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

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Message-ID: <60074bf4-8fe5-bbc2-aaf4-df694a8518dd@iogearbox.net>
Date:   Fri, 1 Mar 2019 21:33:45 +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 09:25 PM, Yonghong Song wrote:
> On 3/1/19 12:06 PM, Daniel Borkmann wrote:
>> 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. :)
> 
> Yes, llvm can do that. Let me prototype it and will let you know
> if it landed in llvm trunk.

Awesome, thanks!