Subject: Re: [PATCH v7 bpf-next 6/6] selftest: bpf: Test bpf_sk_assign_tcp_reqsk().

From: Yonghong Song <[email-masked]>
Date: Tue, 2 Jan 2024 11:17:19 -0800

On 12/20/23 10:35 PM, Martin KaFai Lau wrote:
> On 12/20/23 5:28 PM, Kuniyuki Iwashima wrote:
>> +static int tcp_validate_header(struct tcp_syncookie *ctx)
>> +{
>> +    s64 csum;
>> +
>> +    if (tcp_reload_headers(ctx))
>> +        goto err;
>> +
>> +    csum = bpf_csum_diff(0, 0, (void *)ctx->tcp, ctx->tcp->doff * 4, 
>> 0);
>> +    if (csum < 0)
>> +        goto err;
>> +
>> +    if (ctx->ipv4) {
>> +        /* check tcp_v4_csum(csum) is 0 if not on lo. */
>> +
>> +        csum = bpf_csum_diff(0, 0, (void *)ctx->ipv4, ctx->ipv4->ihl 
>> * 4, 0);
>> +        if (csum < 0)
>> +            goto err;
>> +
>> +        if (csum_fold(csum) != 0)
>> +            goto err;
>> +    } else if (ctx->ipv6) {
>> +        /* check tcp_v6_csum(csum) is 0 if not on lo. */
>> +    }
>> +
>> +    return 0;
>> +err:
>> +    return -1;
>> +}
>> +
>> +static int tcp_parse_option(__u32 index, struct tcp_syncookie *ctx)
>> +{
>> +    char opcode, opsize;
>> +
>> +    if (ctx->ptr + 1 > ctx->data_end)
>> +        goto stop;
>> +
>> +    opcode = *ctx->ptr++;
>> +
>> +    if (opcode == TCPOPT_EOL)
>> +        goto stop;
>> +
>> +    if (opcode == TCPOPT_NOP)
>> +        goto next;
>> +
>> +    if (ctx->ptr + 1 > ctx->data_end)
>> +        goto stop;
>> +
>> +    opsize = *ctx->ptr++;
>> +
>> +    if (opsize < 2)
>> +        goto stop;
>> +
>> +    switch (opcode) {
>> +    case TCPOPT_MSS:
>> +        if (opsize == TCPOLEN_MSS && ctx->tcp->syn &&
>> +            ctx->ptr + (TCPOLEN_MSS - 2) < ctx->data_end)
>> +            ctx->attrs.mss = get_unaligned_be16(ctx->ptr);
>> +        break;
>> +    case TCPOPT_WINDOW:
>> +        if (opsize == TCPOLEN_WINDOW && ctx->tcp->syn &&
>> +            ctx->ptr + (TCPOLEN_WINDOW - 2) < ctx->data_end) {
>> +            ctx->attrs.wscale_ok = 1;
>> +            ctx->attrs.snd_wscale = *ctx->ptr;
>> +        }
>> +        break;
>> +    case TCPOPT_TIMESTAMP:
>> +        if (opsize == TCPOLEN_TIMESTAMP &&
>> +            ctx->ptr + (TCPOLEN_TIMESTAMP - 2) < ctx->data_end) {
>> +            ctx->attrs.rcv_tsval = get_unaligned_be32(ctx->ptr);
>> +            ctx->attrs.rcv_tsecr = get_unaligned_be32(ctx->ptr + 4);
>> +
>> +            if (ctx->tcp->syn && ctx->attrs.rcv_tsecr)
>> +                ctx->attrs.tstamp_ok = 0;
>> +            else
>> +                ctx->attrs.tstamp_ok = 1;
>> +        }
>> +        break;
>> +    case TCPOPT_SACK_PERM:
>> +        if (opsize == TCPOLEN_SACK_PERM && ctx->tcp->syn &&
>> +            ctx->ptr + (TCPOLEN_SACK_PERM - 2) < ctx->data_end)
>> +            ctx->attrs.sack_ok = 1;
>> +        break;
>> +    }
>> +
>> +    ctx->ptr += opsize - 2;
>> +next:
>> +    return 0;
>> +stop:
>> +    return 1;
>> +}
>> +
>> +static void tcp_parse_options(struct tcp_syncookie *ctx)
>> +{
>> +    ctx->ptr = (char *)(ctx->tcp + 1);
>> +
>> +    bpf_loop(40, tcp_parse_option, ctx, 0);
>> +}
>> +
>> +static int tcp_validate_sysctl(struct tcp_syncookie *ctx)
>> +{
>> +    if ((ctx->ipv4 && ctx->attrs.mss != MSS_LOCAL_IPV4) ||
>> +        (ctx->ipv6 && ctx->attrs.mss != MSS_LOCAL_IPV6))
>> +        goto err;
>> +
>> +    if (!ctx->attrs.wscale_ok || ctx->attrs.snd_wscale != 7)
>> +        goto err;
>> +
>> +    if (!ctx->attrs.tstamp_ok)
>
> The bpf-ci reported error in cpuv4. The email from 
> [email-masked] has the link.
>
> I tried the following:
>
>     if (!ctx->attrs.tstamp_ok) {
>         bpf_printk("ctx->attrs.tstamp_ok %u",
>             ctx->attrs.tstamp_ok);
>         goto err;
>     }
>
>
> The above prints tstamp_ok as 1 while there is a "if 
> (!ctx->attrs.tstamp_ok)" test before it.
>
> Yonghong and I debugged it quite a bit. verifier concluded the 
> ctx->attrs.tstamp_ok is 0. We knew some red herring like cpuv4 has 
> fewer register spilling but not able to root cause it yet.
>
> In the mean time, there are existing selftests parsing the tcp header. 
> For example, the test_parse_tcp_hdr_opt[_dynptr].c. Not as complete as 
> your tcp_parse_option() but should be pretty close. It does not use 
> bpf_loop. It uses a bounded loop + a subprog (the parse_hdr_opt in the 
> selftests) instead. You can consider a similar construct to see if it 
> works around the cpuv4 CI issue for the time being.

I did some investigation and found some issues in verifier. I need to dig more into details.
On the other hand, with the following patch
   https://lore.kernel.org/bpf/[email-masked]/
the selftest can run successfully with cpuv2/v3/v4.

>
> pw-bot: cr
>
>> +        goto err;
>> +
>> +    if (!ctx->attrs.sack_ok)
>> +        goto err;
>> +
>> +    if (!ctx->tcp->ece || !ctx->tcp->cwr)
>> +        goto err;
>> +
>> +    return 0;
>> +err:
>> +    return -1;
>> +}
>> +
>
> [ ... ]
>
>> +static int tcp_handle_syn(struct tcp_syncookie *ctx)
>> +{
>> +    s64 csum;
>> +
>> +    if (tcp_validate_header(ctx))
>> +        goto err;
>> +
>> +    tcp_parse_options(ctx);
>> +
>> +    if (tcp_validate_sysctl(ctx))
>> +        goto err;
>> +
>