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
