non-Ethernet packets" has been pulled into net-next.
Please review; thank you!

Peilin Ye

include/uapi/linux/tc_act/tc_skbmod.h | 1 +
net/sched/act_skbmod.c | 44 +++++++++++++++++++--------
2 files changed, 33 insertions(+), 12 deletions(-)

diff --git a/include/uapi/linux/tc_act/tc_skbmod.h b/include/uapi/linux/tc_act/tc_skbmod.h
index c525b3503797..af6ef2cfbf3d 100644
--- a/include/uapi/linux/tc_act/tc_skbmod.h
+++ b/include/uapi/linux/tc_act/tc_skbmod.h
@@ -17,6 +17,7 @@
#define SKBMOD_F_SMAC 0x2
#define SKBMOD_F_ETYPE 0x4
#define SKBMOD_F_SWAPMAC 0x8
+#define SKBMOD_F_ECN 0x10

struct tc_skbmod {
diff --git a/net/sched/act_skbmod.c b/net/sched/act_skbmod.c
index 8d17a543cc9f..762ceec3e6f6 100644
--- a/net/sched/act_skbmod.c
+++ b/net/sched/act_skbmod.c
@@ -11,6 +11,7 @@
#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>
@@ -21,15 +22,13 @@
static unsigned int skbmod_net_id;
static struct tc_action_ops act_skbmod_ops;

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;

bstats_cpu_update(this_cpu_ptr(d->common.cpu_bstats), skb);
@@ -38,19 +37,34 @@ static int tcf_skbmod_act(struct sk_buff *skb, const struct tc_action *a,
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;
+ /* tcf_skbmod_init() guarantees "flags" to be one of the following:
+ * 1. a combination of SKBMOD_F_{DMAC,SMAC,ETYPE}
+ * 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;
+ }

- /* 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);
+ 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)
@@ -66,6 +80,10 @@ static int tcf_skbmod_act(struct sk_buff *skb, const struct tc_action *a,
ether_addr_copy(eth_hdr(skb)->h_source, (u8 *)tmpaddr);

+ if (flags & SKBMOD_F_ECN)
+ INET_ECN_set_ce(skb);
return action;

@@ -129,6 +147,8 @@ static int tcf_skbmod_init(struct net *net, struct nlattr *nla,
index = parm->index;
if (parm->flags & 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)