netdev - Re: [PATCH v2 bpf-next 02/12] libbpf: implement BPF CO-RE offset relocation algorithm

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Date:   Wed, 31 Jul 2019 08:29:56 +0000
From:   Song Liu <songliubraving@...com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
CC:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        Yonghong Song <yhs@...com>, Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH v2 bpf-next 02/12] libbpf: implement BPF CO-RE offset
 relocation algorithm



> On Jul 30, 2019, at 11:52 PM, Andrii Nakryiko <andrii.nakryiko@...il.com> wrote:
> 
> On Tue, Jul 30, 2019 at 10:19 PM Song Liu <songliubraving@...com> wrote:
>> 
>> 
>> 
>>> On Jul 30, 2019, at 6:00 PM, Andrii Nakryiko <andrii.nakryiko@...il.com> wrote:
>>> 
>>> On Tue, Jul 30, 2019 at 5:39 PM Song Liu <songliubraving@...com> wrote:
>>>> 
>>>> 
>>>> 
>>>>> On Jul 30, 2019, at 12:53 PM, Andrii Nakryiko <andriin@...com> wrote:
>>>>> 
>>>>> This patch implements the core logic for BPF CO-RE offsets relocations.
>>>>> Every instruction that needs to be relocated has corresponding
>>>>> bpf_offset_reloc as part of BTF.ext. Relocations are performed by trying
>>>>> to match recorded "local" relocation spec against potentially many
>>>>> compatible "target" types, creating corresponding spec. Details of the
>>>>> algorithm are noted in corresponding comments in the code.
>>>>> 
>>>>> Signed-off-by: Andrii Nakryiko <andriin@...com>

[...]

>>>> 
>>> 
>>> I just picked the most succinct and non-repetitive form. It's
>>> immediately apparent which type it's implicitly converted to, so I
>>> felt there is no need to repeat it. Also, just (void *) is much
>>> shorter. :)
>> 
>> _All_ other code in btf.c converts the pointer to the target type.
> 
> Most in libbpf.c doesn't, though. Also, I try to preserve pointer
> constness for uses that don't modify BTF types (pretty much all of
> them in libbpf), so it becomes really verbose, despite extremely short
> variable names:
> 
> const struct btf_member *m = (const struct btf_member *)(t + 1);

I don't think being verbose is a big problem here. Overusing 
(void *) feels like a bigger problem. 

> 
> Add one or two levels of nestedness and you are wrapping this line.
> 
>> In some cases, it is not apparent which type it is converted to,
>> for example:
>> 
>> +       m = (void *)(targ_type + 1);
>> 
>> I would suggest we do implicit conversion whenever possible.
> 
> Implicit conversion (`m = targ_type + 1;`) is a compilation error,
> that won't work.

I misused "implicit" here. I actually meant to say

	m = ((const struct btf_member *)(t + 1);