On Mon, Dec 30, 2024 at 12:59 PM Ihor Solodrai <ihor.solodrai@xxxxx> wrote:
>
> On Monday, December 30th, 2024 at 12:36 PM, Sam James <sam@xxxxxxxxxx> wrote:
>
> >
> >
> > Andrew Pinski via Gcc gcc@xxxxxxxxxxx writes:
> >
> > > On Mon, Dec 30, 2024 at 12:11 PM Ihor Solodrai via Gcc gcc@xxxxxxxxxxx wrote:
> > >
> > > > Hello everyone.
> > > >
> > > > I picked up the work adding GCC BPF backend to BPF CI pipeline [1],
> > > > originally done by Cupertino Miranda [2].
> > > >
> > > > I encountered issues compiling BPF objects for selftests/bpf with
> > > > recent GCC 15 snapshots. An additional test runner binary is supposed
> > > > to be generated by tools/testing/selftests/bpf/Makefile if BPF_GCC is
> > > > set to a directory with GCC binaries for BPF backend. The runner
> > > > binary depends on BPF binaries, which are produced by GCC.
> > > >
> > > > The first issue is compilation errors on vmlinux.h:
> > > >
> > > > In file included from progs/linked_maps1.c:4:
> > > > /ci/workspace/tools/testing/selftests/bpf/tools/include/vmlinux.h:8483:9: error: expected identifier before ‘false’
> > > > 8483 | false = 0,
> > > > | ^~~~~
> > > >
> > > > A snippet from vmlinux.h:
> > > >
> > > > enum {
> > > > false = 0,
> > > > true = 1,
> > > > };
> > > >
> > > > And:
> > > >
> > > > /ci/workspace/tools/testing/selftests/bpf/tools/include/vmlinux.h:23539:15: error: two or more data types in declaration specifiers
> > > > 23539 | typedef _Bool bool;
> > > > | ^~~~
> > > >
> > > > Full log at [3], and also at [4].
> > >
> > > These are simple, the selftests/bpf programs need to compile with
> > > -std=gnu17 or -std=gnu11 since GCC has changed the default to C23
> > > which defines false and bool as keywords now and can't be redeclared
> > > like before.
> >
> >
> > Yes, the kernel has various issues like this:
> > https://lore.kernel.org/linux-kbuild/20241119044724.GA2246422@thelio-3990X/.
> >
> > Unfortunately, not all the Makefiles correctly declare that they need
> > gnu11.
> >
> > Clang will hit issues like this too when they change default to gnu23.
>
> Andrew, Sam, thank you for a swift response.
>
> vmlinux.h is generated code, so for the booleans perhaps it's more
> appropriate to generate a condition, for example:
>
> #if __STDC_VERSION__ < 202311L
> enum {
> false = 0,
> true = 1,
> };
> #endif
>
> Any drawbacks to this?
By special hacking this specific enum in bpftool ?
Feels like overkill when just adding -std=gnu17 will do.
>
> Also if vmlinux was built with GCC C23 then I assume DWARF wouldn't
> contain the debug info for the enum, hence it wouldn't be present in
> vmlinux.h.
>
> I don't think downgrading the standard for a relatively new backend
> makes sense, especially in the context of CI testing.
I don't see why not. The flag affects the front-end while CI adds
the test coverage to gcc bpf backend.
