Message-ID: <CAEf4BzZKKd+0Hwok=BxyBsWMUnBBTUGj-ZfJK-voMXowRKPgNQ@mail.gmail.com>
Date: Tue, 6 May 2025 15:23:23 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Ihor Solodrai <ihor.solodrai@...ux.dev>
Cc: Holger Hoffstätte <holger@...lied-asynchrony.com>, 
	Alexei Starovoitov <alexei.starovoitov@...il.com>, 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 <bpf@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] bpftool: build bpf bits with -std=gnu11

On Tue, May 6, 2025 at 2:41 PM Ihor Solodrai <ihor.solodrai@...ux.dev> wrote:
>
> On 2025-05-06 2:04 p.m., Andrii Nakryiko wrote:
>  > On Sun, May 4, 2025 at 3:24 AM Holger Hoffstätte
>  > <holger@...lied-asynchrony.com> wrote:
>  >>
>  >> On 2025-05-03 04:36, Alexei Starovoitov wrote:
>  >>> On Fri, May 2, 2025 at 2:53 AM Holger Hoffstätte
>  >>> <holger@...lied-asynchrony.com> wrote:
>  >>>>
>  >>>> 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
>  >>>
>  >>> So you're fixing this build error:
>  >>>
>  >>> bpf-unknown-none-gcc \
>  >>>           -I. \
>  >>>
> -I/var/tmp/portage/dev-util/bpftool-7.5.0/work/bpftool-libbpf-v7.5.0-sources/include/uapi/
>  >>> \
>  >>>
> -I/var/tmp/portage/dev-util/bpftool-7.5.0/work/bpftool-libbpf-v7.5.0-sources/src/bootstrap/libbpf/include
>  >>> \
>  >>>           -g -O2 -Wall -fno-stack-protector \
>  >>>            -c skeleton/profiler.bpf.c -o profiler.bpf.o
>  >>> In file included from skeleton/profiler.bpf.c:3:
>  >>> ./vmlinux.h:5: warning: ignoring '#pragma clang attribute'
> [-Wunknown-pragmas]
>  >>>       5 | #pragma clang attribute push
>  >>> (__attribute__((preserve_access_index)), apply_to = record)
>  >>> ./vmlinux.h:9845:9: error: cannot use keyword 'false' as
> enumeration constant
>  >>>    9845 |         false = 0,
>  >>>         |         ^~~~~
>  >>> ./vmlinux.h:9845:9: note: 'false' is a keyword with '-std=c23' onwards
>  >>> ./vmlinux.h:31137:15: error: 'bool' cannot be defined via 'typedef'
>  >>> 31137 | typedef _Bool bool;
>  >>>         |               ^~~~
>  >>>
>  >>> with -std=gnu11 flag and
>  >>
>  >> Yes, correct. This is the same as all over the kernel or the bpf tests
>  >> for handling C23. I fully understand that this particular patch is only
>  >> one piece of the puzzle.
>  >>
>  >
>  > What's the best way to detect (at compile time) whether bool, false,
>  > and true are treated as reserved keywords? To solve this properly
>  > vmlinux.h would have to be adjusted by vmlinux.h to avoid emitting
>  > bool/false/true *iff* compiler version/mode doesn't like that
> I ran into this when adding GCC BPF to CI [1].
>
> One can do something like:
>
>       #if __STDC_VERSION__ < 202311L
>       enum {
>           false = 0,
>           true = 1,
>       };
>       #endif
>
> But in case of vmlinux.h this would require hacking bpftool, and so for
> selftests/bpf we decided to pass -std=gnu11 [2].

We can adjust btf_dump_is_blacklisted() to ignore bool typedef
(unconditionally), and we'll need to ignore anon enum with false/true
(which is annoying), and then bpftool will unconditionally add the
above block plus typedef _Bool bool.

Would that work?

>
> [1]
> https://lore.kernel.org/bpf/CAADnVQKNqdLW1bpvCpVV3yNizwra0cCkBnAbsNp3rTmi8WFcvQ@mail.gmail.com/
> [2]
> https://lore.kernel.org/bpf/20250107235813.2964472-1-ihor.solodrai@pm.me/
>
>  >
>  >>> ignoring an important warning ?
>  >>
>  >> Nobody is (or was) ignoring the warning - it was under discussion when
>  >> I posted the patch. After reaching out to Oracle to verify, we have now
>  >> added the BPF_NO_PRESERVE_ACCESS_INDEX define when building with
> gcc-bpf;
>  >> this resolves the warning, just like in the bpf self-tests.
>  >>
>  >> You are right that such an addition to the in-kernel bpftool build is
>  >> still missing. If you have a suggestion on how best to do that via the
>  >> existing Makefile I'm all ears.
>  >>
>  >> As for the remaining warnings - we are also very aware of the ongoing
>  >> upstream work to support btf_type_tag:
>  >> https://gcc.gnu.org/pipermail/gcc-patches/2025-April/682340.html.
>  >>
>  >>> End result: partially functional bpftool,
>  >>> and users will have no idea why some features of bpftool are not
> working.
>  >>
>  >> First of all this is never shipped to any users; using gcc-bpf requires
>  >> active opt-in by developers or users, and now also warns that such a
> setup
>  >> may result in unexpected bugs due to ongoing work in both Linux and
> bpftool.
>  >> Like I said before, by default everyone builds with clang and that
> is also
>  >> true for our distributed binaries.
>  >>
>  >> If you think adding the -std=gnu11 bit is inappropriate at this time
> then
>  >> just ignore this patch for now. Sooner or later the bpftool build
> will have
>  >> to be adapted with BPF_CFLAGS (liek in the selftests) and hopefuilly an
>  >> abstracted BPF_CC so that we no longer have to pretend to be clang when
>  >> using gcc.
>  >>
>  >> cheers
>  >> Holger

Powered by blists - more mailing lists