Message-ID: <f08fa9aa-8b0d-8217-1823-2830b2b2587c@fb.com>
Date:   Thu, 25 Nov 2021 21:43:54 -0800
From:   Yonghong Song <yhs@...com>
To:     Maxim Mikityanskiy <maximmi@...dia.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Lorenz Bauer <lmb@...udflare.com>
CC:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Brendan Jackman <jackmanb@...gle.com>,
        Florent Revest <revest@...omium.org>,
        Joe Stringer <joe@...ium.io>, Tariq Toukan <tariqt@...dia.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH bpf-next 09/10] bpf: Add a helper to issue timestamp
 cookies in XDP



On 11/25/21 6:34 AM, Maxim Mikityanskiy wrote:
> On 2021-11-09 09:11, Yonghong Song wrote:
>>
>>
>> On 11/3/21 7:02 AM, Maxim Mikityanskiy wrote:
>>> On 2021-11-03 04:10, Yonghong Song wrote:
>>>>
>>>>
>>>> On 11/1/21 4:14 AM, Maxim Mikityanskiy wrote:
>>>>> On 2021-10-20 19:16, Toke Høiland-Jørgensen wrote:
>>>>>> Lorenz Bauer <lmb@...udflare.com> writes:
>>>>>>
>>>>>>>> +bool cookie_init_timestamp_raw(struct tcphdr *th, __be32 
>>>>>>>> *tsval, __be32 *tsecr)
>>>>>>>
>>>>>>> I'm probably missing context, Is there something in this function 
>>>>>>> that
>>>>>>> means you can't implement it in BPF?
>>>>>>
>>>>>> I was about to reply with some other comments but upon closer 
>>>>>> inspection
>>>>>> I ended up at the same conclusion: this helper doesn't seem to be 
>>>>>> needed
>>>>>> at all?
>>>>>
>>>>> After trying to put this code into BPF (replacing the underlying 
>>>>> ktime_get_ns with ktime_get_mono_fast_ns), I experienced issues 
>>>>> with passing the verifier.
>>>>>
>>>>> In addition to comparing ptr to end, I had to add checks that 
>>>>> compare ptr to data_end, because the verifier can't deduce that end 
>>>>> <= data_end. More branches will add a certain slowdown (not measured).
>>>>>
>>>>> A more serious issue is the overall program complexity. Even though 
>>>>> the loop over the TCP options has an upper bound, and the pointer 
>>>>> advances by at least one byte every iteration, I had to limit the 
>>>>> total number of iterations artificially. The maximum number of 
>>>>> iterations that makes the verifier happy is 10. With more 
>>>>> iterations, I have the following error:
>>>>>
>>>>> BPF program is too large. Processed 1000001 insn
>>>>>
>>>>>                         processed 1000001 insns (limit 1000000) 
>>>>> max_states_per_insn 29 total_states 35489 peak_states 596 mark_read 45
>>>>>
>>>>> I assume that BPF_COMPLEXITY_LIMIT_INSNS (1 million) is the 
>>>>> accumulated amount of instructions that the verifier can process in 
>>>>> all branches, is that right? It doesn't look realistic that my 
>>>>> program can run 1 million instructions in a single run, but it 
>>>>> might be that if you take all possible flows and add up the 
>>>>> instructions from these flows, it will exceed 1 million.
>>>>>
>>>>> The limitation of maximum 10 TCP options might be not enough, given 
>>>>> that valid packets are permitted to include more than 10 NOPs. An 
>>>>> alternative of using bpf_load_hdr_opt and calling it three times 
>>>>> doesn't look good either, because it will be about three times 
>>>>> slower than going over the options once. So maybe having a helper 
>>>>> for that is better than trying to fit it into BPF?
>>>>>
>>>>> One more interesting fact is the time that it takes for the 
>>>>> verifier to check my program. If it's limited to 10 iterations, it 
>>>>> does it pretty fast, but if I try to increase the number to 11 
>>>>> iterations, it takes several minutes for the verifier to reach 1 
>>>>> million instructions and print the error then. I also tried 
>>>>> grouping the NOPs in an inner loop to count only 10 real options, 
>>>>> and the verifier has been running for a few hours without any 
>>>>> response. Is it normal? 
>>>>
>>>> Maxim, this may expose a verifier bug. Do you have a reproducer I 
>>>> can access? I would like to debug this to see what is the root case. 
>>>> Thanks!
>>>
>>> Thanks, I appreciate your help in debugging it. The reproducer is 
>>> based on the modified XDP program from patch 10 in this series. 
>>> You'll need to apply at least patches 6, 7, 8 from this series to get 
>>> new BPF helpers needed for the XDP program (tell me if that's a 
>>> problem, I can try to remove usage of new helpers, but it will affect 
>>> the program length and may produce different results in the verifier).
>>>
>>> See the C code of the program that passes the verifier (compiled with 
>>> clang version 12.0.0-1ubuntu1) in the bottom of this email. If you 
>>> increase the loop boundary from 10 to at least 11 in 
>>> cookie_init_timestamp_raw(), it fails the verifier after a few minutes. 
>>
>> I tried to reproduce with latest llvm (llvm-project repo),
>> loop boundary 10 is okay and 11 exceeds the 1M complexity limit. For 10,
>> the number of verified instructions is 563626 (more than 0.5M) so it is
>> totally possible that one more iteration just blows past the limit.
> 
> So, does it mean that the verifying complexity grows exponentially with 
> increasing the number of loop iterations (options parsed)?

Depending on verification time pruning results, it is possible slightly 
increase number of branches could result quite some (2x, 4x, etc.) of
to-be-verified dynamic instructions.

> 
> Is it a good enough reason to keep this code as a BPF helper, rather 
> than trying to fit it into the BPF program?

Another option is to use global function, which is verified separately
from the main bpf program.

> 
>>
>>> If you apply this tiny change, it fails the verifier after about 3 
>>> hours:
>>>
[...]

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