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

Song Liu
2019-07-31T05:19:35Z

> On Jul 30, 2019, at 6:00 PM, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
>
> On Tue, Jul 30, 2019 at 5:39 PM Song Liu <songliubraving@fb.com> wrote:
>>
>>
>>
>>> On Jul 30, 2019, at 12:53 PM, Andrii Nakryiko <andriin@fb.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@fb.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