Re: parallel pahole hangs while building modules from nvidia-open-kernel-dkms

["Ihor Solodrai" <ihor.solodrai@linux.dev>]

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].

[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@linux.dev/

> Error while encoding BTF.
> 0
> ===== >8 =====
>
> [...]