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

["D. Wythe" <alibuda@linux.alibaba.com>]

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