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

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

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