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Date:   Mon, 11 Jul 2022 16:35:55 -0700
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Subject: Re: [PATCH v2] bpf/scripts: Generate GCC compatible helpers
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To:     James Hilliard <james.hilliard1@gmail.com>, bpf@vger.kernel.org
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Tom Rix <trix@redhat.com>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, llvm@lists.linux.dev
References: <20220706172814.169274-1-james.hilliard1@gmail.com>
From:   Yonghong Song <yhs@fb.com>
In-Reply-To: <20220706172814.169274-1-james.hilliard1@gmail.com>
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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.

Do you mind to help implement similar attribute in clang so we
don't need "#if" here?

> 
> 	#if __GNUC__ && !__clang__
> 	long bpf_map_update_elem(void *map, const void *key, const void *value, __u64 flags) __attribute__((kernel_helper(2)));
> 	#else
> 	static long (*bpf_map_update_elem)(void *map, const void *key, const void *value, __u64 flags) = (void *) 2;
> 	#endif
> 
> 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@gmail.com>
> ---
> Changes v1 -> v2:
>    - more details in commit log
> ---
>   scripts/bpf_doc.py | 43 ++++++++++++++++++++++++++-----------------
>   1 file changed, 26 insertions(+), 17 deletions(-)
> 
[...]