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

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Date:   Tue, 12 Jul 2022 10:48:20 +0100
From:   Quentin Monnet <quentin@...valent.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        James Hilliard <james.hilliard1@...il.com>
Cc:     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 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.

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