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

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Message-ID: <CAEf4BzZrN-GKMHmLSkdh3JZT_h226RK_F3Tu3tPxrVqKYNSoUQ@mail.gmail.com>
Date:   Sat, 27 Jul 2019 14:36:21 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     Alexei Starovoitov <ast@...com>,
        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 Sat, Jul 27, 2019 at 2:29 PM Yonghong Song <yhs@...com> wrote:
>
>
>
> 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.

Well then, I'll just drop it for now. Thanks!

>
> >>
> >>