Re: Errors compiling BPF programs from Linux selftests/bpf with GCC

[Eduard Zingerman <eddyz87@gmail.com>]

On Thu, 2025-01-02 at 10:47 +0100, Jose E. Marchesi wrote:
> Hi Ihor.
> Thanks for working on this! :)
> 
> > [...]
> > Older versions compile the dummy program without errors, however on
> > attempt to build the selftests there is a different issue: conflicting
> > int64 definitions (full log at [6]).
> > 
> >     In file included from /usr/include/x86_64-linux-gnu/sys/types.h:155,
> >                      from /usr/include/x86_64-linux-gnu/bits/socket.h:29,
> >                      from /usr/include/x86_64-linux-gnu/sys/socket.h:33,
> >                      from /usr/include/linux/if.h:28,
> >                      from /usr/include/linux/icmp.h:23,
> >                      from progs/test_cls_redirect_dynptr.c:10:
> >     /usr/include/x86_64-linux-gnu/bits/stdint-intn.h:27:19: error: conflicting types for ‘int64_t’; have ‘__int64_t’ {aka ‘long long int’}
> >        27 | typedef __int64_t int64_t;
> >           |                   ^~~~~~~
> >     In file included from progs/test_cls_redirect_dynptr.c:6:
> >     /ci/workspace/bpfgcc.20240922/lib/gcc/bpf-unknown-none/15.0.0/include/stdint.h:43:24:
> > note: previous declaration of ‘int64_t’ with type ‘int64_t’ {aka ‘long
> > int’}
> >        43 | typedef __INT64_TYPE__ int64_t;
> >           |                        ^~~~~~~
> 
> I think this is what is going on:
> 
> The BPF selftest is indirectly including glibc headers from the host
> where it is being compiled.  In this case your x86_64 ubuntu system.
> 
> Many glibc headers include bits/wordsize.h, which in the case of x86_64
> is:
> 
>   #if defined __x86_64__ && !defined __ILP32__
>   # define __WORDSIZE	64
>   #else
>   # define __WORDSIZE	32
>   #define __WORDSIZE32_SIZE_ULONG		0
>   #define __WORDSIZE32_PTRDIFF_LONG	0
>   #endif
> 
> and then in bits/types.h:
> 
>   #if __WORDSIZE == 64
>   typedef signed long int __int64_t;
>   typedef unsigned long int __uint64_t;
>   #else
>   __extension__ typedef signed long long int __int64_t;
>   __extension__ typedef unsigned long long int __uint64_t;
>   #endif
> 
> i.e. your BPF program ends using __WORDSIZE 32.  This eventually leads
> to int64_t being defined as `signed long long int' in stdint-intn.h, as
> it would correspond to a x86_64 program running in 32-bit mode.
> 
> GCC BPF, on the other hand, is a "baremetal" compiler and it provides a
> small set of headers (including stdint.h) that implement standard C99
> types like int64_t, adjusted to the BPF architecture.
> 
> In this case there is a conflict between the 32-bit x86_64 definition of
> int64_t and the one of BPF.
> 
> PS: the other headers installed by GCC BPF are:
>     float.h iso646.h limits.h stdalign.h stdarg.h stdatomic.h stdbool.h
>     stdckdint.h stddef.h stdfix.h stdint.h stdnoreturn.h syslimits.h
>     tgmath.h unwind.h varargs.h

I wondered how this works with clang, because it does not define
__x86_64__ for bpf target. After staring and the output of -E:
- for clang int64_t is defined once and definition originate from
  /usr/include/bits/stdint-intn.h included from /usr/include/stdint.h;
- for gcc int64_t is defined two times, definitions originate from:
  - <gcc-install-path>/bpf-unknown-none/15.0.0/include/stdint.h
  - /usr/include/bits/stdint-intn.h included from /usr/include/sys/types.h.

So, both refer to stdint-intn.h, but only gcc refers to
compiler-specific stdint.h. This is so because of the structure of the
clang's /usr/lib/clang/19/include/stdint.h:

    ...
    #if __STDC_HOSTED__ && __has_include_next(<stdint.h>)
      ...
      # include_next <stdint.h>
      ...
    #else
      ...
      typedef __INT64_TYPE__ int64_t;
      ...
    #endif
    ...

The __STDC_HOSTED__ is defined as 1, thus when clang compiles the test case,
compiler-specific stdint.h is included, but it's content is ifdef'ed out and
it refers to system stdint.h instead. On the other hand, gcc-specific stdint.h
unconditionally typedefs int64_t.

Links:
- test case pre-processed by clang and gcc:
  https://gist.github.com/eddyz87/d381094d67979291bd8338655b15dd5e
- LLVM source code for stdint.h:
  https://github.com/llvm/llvm-project/blob/c703b4645c79e889fd6a0f3f64f01f957d981aa4/clang/lib/Headers/stdint.h#L24