Re: [PATCH bpf-next 06/12] selftests/bpf: Fix selftests failure

[Yonghong Song <yhs@fb.com>]

On 5/8/22 7:21 PM, Dave Marchevsky wrote:
> On 5/1/22 3:00 PM, Yonghong Song wrote:
>> The kflag is supported now for BTF_KIND_ENUM.
>> So remove the test which tests verifier failure
>> due to existence of kflag.
>>
>> With enum64 support in kernel and libbpf,
>> selftest btf_dump/btf_dump failed with
>> no-enum64 support llvm for the following
>> enum definition:
>>   enum e2 {
>>          C = 100,
>>          D = 4294967295,
>>          E = 0,
>>   };
>>
>> With the no-enum64 support llvm, the signedness is
>> 'signed' by default, and D (4294967295 = 0xffffffff)
>> will print as -1. With enum64 support llvm, the signedness
>> is 'unsigned' and the value of D will print as 4294967295.
> 
> To confirm my understanding, this signedness-by-default change is for _printing
> btf in c format only_ and doesn't correspond to any difference in interpretation
> , since all BTF enums before addition of new kflag were assumed to be signed,
> and new kflag's default value is signed as well.

Yes, the signedness is only used for printing in C format. For
values, the signedness is really depending on corresponding declaration
type or the context where the value is used.

In the next revision, I may change the default signedness to be unsigned
to cover common case like
    enum {
       A = 0,
       B, C, D, ...
    }

> 
>> To support both old and new compilers, this patch
>> changed the value to 268435455 = 0xfffffff which works
>> with both enum64 or non-enum64 support llvm.
>>
>> Signed-off-by: Yonghong Song <yhs@fb.com>
>> ---
> 
> Aside from the general question, for changes in this patch:
> 
> Acked-by: Dave Marchevsky <davemarchevsky@fb.com>