netdev - Re: [PATCH v2 bpf-next 10/12] bpf: check types of arguments passed into helpers

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Date:   Sat, 12 Oct 2019 01:39:13 +0000
From:   Alexei Starovoitov <ast@...com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Alexei Starovoitov <ast@...nel.org>
CC:     "David S. Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        "x86@...nel.org" <x86@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH v2 bpf-next 10/12] bpf: check types of arguments passed
 into helpers

On 10/11/19 12:02 PM, Andrii Nakryiko wrote:
> On Wed, Oct 9, 2019 at 9:15 PM Alexei Starovoitov <ast@...nel.org> wrote:
>>
>>   /* type of values returned from helper functions */
>> @@ -235,11 +236,17 @@ struct bpf_func_proto {
>>          bool gpl_only;
>>          bool pkt_access;
>>          enum bpf_return_type ret_type;
>> -       enum bpf_arg_type arg1_type;
>> -       enum bpf_arg_type arg2_type;
>> -       enum bpf_arg_type arg3_type;
>> -       enum bpf_arg_type arg4_type;
>> -       enum bpf_arg_type arg5_type;
>> +       union {
>> +               struct {
>> +                       enum bpf_arg_type arg1_type;
>> +                       enum bpf_arg_type arg2_type;
>> +                       enum bpf_arg_type arg3_type;
>> +                       enum bpf_arg_type arg4_type;
>> +                       enum bpf_arg_type arg5_type;
>> +               };
>> +               enum bpf_arg_type arg_type[5];
>> +       };
>> +       u32 *btf_id; /* BTF ids of arguments */
> 
> are you trying to save memory with this? otherwise not sure why it's
> not just `u32 btf_id[5]`? Even in that case it will save at most 12
> bytes (and I haven't even check alignment padding and stuff). So
> doesn't seem worth it?

Glad you asked :)
It cannot be "u32 btf_id[5];".
Guess why?
I think it's a cool trick.
I was happy when I finally figured out to solve it this way
after analyzing a bunch of ugly solutions.

>> + *
>> + *             The value to write, of *size*, is passed through eBPF stack and
>> + *             pointed by *data*.
> 
> typo? pointed __to__ by *data*?

I'm not an grammar expert. That was a copy paste from existing comment.

>> + *
>> + *             *ctx* is a pointer to in-kernel sutrct sk_buff.
>> + *
>> + *             This helper is similar to **bpf_perf_event_output**\ () but
>> + *             restricted to raw_tracepoint bpf programs.
> 
> nit: with BTF type tracking enabled?

sure.

>> +       for (i = 0; i < 5; i++) {
>> +               if (fn->arg_type[i] == ARG_PTR_TO_BTF_ID) {
>> +                       if (!fn->btf_id[i])
>> +                               fn->btf_id[i] = btf_resolve_helper_id(&env->log, fn->func, 0);
> 
> bug: 0 -> i  :)

Nice catch.
Clearly I don't have a use case yet for 2nd arg being ptr_to_btf.