Re: Enum relocations against zero values

On Fri, Feb 26, 2021 at 10:08 AM Yonghong Song [off-list ref] wrote:

On 2/26/21 9:47 AM, Lorenz Bauer wrote:

quoted

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.

quoted

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.

quoted

Best
Lorenz