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

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Date:   Mon, 11 Jul 2022 16:35:55 -0700
From:   Yonghong Song <yhs@...com>
To:     James Hilliard <james.hilliard1@...il.com>, bpf@...r.kernel.org
Cc:     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>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH v2] bpf/scripts: Generate GCC compatible helpers



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@...il.com>
> ---
> Changes v1 -> v2:
>    - more details in commit log
> ---
>   scripts/bpf_doc.py | 43 ++++++++++++++++++++++++++-----------------
>   1 file changed, 26 insertions(+), 17 deletions(-)
> 
[...]