Re: [PATCH net-next] net/smc: Introduce a hook to modify syn_smc at runtime

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On 10/11/24 12:21 AM, Alexei Starovoitov wrote:
On Wed, Oct 9, 2024 at 8:58 PM D. Wythe <alibuda@xxxxxxxxxxxxxxxxx> wrote:


+__bpf_hook_start();
+
+__weak noinline int select_syn_smc(const struct sock *sk, struct sockaddr *peer)
+{
+       return 1;
+}
+
+__bpf_hook_end();
+
  int smc_nl_dump_hs_limitation(struct sk_buff *skb, struct netlink_callback *cb)
  {
         struct smc_nl_dmp_ctx *cb_ctx = smc_nl_dmp_ctx(cb);
@@ -156,19 +165,43 @@ static struct sock *smc_tcp_syn_recv_sock(const struct sock *sk,
         return NULL;
  }

-static bool smc_hs_congested(const struct sock *sk)
+static void smc_openreq_init(struct request_sock *req,
+                            const struct tcp_options_received *rx_opt,
+                            struct sk_buff *skb, const struct sock *sk)
  {
+       struct inet_request_sock *ireq = inet_rsk(req);
+       struct sockaddr_storage rmt_sockaddr = {};
         const struct smc_sock *smc;

         smc = smc_clcsock_user_data(sk);

         if (!smc)
-               return true;
+               return;

-       if (workqueue_congested(WORK_CPU_UNBOUND, smc_hs_wq))
-               return true;
+       if (smc->limit_smc_hs && workqueue_congested(WORK_CPU_UNBOUND, smc_hs_wq))
+               goto out_no_smc;

-       return false;
+       rmt_sockaddr.ss_family = sk->sk_family;
+
+       if (rmt_sockaddr.ss_family == AF_INET) {
+               struct sockaddr_in *rmt4_sockaddr =  (struct sockaddr_in *)&rmt_sockaddr;
+
+               rmt4_sockaddr->sin_addr.s_addr = ireq->ir_rmt_addr;
+               rmt4_sockaddr->sin_port = ireq->ir_rmt_port;
+#if IS_ENABLED(CONFIG_IPV6)
+       } else {
+               struct sockaddr_in6 *rmt6_sockaddr =  (struct sockaddr_in6 *)&rmt_sockaddr;
+
+               rmt6_sockaddr->sin6_addr = ireq->ir_v6_rmt_addr;
+               rmt6_sockaddr->sin6_port = ireq->ir_rmt_port;
+#endif /* CONFIG_IPV6 */
+       }
+
+       ireq->smc_ok = select_syn_smc(sk, (struct sockaddr *)&rmt_sockaddr);
+       return;
+out_no_smc:
+       ireq->smc_ok = 0;
+       return;
  }

  struct smc_hashinfo smc_v4_hashinfo = {
@@ -1671,7 +1704,7 @@ int smc_connect(struct socket *sock, struct sockaddr *addr,
         }

         smc_copy_sock_settings_to_clc(smc);
-       tcp_sk(smc->clcsock->sk)->syn_smc = 1;
+       tcp_sk(smc->clcsock->sk)->syn_smc = select_syn_smc(sk, addr);
         if (smc->connect_nonblock) {
                 rc = -EALREADY;
                 goto out;
@@ -2650,8 +2683,7 @@ int smc_listen(struct socket *sock, int backlog)

         inet_csk(smc->clcsock->sk)->icsk_af_ops = &smc->af_ops;

-       if (smc->limit_smc_hs)
-               tcp_sk(smc->clcsock->sk)->smc_hs_congested = smc_hs_congested;
+       tcp_sk(smc->clcsock->sk)->smc_openreq_init = smc_openreq_init;

         rc = kernel_listen(smc->clcsock, backlog);
         if (rc) {
@@ -3475,6 +3507,24 @@ static void __net_exit smc_net_stat_exit(struct net *net)
         .exit = smc_net_stat_exit,
  };

+#if IS_ENABLED(CONFIG_BPF_SYSCALL)
+BTF_SET8_START(bpf_smc_fmodret_ids)
+BTF_ID_FLAGS(func, select_syn_smc)
+BTF_SET8_END(bpf_smc_fmodret_ids)
+
+static const struct btf_kfunc_id_set bpf_smc_fmodret_set = {
+       .owner = THIS_MODULE,
+       .set   = &bpf_smc_fmodret_ids,
+};
+
+static int bpf_smc_kfunc_init(void)
+{
+       return register_btf_fmodret_id_set(&bpf_smc_fmodret_set);
+}

fmodret was an approach that hid-bpf took initially,
but eventually they removed it all and switched to struct-ops approach.
Please learn that lesson.
Use struct_ops from the beginning.

I did a presentation recently explaining the motivation behind
struct_ops and tips on how to extend the kernel.
TLDR: the step one is to design the extension _without_ bpf.
The interface should be usable for kernel modules.
And then when you have *_ops style api in place
the bpf progs will plug-in without extra work.

Slides:
https://github.com/4ast/docs/blob/main/BPF%20struct-ops.pdf


Hi Alexei,

Thanks very much for your suggestion.

In fact, I tried struct_ops in SMC about a year ago. Unfortunately, at that time struct_ops did not support registration from modules, and I had to move some smc dependencies into bpf, which met with community opposition. However, I noticed that this feature is now supported, so perhaps this is an opportunity.

But on the other hand, given the current functionality, I wonder if struct_ops might be an overkill. I haven't been able to come up with a suitable abstraction to define this ops, and in the future, this ops might only contain the very one callback (select_syn_smc).

Looking forward for your advises.

Thanks,
D. Wythe







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