> 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.
Yes, in the GCC BPF backend we are using
use_gcc_stdint=provide
which makes GCC to provide the version of stdint.h that assumes
freestanding ("baremetal") mode. If we changed it to use
use_gcc_stdint=wrap
then it would install a stdint.h that does somethins similar to what
clang does, at least in hosts providing C99 headers (note the lack of
__has_include_next):
#ifndef _GCC_WRAP_STDINT_H
#if __STDC_HOSTED__
# if defined __cplusplus && __cplusplus >= 201103L
# undef __STDC_LIMIT_MACROS
# define __STDC_LIMIT_MACROS
# undef __STDC_CONSTANT_MACROS
# define __STDC_CONSTANT_MACROS
# endif
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wpedantic" // include_next
# include_next <stdint.h>
#pragma GCC diagnostic pop
#else
# include "stdint-gcc.h"
#endif
#define _GCC_WRAP_STDINT_H
#endif
We could switch to "wrap" to align with clang, but in that case it would
be up to the user to provide a "host" stdint.h that contains sensible
definitions for BPF. The kernel selftests, for example, would need to
do so to avoid including /usr/include/stdint.h that more likely than not
will provide incorrect definitions for int64_t and friends...
>
> 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
