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

Song Liu
2019-07-30T21:25:02Z

> 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))"