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

[Holger Hoffstätte]

On 2025-05-03 04:36, Alexei Starovoitov wrote:
> On Fri, May 2, 2025 at 2:53 AM Holger Hoffstätte
> <holger@xxxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxx>
> > >
> > > Thanks! I tested that it still works with clang.
> > >
> > > Acked-by: Quentin Monnet <qmo@xxxxxxxxxx>
> >
> > 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.

> 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