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

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Date:   Sat, 27 Jul 2019 11:24:40 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     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>,
        Yonghong Song <yhs@...com>, 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 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.

>
>