On 28/03/2025 20:25, Ihor Solodrai wrote:
> On 3/25/25 2:10 AM, Domenico Andreoli wrote:
>> Hi,
>>
>> This a forward of Debian bug report [0] where you can find more
>> details. At [1] and [2] you can get the kernel and module to reproduce.
>> I could reproduce on both amd64 and arm64 using pahole 1.29.
>>
>> This is marked as serious severity because it makes the autobuilder hang
>> as well [3].
>>
>> Could you please help?
>>
>> Regards,
>> Domenico
>>
>>
>> The command to succeed:
>>
>> This simplified (sequential) command succeeds:
>>
>> cp nvidia-modeset.base.ko nvidia-modeset.ko
>> LLVM_OBJCOPY="x86_64-linux-gnu-objcopy" pahole -J --btf_features=encode_force,var,float,enum64,decl_tag,type_tag,optimized_func,consistent_func,decl_tag_kfuncs --btf_features=distilled_base --btf_base vmlinux nvidia-modeset.ko -j1
>> echo $?
>>
>> producing this output:
>> ===== 8< =====
>> dwarf_expr: unhandled 0x12 DW_OP_ operation
>> Unsupported DW_TAG_reference_type(0x10): type: 0x28172
>
> Domenico, Alan, Arnaldo,
>
> I was able to reproduce this error using the input files provided by
> Domenico [1][2].
>
> ./build/pahole -J --btf_features=encode_force,var,float,enum64,decl_tag,type_tag,optimized_func,consistent_func,decl_tag_kfuncs --btf_features=distilled_base --btf_base debian-repro/vmlinux debian-repro/nvidia-modeset.base.ko -j1
> dwarf_expr: unhandled 0x12 DW_OP_ operation
> Unsupported DW_TAG_reference_type(0x10): type: 0x28172
> Error while encoding BTF.
> libbpf: failed to find '.BTF' ELF section in debian-repro/nvidia-modeset.base.ko
> pahole: debian-repro/nvidia-modeset.base.ko: Invalid argument
>
>
> The unhandled tag points to src/common/displayport/src/dp_auxretry.cpp
> [3] of nvidia-modeset.base.ko
>
> Now, as far as I know, BTF can't represent C++-style references
> directly (maybe indirectly with tags?).
>
> According to the code, pahole simply bails out in case it encounters
> `DW_TAG_reference_type` during BTF encoding. So the question is why
> BTF generation is even attempted for a module written in C++? It does
> not appear to be a supported use-case.
>
> Please correct me if I'm wrong about this.
>
> Alan, sorry for jumping into this uninvited. I trust you'll take over
> from here. Thanks!
>
> I've sent a patch with a fix for the hanging [4].
>
Thanks for the update; I'll test the fix shortly and I can reproduce
this failure at my end now too. For me, adding --lang_excude=c++11
resolves the issue and BTF encoding is successful, i.e. the following:
pahole -J
--btf_features=encode_force,var,float,enum64,decl_tag,type_tag,optimized_func,consistent_func,decl_tag_kfuncs
--btf_features=distilled_base --lang_exclude=rust,c++11 --btf_base
vmlinux nvidia-modeset.base.ko
...works. I see though there is some support in pahole for C++
constructs, so we should figure out what we can/should support too. In
this particular case, the reference types and rvalue reference types all
appear to be toplevel DWARF tags, e.g.
<1></>: Abbrev Number: 16 (DW_TAG_reference_type)
<2b6053> DW_AT_byte_size : 8
<2b6053> DW_AT_type : <0x2b5805>
...refers to a const type:
<2><2b5805>: Abbrev Number: 10 (DW_TAG_const_type)
<2b5806> DW_AT_type : <0x2b561f>
...which in turn refers to a buffer:
<2><2b561f>: Abbrev Number: 43 (DW_TAG_class_type)
<2b5620> DW_AT_name : (indirect string, offset: 0x20bf79):
Buffer
<2b5624> DW_AT_byte_size : 24
<2b5625> DW_AT_decl_file : 10
<2b5626> DW_AT_decl_line : 38
<2b5627> DW_AT_decl_column : 11
<2b5627> DW_AT_sibling : <0x2b5805>
I tried doing the simple thing and skipping them for BTF encoding and we
end up falling over during deduplication, so that tells us they are
getting swept up into the type hierarchy.
Further investigation shows the DW_TAG_subprogram associated with Buffer
refers to the above reference type as a formal parameter.
I think these are class constructors, possibly for
https://github.com/NVIDIA/open-gpu-kernel-modules/blob/main/src/common/displayport/inc/dp_buffer.h
so
Buffer(const Buffer & other);
The & connotes a reference type in C++ I think (&& is a rvalue reference
type, which we'd need to handle too)..
> [1] https://bugs.debian.org/cgi-bin/bugreport.cgi?att=1;bug=1100503;filename=vmlinux.zst;msg=19
> [2] https://bugs.debian.org/cgi-bin/bugreport.cgi?att=1;bug=1100503;filename=nvidia-modeset.base.ko.zst;msg=12
> [3] https://github.com/NVIDIA/open-gpu-kernel-modules/blob/main/src/common/displayport/src/dp_auxretry.cpp
> [4] https://lore.kernel.org/bpf/20250328174003.3945581-1-ihor.solodrai@xxxxxxxxx/
>
>> Error while encoding BTF.
>> 0
>> ===== >8 =====
>>
>> [...]
>
