Re: [PATCH bpf-next v4 07/10] bpf: Support calling non-tailcall bpf prog

[Yonghong Song <yonghong.song@linux.dev>]

On 10/10/24 1:28 PM, Alexei Starovoitov wrote:
> On Thu, Oct 10, 2024 at 10:56 AM Yonghong Song <yonghong.song@linux.dev> wrote:
>> A kfunc bpf_prog_call() is introduced such that it can call another bpf
>> prog within a bpf prog. It has the same parameters as bpf_tail_call()
>> but acts like a normal function call.
>>
>> But bpf_prog_call() could recurse to the caller prog itself. So if a bpf
>> prog calls bpf_prog_call(), that bpf prog will use private stacks with
>> maximum recursion level 4. The 4 level recursion should work for most
>> cases.
>>
>> bpf_prog_call() cannot be used if tail_call exists in the same prog
>> since tail_call does not use private stack. If both prog_call and
>> tail_call in the same prog, verification will fail.
> ..
>
>> +__bpf_kfunc int bpf_prog_call(void *ctx, struct bpf_map *p__map, u32 index)
>> +{
>> +       struct bpf_array *array;
>> +       struct bpf_prog *prog;
>> +
>> +       if (p__map->map_type != BPF_MAP_TYPE_PROG_ARRAY)
>> +               return -EINVAL;
>> +
>> +       array = container_of(p__map, struct bpf_array, map);
>> +       if (unlikely(index >= array->map.max_entries))
>> +               return -E2BIG;
>> +
>> +       prog = READ_ONCE(array->ptrs[index]);
>> +       if (!prog)
>> +               return -ENOENT;
>> +
>> +       return bpf_prog_run(prog, ctx);
>> +}
> bpf_tail_call() was a hack during the early days,
> since I didn't know any better :(
> I really don't want to use that as a pattern.
> prog life time rules, tail call cnt, prog_array_compatible, etc.
> caused plenty of pain. Don't want to see a repeat.
>
> Progs that need to call another prog can use freplace mechanism already.
> There is no need for bpf_prog_call.

In this case, it could call itself.

>
> Let's get priv_stack in shape first (the first ~6 patches).

I am okay to focus on the first 6 patches. But I would like to get
Tejun's comments about what is the best way to support hierarchical
bpf based scheduler.

>
> pw-bot: cr