Em Wed, Jun 24, 2020 at 10:41:10AM -0700, Andrii Nakryiko escreveu: > 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. yeah, I was thinking this was for a vmlinux generated by clang, which, from the commands below (the suffix _prog.o) should have told me this is a target BPF object file. But then, if one insists for some reason in generating BTF from the DWARF in a BPF target object file, stripping .BTF.ext, if present, is the right thing to do at this point. - Arnaldo > > > > 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 -- - Arnaldo
