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Date:   Sat, 23 Nov 2019 10:24:16 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        john fastabend <john.fastabend@...il.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 7/8] bpf, x86: emit patchable direct jump as
 tail call

On 11/23/19 7:18 AM, Alexei Starovoitov wrote:
> On Fri, Nov 22, 2019 at 09:00:35PM -0800, Andrii Nakryiko wrote:
>> On Fri, Nov 22, 2019 at 6:28 PM Alexei Starovoitov
>> <alexei.starovoitov@...il.com> wrote:
>>> On Fri, Nov 22, 2019 at 3:25 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>>>>>> +       case BPF_MOD_CALL_TO_NOP:
>>>>>> +       case BPF_MOD_JUMP_TO_NOP:
>>>>>> +               if (old_addr && !new_addr) {
>>>>>> +                       memcpy(new_insn, nop_insn, X86_PATCH_SIZE);
>>>>>> +
>>>>>> +                       prog = old_insn;
>>>>>> +                       ret = emit_patch_fn(&prog, old_addr, ip);
>>>>>> +                       if (ret)
>>>>>> +                               return ret;
>>>>>> +                       break;
>>>>>> +               }
>>>>>> +               return -ENXIO;
>>>>>> +       default:
>>>>>
>>>>> There is this redundancy between BPF_MOD_xxx enums and
>>>>> old_addr+new_addr (both encode what kind of transition it is), which
>>>>> leads to this cumbersome logic. Would it be simpler to have
>>>>> old_addr/new_addr determine whether it's X-to-NOP, NOP-to-Y, or X-to-Y
>>>>> transition, while separate bool or simple BPF_MOD_CALL/BPF_MOD_JUMP
>>>>> enum determining whether it's a call or a jump that we want to update.
>>>>> Seems like that should be a simpler interface overall and cleaner
>>>>> implementation?
>>>>
>>>> Right we can probably simplify it further, I kept preserving the original
>>>> switch from Alexei's code where my assumption was that having the transition
>>>> explicitly spelled out was preferred in here and then based on that doing
>>>> the sanity checks to make sure we don't get bad input from any call-site
>>>> since we're modifying kernel text, e.g. in the bpf_trampoline_update() as
>>>> one example the BPF_MOD_* is a fixed constant input there.
>>>
>>> I guess we can try adding one more argument
>>> bpf_arch_text_poke(ip, BPF_MOD_NOP, old_addr, BPF_MOD_INTO_CALL, new_addr);
>>
>> I was thinking along the lines of:
>>
>> bpf_arch_text_poke(ip, BPF_MOD_CALL (or BPF_MOD_JMP), old_addr, new_addr);
>>
>> old_addr/new_addr being possibly NULL determine NOP/not-a-NOP.
> 
> I see. Something like:
> if (BPF_MOD_CALL) {
>     if (old_addr)
>         memcmp(ip, old_call_insn);
>     else
>         memcmp(ip, nop_insn);
> } else if (BPF_MOD_JMP) {
>     if (old_addr)
>         memcmp(ip, old_jmp_insn);
>     else
>         memcmp(ip, nop_insn);
> }
> I guess that can work.

Ok, will see to come up with a clean simplification in the evening.

Thanks,
Daniel

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