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

From: Quentin Monnet
Date: Tue Jul 12 2022 - 05:48:30 EST

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@xxxxxxxxx> wrote:
>> On Mon, Jul 11, 2022 at 5:36 PM Yonghong Song <yhs@xxxxxx> 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).