netdev - Re: [PATCH v2 bpf-next 03/12] selftests/bpf: add BPF_CORE

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Date:   Tue, 30 Jul 2019 21:33:15 +0000
From:   Song Liu <songliubraving@...com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
CC:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <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 03/12] selftests/bpf: add BPF_CORE_READ
 relocatable read macro



> On Jul 30, 2019, at 2:26 PM, Andrii Nakryiko <andrii.nakryiko@...il.com> wrote:
> 
> On Tue, Jul 30, 2019 at 2:24 PM Song Liu <songliubraving@...com> wrote:
>> 
>> 
>> 
>>> On Jul 30, 2019, at 12:53 PM, Andrii Nakryiko <andriin@...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@...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.