Re: [PATCH bpf-next 1/2] libbpf: add option to force-anonymize nested structs for BTF dump

[Alan Maguire <alan.maguire@oracle.com>]

On 17/12/2025 17:52, Alexei Starovoitov wrote:
> On Wed, Dec 17, 2025 at 9:33 AM Alan Maguire <alan.maguire@oracle.com> wrote:
>>
>> On 17/12/2025 17:06, Andrii Nakryiko wrote:
>>> On Wed, Dec 17, 2025 at 8:13 AM Alexei Starovoitov
>>> <alexei.starovoitov@gmail.com> wrote:
>>>>
>>>> On Wed, Dec 17, 2025 at 8:06 AM Alan Maguire <alan.maguire@oracle.com> wrote:
>>>>>
>>>>> struct foo {
>>>>>         struct foo *ptr;
>>>>> };
>>>>>
>>>>> struct bar {
>>>>>
>>>>> #ifdef __MS_EXTENSIONS__
>>>>>         struct foo;
>>>>> #else
>>>>>         struct {
>>>>>                 struct foo *ptr;
>>>>>         };
>>>>> #endif
>>>>
>>>> Did you test it ? I suspect AI invented it.
>>>> I see nothing like this in gcc or llvm sources.
>>>
>>> Grepping a bit I suspect we need to check for _MSC_EXTENSIONS, worst
>>> case - _MSC_VER. But Alan, please double check in practice.
>>
>> Thanks; I tried these too, no luck with either gcc or clang. Looks like the
>> requests to merge them haven't landed yet, latest I could find on this was [1]/[2].
> 
> clang diff landed, but these defines are there only when
> clang is built for windows.
> 
> After studying the code a bit the following works with clang on linux:
> 
> #if __has_builtin(__builtin_FUNCSIG)
> 
> but not with gcc.

So maybe the best we can do here is something like the following at the top
of vmlinux.h:

#ifndef BPF_USE_MS_EXTENSIONS
#if __has_builtin(__builtin_FUNCSIG) || defined(_MSC_EXTENSIONS)
#define BPF_USE_MS_EXTENSIONS
#endif
#endif

...and then guard using #ifdef BPF_USE_MS_EXTENSIONS

That will work on clang and perhaps at some point work on gcc, but also
gives the user the option to supply a macro to force use in cases where
there is no detection available.