Re: [PATCH bpf v1 2/2] selftests/bpf: Add protocol check test for bpf_sk_assign_tcp_reqsk()

Martin KaFai Lau
2026-03-26T06:33:29Z

On 3/25/26 8:24 PM, Jiayuan Chen wrote:
>
> On 3/25/26 5:59 AM, Martin KaFai Lau wrote:
>> On 3/23/26 3:54 AM, Jiayuan Chen wrote:
>>> +void test_tcp_custom_syncookie_protocol_check(void)
>>> +{
>>> +    struct test_tcp_custom_syncookie *skel;
>>> +    struct sockaddr_in tcp_addr, udp_addr;
>>> +    socklen_t addr_len = sizeof(tcp_addr);
>>> +    int tcp_server = -1, udp_client = -1;
>>> +    char buf[32] = "test";
>>> +    int ret;
>>> +
>>> +    if (setup_netns())
>>> +        return;
>>> +
>>> +    skel = test_tcp_custom_syncookie__open_and_load();
>>> +    if (!ASSERT_OK_PTR(skel, "open_and_load"))
>>> +        return;
>>>   +    /* Create a TCP listener so the BPF can find a LISTEN socket */
>>> +    tcp_server = start_server(AF_INET, SOCK_STREAM, "127.0.0.1", 0, 0);
>>> +    if (!ASSERT_NEQ(tcp_server, -1, "start tcp_server"))
>>> +        goto destroy_skel;
>>> +
>>> +    ret = getsockname(tcp_server, (struct sockaddr *)&tcp_addr,
>>> &addr_len);
>>> +    if (!ASSERT_OK(ret, "getsockname"))
>>> +        goto close_tcp;
>>> +
>>> +    skel->bss->tcp_listener_port = ntohs(tcp_addr.sin_port);
>>> +    skel->bss->udp_test_port = 9999;
>>> +
>>> +    ret = bpf_program__fd(skel->progs.tcp_custom_syncookie_badproto);
>>> +    if (setup_tc(ret))
>>> +        goto close_tcp;
>>> +
>>> +    udp_client = socket(AF_INET, SOCK_DGRAM, 0);
>>> +    if (!ASSERT_NEQ(udp_client, -1, "udp socket"))
>>> +        goto cleanup_tc;
>>> +
>>> +    memset(&udp_addr, 0, sizeof(udp_addr));
>>> +    udp_addr.sin_family = AF_INET;
>>> +    udp_addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
>>> +    udp_addr.sin_port = htons(9999);
>>> +
>>> +    ret = sendto(udp_client, buf, sizeof(buf), 0,
>>> +             (struct sockaddr *)&udp_addr, sizeof(udp_addr));
>>> +    ASSERT_EQ(ret, sizeof(buf), "sendto udp");
>>> +
>>> +    /* Wait for TC ingress BPF to process the skb. */
>>> +    kern_sync_rcu();
>>
>> hmm... is it guaranteed to work? Regardless, it checks the error
>> returned from bpf_sk_assign_tcp_reqsk(). Maybe bpf_prog_test_run is
>> simpler?
>>
>> pw-bot: cr
>>
>>> +
>>>
> I looked into using bpf_prog_test_run, but it won't work here
> because bpf_sk_assign_tcp_reqsk() requires the skb to come from TC ingress
> — it checks skb_at_tc_ingress() internally and returns -EINVAL otherwise.

Thanks for trying.

>
> For the synchronization concern, instead of kern_sync_rcu(), I now create
> a UDP server bound to the target port and recv() the packet after sendto().
> Since the BPF program returns TC_ACT_OK, the packet passes through TC
> ingress
> and arrives at the UDP socket. The recv() naturally blocks until the BPF
> program
> has finished processing, so no timing tricks are needed.
>

sgtm.