pahole generates invalid BTF for code compiled with recent clang

[Lorenz Bauer <lmb@xxxxxxxxxxxxxx>]

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.

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 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

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