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

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Date:   Sat, 18 Jul 2020 20:52:27 -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] libbpf bpf_helpers: Use __builtin_offsetof for offsetof
 if available

On Fri, Jul 17, 2020 at 12:24 AM 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 if available.
>
> Signed-off-by: Ian Rogers <irogers@...gle.com>
> ---
>  tools/lib/bpf/bpf_helpers.h | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
> index a510d8ed716f..ed2ac74fc515 100644
> --- a/tools/lib/bpf/bpf_helpers.h
> +++ b/tools/lib/bpf/bpf_helpers.h
> @@ -40,8 +40,12 @@
>   * Helper macro to manipulate data structures
>   */
>  #ifndef offsetof
> +#if __has_builtin(__builtin_offsetof)
> +#define offsetof(TYPE, MEMBER)  __builtin_offsetof(TYPE, MEMBER)
> +#else
>  #define offsetof(TYPE, MEMBER)  ((size_t)&((TYPE *)0)->MEMBER)

Let's either always use __builtin_offsetof (as Yonghong mentioned, it
should always be available on relevant LLVM versions). Or instead of
size_t, just cast to (unsigned long), I think it will have absolutely
the same effect as size_t casting?

>  #endif
> +#endif
>  #ifndef container_of
>  #define container_of(ptr, type, member)                                \
>         ({                                                      \
> --
> 2.28.0.rc0.105.gf9edc3c819-goog
>