On Fri, Feb 26, 2021 at 10:08 AM Yonghong Song <yhs@xxxxxx> wrote:
>
>
>
> On 2/26/21 9:47 AM, Lorenz Bauer wrote:
> > Hi Andrii and Yonghong,
> >
> > I'm playing around with enum CO-RE relocations, and hit the following snag:
> >
> > enum e { TWO };
> > bpf_core_enum_value_exists(enum e, TWO);
> >
> > Compiling this with clang-12
> > (12.0.0-++20210225092616+e0e6b1e39e7e-1~exp1~20210225083321.50) gives
> > me the following:
> >
> > internal/btf/testdata/relocs.c:66:2: error:
> > __builtin_preserve_enum_value argument 1 invalid
> > enum_value_exists(enum e, TWO);
> > ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > internal/btf/testdata/relocs.c:53:8: note: expanded from macro
> > 'enum_value_exists'
> > if (!bpf_core_enum_value_exists(t, v)) { \
> > ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > internal/btf/testdata/bpf_core_read.h:168:32: note: expanded from
> > macro 'bpf_core_enum_value_exists'
> > __builtin_preserve_enum_value(*(typeof(enum_type)
> > *)enum_value, BPF_ENUMVAL_EXISTS)
> > ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> Andrii can comment on MACRO failures.
Yeah, I ran into this a long time ago as well...
I don't actually know why this doesn't work for zeroes. I've tried to
write that macro in a bit different way, but Clang rejects it:
__builtin_preserve_enum_value(({typeof(enum_type) ___xxx =
(enum_value); *(typeof(enum_type)*)&___xxx;}), BPF_ENUMVAL_EXISTS)
And something as straightforward as
__builtin_preserve_enum_value((typeof(enum_type))(enum_value),
BPF_ENUMVAL_EXISTS)
doesn't work as well.
Yonghong, any idea how to write such a macro to work in all cases? Or
why those alternatives don't work? I only get " error:
__builtin_preserve_enum_value argument 1 invalid" with no more
details, so hard to do anything about this.
>
> >
> > Changing the definition of the enum to
> >
> > enum e { TWO = 1 }
> >
> > compiles successfully. I get the same result for any enum value that
> > is zero. Is this expected?
>
> IIRC, libbpf will try to do relocation against vmlinux BTF.
> So here, "enum e" probably does not exist in vmlinux BTF, so
> the builtin will return 0. You can try some enum type
> existing in vmlinux BTF to see what happens.
>
> >
> > Best
> > Lorenz
> >
