Re: pahole generates invalid BTF for code compiled with recent clang

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On Wed, Jun 24, 2020 at 4:07 AM Lorenz Bauer <lmb@xxxxxxxxxxxxxx> wrote:
>
> Hi,
>
> If pahole -J is used on an ELF that has BTF info from clang, it
> produces an invalid
> output. This is because pahole rewrites the .BTF section (which
> includes a new string
> table) but it doesn't touch .BTF.ext at all.

Why do you run `pahole -J` on BPF .o file? Clang already generates
.BTF (and .BTF.ext, of course) for you.

pahole -J is supposed to be used for vmlinux, not for clang-compiled
-target BPF object files.

>
> To demonstrate, on a recent check out of bpf-next:
>     $ cp connect4_prog.o connect4_pahole.o
>     $ pahole -J connect4_pahole.o
>     $ llvm-objcopy-10 --dump-section .BTF=pahole-btf.bin
> --dump-section .BTF.ext=pahole-btf-ext.bin connect4_pahole.o
>     $ llvm-objcopy-10 --dump-section .BTF=btf.bin --dump-section
> .BTF.ext=btf-ext.bin connect4_prog.o
>     $ sha1sum *.bin
>     1b5c7407dd9fd13f969931d32f6b864849e66a68  btf.bin
>     4c43efcc86d3cd908ddc77c15fc4a35af38d842b  btf-ext.bin
>     2a60767a3a037de66a8d963110601769fa0f198e  pahole-btf.bin
>     4c43efcc86d3cd908ddc77c15fc4a35af38d842b  pahole-btf-ext.bin
>
> This problem crops up when compiling old kernels like 4.19 which have
> an extra pahole
> build step with clang-10.

I was under impression that clang generates .BTF and .BTF.ext only for
-target BPF. In this case, kernel is compiled for "real" target arch,
so there shouldn't be .BTF.ext in the first place? If that's not the
case, then I guess it's a bug in Clang.

>
> I think a possible fix is to strip .BTF.ext if .BTF is rewritten.
>
> Best
> Lorenz
>
> --
> Lorenz Bauer  |  Systems Engineer
> 6th Floor, County Hall/The Riverside Building, SE1 7PB, UK
>
> www.cloudflare.com



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