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

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Message-ID: <AA9B5489-425E-4FAE-BE01-F0F65679DF00@fb.com>
Date:   Wed, 31 Jul 2019 05:19:09 +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 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>
>>> ---
>>> tools/lib/bpf/libbpf.c | 915 ++++++++++++++++++++++++++++++++++++++++-
>>> tools/lib/bpf/libbpf.h |   1 +
>>> 2 files changed, 909 insertions(+), 7 deletions(-)
>>> 
>>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> 
> [...]
> 
> Please trim irrelevant parts. It doesn't matter with desktop Gmail,
> but pretty much everywhere else is very hard to work with.

This won't be a problem if the patch is shorter. ;) 

> 
>>> +
>>> +     for (i = 1; i < spec->raw_len; i++) {
>>> +             t = skip_mods_and_typedefs(btf, id, &id);
>>> +             if (!t)
>>> +                     return -EINVAL;
>>> +
>>> +             access_idx = spec->raw_spec[i];
>>> +
>>> +             if (btf_is_composite(t)) {
>>> +                     const struct btf_member *m = (void *)(t + 1);
>> 
>> Why (void *) instead of (const struct btf_member *)? There are a few more
>> in the rest of the patch.
>> 
> 
> 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. 
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. 

Thanks,
Song