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

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Message-ID: <957fff81-d845-ebc9-0e80-dbb1f1736b40@fb.com>
Date:   Sat, 27 Jul 2019 17:00:25 +0000
From:   Alexei Starovoitov <ast@...com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     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 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?