linux-kernel - Re: [PATCH v2] libbpf bpf_helpers: Use __builtin

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Date:   Sun, 19 Jul 2020 23:21:37 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Ian Rogers <irogers@...gle.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Stanislav Fomichev <sdf@...gle.com>
Subject: Re: [PATCH v2] libbpf bpf_helpers: Use __builtin_offsetof for offsetof

On Sun, Jul 19, 2020 at 11:18 PM Ian Rogers <irogers@...gle.com> wrote:
>
> The non-builtin route for offsetof has a dependency on size_t from
> stdlib.h/stdint.h that is undeclared and may break targets.
> The offsetof macro in bpf_helpers may disable the same macro in other
> headers that have a #ifdef offsetof guard. Rather than add additional
> dependencies improve the offsetof macro declared here to use the
> builtin that is available since llvm 3.7 (the first with a BPF backend).
>
> Signed-off-by: Ian Rogers <irogers@...gle.com>
> ---

Acked-by: Andrii Nakryiko <andriin@...com>

>  tools/lib/bpf/bpf_helpers.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
> index a510d8ed716f..bc14db706b88 100644
> --- a/tools/lib/bpf/bpf_helpers.h
> +++ b/tools/lib/bpf/bpf_helpers.h
> @@ -40,7 +40,7 @@
>   * Helper macro to manipulate data structures
>   */
>  #ifndef offsetof
> -#define offsetof(TYPE, MEMBER)  ((size_t)&((TYPE *)0)->MEMBER)
> +#define offsetof(TYPE, MEMBER)  __builtin_offsetof(TYPE, MEMBER)
>  #endif
>  #ifndef container_of
>  #define container_of(ptr, type, member)                                \
> --
> 2.28.0.rc0.105.gf9edc3c819-goog
>