Re: Enum relocations against zero values

[Yonghong Song <yhs@fb.com>]

On 2/26/21 7:31 PM, Yonghong Song wrote:
> 
> 
> On 2/26/21 12:43 PM, Andrii Nakryiko wrote:
>> On Fri, Feb 26, 2021 at 10:08 AM Yonghong Song <yhs@fb.com> 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.
> 
> This is a clang BPF bug. In certain number classification system,
> clang considers 0 as NULL and non-0 as INTEGER. I only checked
> INTEGER and hence only non-0 works. All my tests has non-zero
> enum values :-(
> 
> Will fix the issue soon. Thanks for reporting!

Just pushed the fix (https://reviews.llvm.org/D97659) to llvm trunk
this morning. Also filed a request 
(https://bugs.llvm.org/show_bug.cgi?id=49391) to backport the fix to 
12.0.1 release.
it is too late to be included in 12.0.0 release.
Thanks!

> 
>>
>>
>>>
>>>>
>>>> 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
>>>>