Pahole/BTF issue with __int128

["Alexis Lothoré" <alexis.lothore@bootlin.com>]

Hello,

I am working on some ebpf feature for ARM64 (improving trampolines to
attach tracing programs to functions with more arguments than the current
limit), and I am facing an issue with the generated BTF information when
playing with large int types like __int128 (I need to use those large types
to properly test some architecture-specific alignment expectations). I
suspect the issue to be in pahole, but I would like to get some opinions on
my observations, and maybe some guidance on where to look at to go further.

I would like to attach some fentry/fexit programs to the following kind of
function, which is currently defined in a kernel module (bpf_testmod.ko in
bpf selftests):

  struct bpf_testmod_struct_arg_7 {
  	_int128 a;
  };
  
  noinline int bpf_testmod_test_struct_arg_11(
  	struct bpf_testmod_struct_arg_7 a,
  	struct bpf_testmod_struct_arg_7 b,
  	struct bpf_testmod_struct_arg_7 c,
  	struct bpf_testmod_struct_arg_7 d,
  	short e,
  	struct bpf_testmod_struct_arg_7 f)
  {
  	[...]
  }

This one works well (let's call it case 1), I am able to attach
fentry/fexit programs to such function through libbpf.

However, if, in a case 2, I change the bpf_testmod_test_struct_arg_11
prototype to use __in128 arguments instead of struct arguments, like the
following one:

  noinline int bpf_testmod_test_struct_arg_11(
  	__int128 a,
  	__int128 b,
  	__int128 c,
  	__int128 d,
  	short e,
  	__int128 f)
  {
  	[...]
  }

and rebuild the module/run my test, this does not work anymore, and libbpf
complains with the following error:
  libbpf: prog 'test_struct_many_args_9': failed to find kernel BTF type ID
  of 'bpf_testmod_test_struct_arg_11': -ESRCH

Inspecting the generated BTF information in bpf_testmod.ko file with bpftool, I
indeed find some BTF info related to my target func in case 1 but not in
case 2:

  [...]
  [118] STRUCT 'bpf_testmod_struct_arg_7' size=16 vlen=1
          'a' type_id=10 bits_offset=0
  [...]
  [371] FUNC_PROTO '(anon)' ret_type_id=6 vlen=6
          'a' type_id=118
          'b' type_id=118
          'c' type_id=118
          'd' type_id=118
          'e' type_id=5
          'f' type_id=118
  [372] FUNC 'bpf_testmod_test_struct_arg_11' type_id=371 linkage=static
  [...]

I checked the command executed by the kernel build system to generate BTF
info for the module, and got the following one:
  pahole -J -j\
  --btf_features=encode_force,var,float,enum64,decl_tag,type_tag,optimized_func,consistent_func,decl_tag_kfuncs\
  --btf_features=attributes --lang_exclude=rust\
  --btf_features=distilled_base --btf_base vmlinux\
  tools/testing/selftests/bpf/bpf_testmod.ko

I ran the same command before/after switching the struct arguments to
__int128, and made the same observation (I am running pahole 1.30). I then
took a look at available DWARF info available in bpf_testmod.ko for pahole
to generate BTF info, and AFAICT, it looks ok (to be confirmed ?) in both
cases (I am using an aarch64-linux-gcc toolchain, v13.2.0 from
https://toolchains.bootlin.com/)

Case 1:

  [...]
  <1><262>: Abbrev Number: 106 (DW_TAG_base_type)
     <263>   DW_AT_byte_size   : 16
     <264>   DW_AT_encoding    : 5       (signed)
     <265>   DW_AT_name        : (indirect string, offset: 0x193bc): __int128
  [...]
  <1><23429>: Abbrev Number: 11 (DW_TAG_structure_type)
     <2342a>   DW_AT_name        : (indirect string, offset: 0xe98d): bpf_testmod_struct_arg_7
     <2342e>   DW_AT_byte_size   : 16
     <2342f>   DW_AT_decl_file   : 1
     <23430>   DW_AT_decl_line   : 70
     <23431>   DW_AT_decl_column : 8
     <23432>   DW_AT_sibling     : <0x23442>
  <2><23436>: Abbrev Number: 12 (DW_TAG_member)
     <23437>   DW_AT_name        : a
     <23439>   DW_AT_decl_file   : 1
     <2343a>   DW_AT_decl_line   : 71
     <2343b>   DW_AT_decl_column : 11
     <2343c>   DW_AT_type        : <0x262>
     <23440>   DW_AT_data_member_location: 0
  [...]
  <1><295c1>: Abbrev Number: 99 (DW_TAG_subprogram)
     <295c2>   DW_AT_external    : 1
     <295c2>   DW_AT_name        : (indirect string, offset: 0x5e20): bpf_testmod_test_struct_arg_11
     <295c6>   DW_AT_decl_file   : 1
     <295c7>   DW_AT_decl_line   : 152
     <295c8>   DW_AT_decl_column : 14
     <295c9>   DW_AT_prototyped  : 1
     <295c9>   DW_AT_type        : <0xdd>
     <295cd>   DW_AT_low_pc      : 0x1380
     <295d5>   DW_AT_high_pc     : 0x34
     <295dd>   DW_AT_frame_base  : 1 byte block: 9c      (DW_OP_call_frame_cfa)
     <295df>   DW_AT_GNU_all_call_sites: 1
     <295df>   DW_AT_sibling     : <0x2964a>
  <2><295e3>: Abbrev Number: 45 (DW_TAG_formal_parameter)
     <295e4>   DW_AT_name        : a
     <295e6>   DW_AT_decl_file   : 1
     <295e7>   DW_AT_decl_line   : 152
     <295e8>   DW_AT_decl_column : 77
     <295e9>   DW_AT_type        : <0x23429>
     <295ed>   DW_AT_location    : 0x6196 (location list)
     <295f1>   DW_AT_GNU_locviews: 0x6194
  [...]

Case 2:

  [...]
  <1><262>: Abbrev Number: 106 (DW_TAG_base_type)
     <263>   DW_AT_byte_size   : 16
     <264>   DW_AT_encoding    : 5       (signed)
     <265>   DW_AT_name        : (indirect string, offset: 0x1935d): __int128
  [...]
   <1><29552>: Abbrev Number: 98 (DW_TAG_subprogram)
      <29553>   DW_AT_external    : 1
      <29553>   DW_AT_name        : (indirect string, offset: 0x5e20): bpf_testmod_test_struct_arg_11
      <29557>   DW_AT_decl_file   : 1
      <29558>   DW_AT_decl_line   : 148
      <29559>   DW_AT_decl_column : 14
      <2955a>   DW_AT_prototyped  : 1
      <2955a>   DW_AT_type        : <0xdd>
      <2955e>   DW_AT_low_pc      : 0x1380
      <29566>   DW_AT_high_pc     : 0x34
      <2956e>   DW_AT_frame_base  : 1 byte block: 9c      (DW_OP_call_frame_cfa)
      <29570>   DW_AT_GNU_all_call_sites: 1
      <29570>   DW_AT_sibling     : <0x295d6>
   <2><29574>: Abbrev Number: 46 (DW_TAG_formal_parameter)
      <29575>   DW_AT_name        : a
      <29577>   DW_AT_decl_file   : 1
      <29578>   DW_AT_decl_line   : 148
      <29579>   DW_AT_decl_column : 54
      <2957a>   DW_AT_type        : <0x262>
      <2957e>   DW_AT_location    : 0x6158 (location list)
      <29582>   DW_AT_GNU_locviews: 0x6154
  [...]

Am I missing some constraint or limitation that would prevent the case 2
function from being described with BTF info ? If not, any advice about how
to debug this further ?

Thanks,

Alexis

-- 
Alexis Lothoré, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com