netdev - Re: [PATCH bpf-next v2 5/7] bpf, libbpf: support global data/bss/rodata sections

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Date:   Fri, 1 Mar 2019 21:06:35 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Alexei Starovoitov <ast@...com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "joe@...d.net.nz" <joe@...d.net.nz>,
        "john.fastabend@...il.com" <john.fastabend@...il.com>,
        "tgraf@...g.ch" <tgraf@...g.ch>, Andrii Nakryiko <andriin@...com>,
        "jakub.kicinski@...ronome.com" <jakub.kicinski@...ronome.com>,
        "lmb@...udflare.com" <lmb@...udflare.com>
Subject: Re: [PATCH bpf-next v2 5/7] bpf, libbpf: support global
 data/bss/rodata sections

On 03/01/2019 08:19 PM, Yonghong Song wrote:
> On 3/1/19 11:10 AM, Andrii Nakryiko wrote:
>> On Fri, Mar 1, 2019 at 10:58 AM Yonghong Song <yhs@...com> wrote:
>>> On 3/1/19 10:48 AM, Andrii Nakryiko wrote:
>>>> On Fri, Mar 1, 2019 at 10:31 AM Yonghong Song <yhs@...com> wrote:
>>>>> On 2/28/19 3:18 PM, Daniel Borkmann wrote:
[...]
>>>> Would it be reasonable to just plain disable usage of uninitialized
>>>> global variables, as it kind of goes against BPF's philosophy that
>>>> everything should be written to, before can be read? So while we can
>>>> just implicitly zero-out everything beforehand, it might be a good
>>>> idea to remind and enforce that explictly?
>>>
>>> There will be a verifier error, so the program with "int g" will not
>>> run, the same as today.
>>
>> Yeah, I understand, but with pretty obscure error about not supporting
>> relocations and stuff, right?
>>
>>>
>>> We could improve by flagging the error at compiler error or libbpf time.
>>
>> So that's my point, that having compiler emit nicer error for
>> target=bpf would be nice touch to user experience :)
> 
> I just removed a compiler error for static variables...
> 
> I will wait for this patch lands, hear people complains (either need to
> support "int g;" or need better error messages, etc.) and then decide
> what next to do ...

By the way, from LLVM side, do you think it makes sense for local vars
where you encode the offset into insn->imm to already encode it into
(insn+1)->imm of the ldimm64, so that loaders can just pass this offset
through instead of fixing it up like I did? I'm fine either way though,
just thought might be worth pointing out while we're at it. :)

Thanks,
Daniel