On Wed, Mar 29, 2023 at 1:59 PM Martin KaFai Lau <martin.lau@xxxxxxxxx> wrote:
>
> On 3/29/23 12:12 PM, James Hilliard wrote:
> > On Wed, Mar 29, 2023 at 11:03 AM Martin KaFai Lau <martin.lau@xxxxxxxxx> wrote:
> >>
> >> On 3/27/23 8:51 PM, James Hilliard wrote:
> >>>> diff --git a/tools/testing/selftests/bpf/progs/bench_local_storage_create.c b/tools/testing/selftests/bpf/progs/bench_local_storage_create.c
> >>>> index 2814bab54d28..7c851c9d5e47 100644
> >>>> --- a/tools/testing/selftests/bpf/progs/bench_local_storage_create.c
> >>>> +++ b/tools/testing/selftests/bpf/progs/bench_local_storage_create.c
> >>>> @@ -22,6 +22,13 @@ struct {
> >>>> __type(value, struct storage);
> >>>> } sk_storage_map SEC(".maps");
> >>>>
> >>>> +struct {
> >>>> + __uint(type, BPF_MAP_TYPE_TASK_STORAGE);
> >>>> + __uint(map_flags, BPF_F_NO_PREALLOC);
> >>>> + __type(key, int);
> >>>> + __type(value, struct storage);
> >>>> +} task_storage_map SEC(".maps");
> >>>> +
> >>>> SEC("raw_tp/kmalloc")
> >>>> int BPF_PROG(kmalloc, unsigned long call_site, const void *ptr,
> >>>> size_t bytes_req, size_t bytes_alloc, gfp_t gfp_flags,
> >>>> @@ -32,6 +39,24 @@ int BPF_PROG(kmalloc, unsigned long call_site, const void *ptr,
> >>>> return 0;
> >>>> }
> >>>>
> >>>> +SEC("tp_btf/sched_process_fork")
> >>>> +int BPF_PROG(fork, struct task_struct *parent, struct task_struct *child)
> >>>
> >>> Apparently fork is a built-in function in bpf-gcc:
> >>
> >> It is also failing in a plain C program
> >>
> >> #> gcc -Werror=builtin-declaration-mismatch -o test test.c
> >> test.c:14:35: error: conflicting types for built-in function ‘fork’; expected
> >> ‘int(void)’ [-Werror=builtin-declaration-mismatch]
> >> 14 | int __attribute__((__noinline__)) fork(long x, long y)
> >> | ^~~~
> >> cc1: some warnings being treated as errors
> >>
> >> #> clang -o test test.c
> >> succeed
> >>
> >> I am not too attached to the name but it seems something should be addressed in
> >> the gcc instead.
> >
> > Hmm, so it looks like it's marked as a builtin here:
> > https://github.com/gcc-mirror/gcc/blob/releases/gcc-12.1.0/gcc/builtins.def#L875
> >
> > The macro for that is here:
> > https://github.com/gcc-mirror/gcc/blob/releases/gcc-12.1.0/gcc/builtins.def#L104-L111
> >
> > Which has this comment:
> > /* Like DEF_LIB_BUILTIN, except that the function is not one that is
> > specified by ANSI/ISO C. So, when we're being fully conformant we
> > ignore the version of these builtins that does not begin with
> > __builtin. */
> >
> > Looks like this builtin was originally added here:
> > https://github.com/gcc-mirror/gcc/commit/d1c38823924506d389ca58d02926ace21bdf82fa
> >
> > Based on this issue it looks like fork is treated as a builtin for
> > libgcov support:
> > https://gcc.gnu.org/bugzilla//show_bug.cgi?id=82457
> >
> > So from my understanding fork is a gcc builtin when building with -std=gnu11
> > but is not a builtin when building with -std=c11.
>
> That sounds like there is a knob to turn this behavior on and off. Do the same
> for the bpf target?
I don't think we want to have to do that.
>
> >
> > So it looks like fork is translated to __gcov_fork when -std=gnu* is set which
> > is why we get this error.
> >
> > As this appears to be intended behavior for gcc I think the best option is
> > to just rename the function so that we don't run into issues when building
> > with gnu extensions like -std=gnu11.
>
> Is it sure 'fork' is the only culprit? If not, it is better to address it
> properly because this unnecessary name change is annoying when switching bpf
> prog from clang to gcc. Like changing the name in this .c here has to make
> another change to the .c in the prog_tests/ directory.
We've fixed a similar issue in the past by renaming to avoid a
conflict with the builtin:
https://github.com/torvalds/linux/commit/ab0350c743d5c93fd88742f02b3dff12168ab435
>
> >
> >>
> >>>
> >>> In file included from progs/bench_local_storage_create.c:6:
> >>> progs/bench_local_storage_create.c:43:14: error: conflicting types for
> >>> built-in function 'fork'; expected 'int(void)'
> >>> [-Werror=builtin-declaration-mismatch]
> >>> 43 | int BPF_PROG(fork, struct task_struct *parent, struct
> >>> task_struct *child)
> >>> | ^~~~
> >>>
> >>> I haven't been able to find this documented anywhere however.
> >>
>
