linux-kernel - Re: [PATCH v2] bpf/scripts: Generate GCC compatible helpers

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Message-ID: <CADvTj4qrKkyGBzxVk-Ddtv5fAs8wUD7L-cwZtqNE=7CMF+O0Eg@mail.gmail.com>
Date:   Tue, 12 Jul 2022 17:29:53 -0600
From:   James Hilliard <james.hilliard1@...il.com>
To:     Quentin Monnet <quentin@...valent.com>
Cc:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Yonghong Song <yhs@...com>, bpf <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, Networking <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        llvm@...ts.linux.dev
Subject: Re: [PATCH v2] bpf/scripts: Generate GCC compatible helpers

On Tue, Jul 12, 2022 at 3:48 AM Quentin Monnet <quentin@...valent.com> wrote:
>
> On 12/07/2022 05:40, Andrii Nakryiko wrote:
> > CC Quentin as well
> >
> > On Mon, Jul 11, 2022 at 5:11 PM James Hilliard
> > <james.hilliard1@...il.com> wrote:
> >>
> >> On Mon, Jul 11, 2022 at 5:36 PM Yonghong Song <yhs@...com> wrote:
> >>>
> >>>
> >>>
> >>> On 7/6/22 10:28 AM, James Hilliard 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 adds conditional blocks for GCC while leaving clang codepaths
> >>>> unchanged, for example:
> >>>>       #if __GNUC__ && !__clang__
> >>>>       void *bpf_map_lookup_elem(void *map, const void *key) __attribute__((kernel_helper(1)));
> >>>>       #else
> >>>>       static void *(*bpf_map_lookup_elem)(void *map, const void *key) = (void *) 1;
> >>>>       #endif
> >>>
> >>> It does look like that gcc kernel_helper attribute is better than
> >>> '(void *) 1' style. The original clang uses '(void *) 1' style is
> >>> just for simplicity.
> >>
> >> Isn't the original style going to be needed for backwards compatibility with
> >> older clang versions for a while?
> >
> > I'm curious, is there any added benefit to having this special
> > kernel_helper attribute vs what we did in Clang for a long time? Did
> > GCC do it just to be different and require workarounds like this or
> > there was some technical benefit to this?
> >
> > This duplication of definitions with #if for each one looks really
> > awful, IMO. I'd rather have a macro invocation like below (or
> > something along those lines) for each helper:
> >
> > BPF_HELPER_DEF(2, void *, bpf_map_update_elem, void *map, const void
> > *key, const void *value, __u64 flags);
> >
> > And then define BPF_HELPER_DEF() once based on whether it's Clang or GCC.
>
> Hi, for what it's worth I agree with Andrii, I would rather avoid the
> #if/else/endif and dual definition for each helper in the header, using
> a macro should keep it more readable indeed. The existing one
> (BPF_HELPER(return_type, name, args, id)) can likely be adapted.

Yeah, seems a bit cleaner, think I got it working:
https://lore.kernel.org/bpf/20220712232556.248863-1-james.hilliard1@gmail.com/

>
> Also I note that contrarily to clang's helpers, you don't declare GCC's
> as "static" (although I'm not sure of the effect of declaring them
> static in this case).
>
> Thanks,
> Quentin