Re: [PATCH v2 bpf-next 03/12] selftests/bpf: add BPF_CORE_READ relocatable read macro

Song Liu
2019-07-30T21:34:19Z

> On Jul 30, 2019, at 2:26 PM, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
>
> On Tue, Jul 30, 2019 at 2:24 PM Song Liu <songliubraving@fb.com> wrote:
>>
>>
>>
>>> On Jul 30, 2019, at 12:53 PM, Andrii Nakryiko <andriin@fb.com> wrote:
>>>
>>> Add BPF_CORE_READ macro used in tests to do bpf_core_read(), which
>>> automatically captures offset relocation.
>>>
>>> Signed-off-by: Andrii Nakryiko <andriin@fb.com>
>>> ---
>>> tools/testing/selftests/bpf/bpf_helpers.h | 19 +++++++++++++++++++
>>> 1 file changed, 19 insertions(+)
>>>
>>> diff --git a/tools/testing/selftests/bpf/bpf_helpers.h b/tools/testing/selftests/bpf/bpf_helpers.h
>>> index f804f210244e..81bc51293d11 100644
>>> --- a/tools/testing/selftests/bpf/bpf_helpers.h
>>> +++ b/tools/testing/selftests/bpf/bpf_helpers.h
>>> @@ -501,4 +501,23 @@ struct pt_regs;
>>> (void *)(PT_REGS_FP(ctx) + sizeof(ip))); })
>>> #endif
>>>
>>> +/*
>>> + * BPF_CORE_READ abstracts away bpf_probe_read() call and captures offset
>>> + * relocation for source address using __builtin_preserve_access_index()
>>> + * built-in, provided by Clang.
>>> + *
>>> + * __builtin_preserve_access_index() takes as an argument an expression of
>>> + * taking an address of a field within struct/union. It makes compiler emit
>>> + * a relocation, which records BTF type ID describing root struct/union and an
>>> + * accessor string which describes exact embedded field that was used to take
>>> + * an address. See detailed description of this relocation format and
>>> + * semantics in comments to struct bpf_offset_reloc in libbpf_internal.h.
>>> + *
>>> + * This relocation allows libbpf to adjust BPF instruction to use correct
>>> + * actual field offset, based on target kernel BTF type that matches original
>>> + * (local) BTF, used to record relocation.
>>> + */
>>> +#define BPF_CORE_READ(dst, src) \
>>> + bpf_probe_read(dst, sizeof(*src), __builtin_preserve_access_index(src))
>>
>> We should use "sizeof(*(src))"
>>
>
> Good point. Also (dst) instead of just (dst). Will update.

I think dst as-is is fine. "," is the very last in precedence list.