Re: Enum relocations against zero values

[Yonghong Song <yhs@fb.com>]

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.

> 
> 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
>