Re: [PATCH bpf-next] selftests/bpf: fix btf_dump __int128 test failure with clang build kernel

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Fri, Sep 24, 2021 at 11:13 AM Alan Maguire <alan.maguire@oracle.com> wrote:
>
> On Fri, 24 Sep 2021, Yonghong Song wrote:
>
> > With clang build kernel (adding LLVM=1 to kernel and selftests/bpf build
> > command line), I hit the following test failure:
> >
> >   $ ./test_progs -t btf_dump
> >   ...
> >   btf_dump_data:PASS:ensure expected/actual match 0 nsec
> >   btf_dump_data:FAIL:find type id unexpected find type id: actual -2 < expected 0
> >   btf_dump_data:FAIL:find type id unexpected find type id: actual -2 < expected 0
> >   test_btf_dump_int_data:FAIL:dump __int128 unexpected error: -2 (errno 2)
> >   #15/9 btf_dump/btf_dump: int_data:FAIL
> >
> > Further analysis showed gcc build kernel has type "__int128" in dwarf/BTF
> > and it doesn't exist in clang build kernel. Code searching for kernel code
> > found the following:
> >   arch/s390/include/asm/types.h:  unsigned __int128 pair;
> >   crypto/ecc.c:   unsigned __int128 m = (unsigned __int128)left * right;
> >   include/linux/math64.h: return (u64)(((unsigned __int128)a * mul) >> shift);
> >   include/linux/math64.h: return (u64)(((unsigned __int128)a * mul) >> shift);
> >   lib/ubsan.h:typedef __int128 s_max;
> >   lib/ubsan.h:typedef unsigned __int128 u_max;
> >
> > In my case, CONFIG_UBSAN is not enabled. Even if we only have "unsigned __int128"
> > in the code, somehow gcc still put "__int128" in dwarf while clang didn't.
> > Hence current test works fine for gcc but not for clang.
> >
> > Enabling CONFIG_UBSAN is an option to provide __int128 type into dwarf
> > reliably for both gcc and clang, but not everybody enables CONFIG_UBSAN
> > in their kernel build. So the best choice is to use "unsigned __int128" type
> > which is available in both clang and gcc build kernels. But clang and gcc
> > dwarf encoded names for "unsigned __int128" are different:
> >
> >   [$ ~] cat t.c
> >   unsigned __int128 a;
> >   [$ ~] gcc -g -c t.c && llvm-dwarfdump t.o | grep __int128
> >                   DW_AT_type      (0x00000031 "__int128 unsigned")
> >                   DW_AT_name      ("__int128 unsigned")
> >   [$ ~] clang -g -c t.c && llvm-dwarfdump t.o | grep __int128
> >                   DW_AT_type      (0x00000033 "unsigned __int128")
> >                   DW_AT_name      ("unsigned __int128")
> >
> > The test change in this patch tries to test type name before
> > doing actual test.
> >
>
> Thanks for finding and fixing this!
>
> > Signed-off-by: Yonghong Song <yhs@fb.com>
>
> Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
>

Applied to bpf-next, thanks.

> > ---
> >  .../selftests/bpf/prog_tests/btf_dump.c       | 27 ++++++++++++++-----
> >  1 file changed, 21 insertions(+), 6 deletions(-)
> >

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