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

[Jiayuan Chen <jiayuan.chen@linux.dev>]

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.

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.