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

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Date:   Tue, 30 Jul 2019 21:24:36 +0000
From:   Song Liu <songliubraving@...com>
To:     Andrii Nakryiko <andriin@...com>
CC:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        "Alexei Starovoitov" <ast@...com>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        Yonghong Song <yhs@...com>,
        "andrii.nakryiko@...il.com" <andrii.nakryiko@...il.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 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))"