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

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On Thursday, January 2nd, 2025 at 1:47 AM, Jose E. Marchesi <jose.marchesi@xxxxxxxxxx> 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.

Hi Jose, thanks for breaking this down.

I was able to mitigate int64_t declaration conflict by passing
-nostdinc to gcc.

Currently system-installed headers are being passed via -idirafter in
a compilation command:

    /ci/workspace/bpfgcc.20241229/bin/bpf-unknown-none-gcc \
        -g -Wall -Werror -D__TARGET_ARCH_x86 -mlittle-endian \
        -I/ci/workspace/tools/testing/selftests/bpf/tools/include
        -I/ci/workspace/tools/testing/selftests/bpf \
        -I/ci/workspace/tools/include/uapi \
        -I/ci/workspace/tools/testing/selftests/usr/include \
        -Wno-compare-distinct-pointer-types \
        -idirafter /usr/lib/gcc/x86_64-linux-gnu/13/include \
        -idirafter /usr/local/include \
        -idirafter /usr/include/x86_64-linux-gnu \
        -idirafter /usr/include \
        -DBPF_NO_PRESERVE_ACCESS_INDEX \
        -Wno-attributes \
        -O2 -std=gnu17 \                  # -nostdinc here helps
        -c progs/test_cls_redirect.c \
        -o /ci/workspace/tools/testing/selftests/bpf/bpf_gcc/test_cls_redirect.bpf.o

Passing -nostdinc makes gcc to pick compiler-installed header, if I
understand correctly.

> 
> 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


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