netdev - Re: [PATCH bpf-next 02/10] libbpf: implement BPF CO-RE offset relocation algorithm

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Message-ID: <363f7363-7031-3160-9f5f-583a1662fe25@fb.com>
Date:   Sat, 27 Jul 2019 21:29:22 +0000
From:   Yonghong Song <yhs@...com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Alexei Starovoitov <ast@...com>
CC:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf-next 02/10] libbpf: implement BPF CO-RE offset
 relocation algorithm



On 7/27/19 11:24 AM, Andrii Nakryiko wrote:
> On Sat, Jul 27, 2019 at 10:00 AM Alexei Starovoitov <ast@...com> wrote:
>>
>> On 7/26/19 11:25 PM, Andrii Nakryiko wrote:
>>>>> +     } else if (class == BPF_ST && BPF_MODE(insn->code) == BPF_MEM) {
>>>>> +             if (insn->imm != orig_off)
>>>>> +                     return -EINVAL;
>>>>> +             insn->imm = new_off;
>>>>> +             pr_debug("prog '%s': patched insn #%d (ST | MEM) imm %d -> %d\n",
>>>>> +                      bpf_program__title(prog, false),
>>>>> +                      insn_idx, orig_off, new_off);
>>>> I'm pretty sure llvm was not capable of emitting BPF_ST insn.
>>>> When did that change?
>>> I just looked at possible instructions that could have 32-bit
>>> immediate value. This is `*(rX) = offsetof(struct s, field)`, which I
>>> though is conceivable. Do you think I should drop it?
>>
>> Just trying to point out that since it's not emitted by llvm
>> this code is likely untested ?
>> Or you've created a bpf asm test for this?
> 
> 
> Yeah, it's untested right now. Let me try to come up with LLVM
> assembly + relocation (not yet sure how/whether builtin works with
> inline assembly), if that works out, I'll leave this, if not, I'll
> drop BPF_ST|BPF_MEM part.

FYI. The llvm does not have assembly code format for BPF_ST instructions 
as it does not generate code for it. So inline asm through llvm won't 
work. llvm disasseembler won't be able to decode BPF_ST either.

>>
>>