linux-kernel - Re: [PATCH] bpftool: build bpf bits with -std=gnu11

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Message-ID: <913f66a8-6745-0e30-b5b8-96d23bf05b90@applied-asynchrony.com>
Date: Fri, 2 May 2025 11:53:35 +0200
From: Holger Hoffstätte <holger@...lied-asynchrony.com>
To: Quentin Monnet <qmo@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>,
 Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman
 <eddyz87@...il.com>, Song Liu <song@...nel.org>,
 Yonghong Song <yonghong.song@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>, bpf@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bpftool: build bpf bits with -std=gnu11

On 2025-05-02 11:26, Quentin Monnet wrote:
> On 02/05/2025 09:57, Holger Hoffstätte wrote:
>> A gcc-15-based bpf toolchain defaults to C23 and fails to compile various
>> kernel headers due to their use of a custom 'bool' type.
>> Explicitly using -std=gnu11 works with both clang and bpf-toolchain.
>>
>> Signed-off-by: Holger Hoffstätte <holger@...lied-asynchrony.com>
> 
> Thanks! I tested that it still works with clang.
> 
> Acked-by: Quentin Monnet <qmo@...nel.org>

Thanks!

> I didn't manage to compile with gcc, though. I tried with gcc 15.1.1 but
> option '--target=bpf' is apparently unrecognised by the gcc version on
> my setup.
> 
> Out of curiosity, how did you build using gcc for the skeleton? Was it
> enough to run "CLANG=gcc make"? Does it pass the clang-bpf-co-re build
> probe successfully?

I'm on Gentoo where we have a gcc-14/15 based "bpf-toolchain" package,
which is just gcc configured & packaged for the bpf target.
Our bpftool package can be built with clang (default) or without, in
which case it depend on the bpf-toolchain. The idea is to gradually
allow bpf/xdp tooling to build/run without requiring clang.

The --target definition is conditional and removed when not using clang:
https://gitweb.gentoo.org/repo/gentoo.git/tree/dev-util/bpftool/bpftool-7.5.0.ebuild?id=bf70fbf7b0dc97fbc97af579954ea81a8df36113#n94

The bug for building with the new gcc-15 based toolchain where this
patch originated is here: https://bugs.gentoo.org/955156

Hope this helps!

cheers
Holger