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

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Message-ID: <0120903c-bef7-51b8-44b8-5f4288a78107@fb.com>
Date:   Fri, 1 Mar 2019 20:25:41 +0000
From:   Yonghong Song <yhs@...com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        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 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.

> 
> Thanks,
> Daniel
>