linux-kernel - Re: [PATCH v3] bpf/scripts: Generate GCC compatible helpers

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Date:   Tue, 12 Jul 2022 16:57:49 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     James Hilliard <james.hilliard1@...il.com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, LKML <linux-kernel@...r.kernel.org>,
        llvm@...ts.linux.dev
Subject: Re: [PATCH v3] bpf/scripts: Generate GCC compatible helpers

On Tue, Jul 12, 2022 at 4:26 PM James Hilliard
<james.hilliard1@...il.com> wrote:
>
> The current bpf_helper_defs.h helpers are llvm specific and don't work
> correctly with gcc.
>
> GCC appears to required kernel helper funcs to have the following
> attribute set: __attribute__((kernel_helper(NUM)))
>
> Generate gcc compatible headers based on the format in bpf-helpers.h.
>
> This generates GCC/Clang compatible helpers, for example:
>         /* Helper macro for GCC/Clang compatibility */
>         #define NOARG
>         #if __GNUC__ && !__clang__
>         #define BPF_HELPER_DEF(num, ret_star, ret_type, name, ...) \
>         ret_type ret_star name(__VA_ARGS__) __attribute__((kernel_helper(num)));
>         #else
>         #define BPF_HELPER_DEF(num, ret_star, ret_type, name, ...) \
>         static ret_type ret_star(*name)(__VA_ARGS__) = (void *) num;
>         #endif
>
>         BPF_HELPER_DEF(1, *, void, bpf_map_lookup_elem, void *map, const void *key)
>
>         BPF_HELPER_DEF(2, NOARG, long, bpf_map_update_elem, void *map, const void *key, const void *value, __u64 flags)
>
> See:
> https://github.com/gcc-mirror/gcc/blob/releases/gcc-12.1.0/gcc/config/bpf/bpf-helpers.h#L24-L27
>
> This fixes the following build error:
> error: indirect call in function, which are not supported by eBPF
>
> Signed-off-by: James Hilliard <james.hilliard1@...il.com>
> ---
> Changes v2 -> v3:
>   - use a conditional helper macro
> Changes v1 -> v2:
>   - more details in commit log
> ---
>  scripts/bpf_doc.py | 21 ++++++++++++++++++---
>  1 file changed, 18 insertions(+), 3 deletions(-)
>
> diff --git a/scripts/bpf_doc.py b/scripts/bpf_doc.py
> index a0ec321469bd..45f51ff1318c 100755
> --- a/scripts/bpf_doc.py
> +++ b/scripts/bpf_doc.py
> @@ -717,6 +717,16 @@ class PrinterHelpers(Printer):
>          header = '''\
>  /* This is auto-generated file. See bpf_doc.py for details. */
>
> +/* Helper macro for GCC/Clang compatibility */
> +#define NOARG
> +#if __GNUC__ && !__clang__
> +#define BPF_HELPER_DEF(num, ret_star, ret_type, name, ...) \\
> +ret_type ret_star name(__VA_ARGS__) __attribute__((kernel_helper(num)));
> +#else
> +#define BPF_HELPER_DEF(num, ret_star, ret_type, name, ...) \\
> +static ret_type ret_star(*name)(__VA_ARGS__) = (void *) num;
> +#endif

Nack for the reasons stated earlier.