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

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 <redacted>

Reviewed-by: Alan Maguire <redacted>