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

[Ihor Solodrai <ihor.solodrai@linux.dev>]

On 2025-05-06 2:04 p.m., Andrii Nakryiko wrote:
 > On Sun, May 4, 2025 at 3:24 AM Holger Hoffstätte
 > <holger@applied-asynchrony.com> wrote:
 >>
 >> On 2025-05-03 04:36, Alexei Starovoitov wrote:
 >>> On Fri, May 2, 2025 at 2:53 AM Holger Hoffstätte
 >>> <holger@applied-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@applied-asynchrony.com>
 >>>>>
 >>>>> Thanks! I tested that it still works with clang.
 >>>>>
 >>>>> Acked-by: Quentin Monnet <qmo@kernel.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].

[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