On Wed, 24 Jun 2020 at 18:41, Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote: > > 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. You're missing the point. The kernel build system does it. Try the following: * Get the v4.19 sources * Make sure that clang --version is 10 * Make sure you have pahole (I used v1.17) * Build selftests The resulting object files will have bogus .BTF.ext sections due the bug I have described. Does it make sense to run pahole -J on these? No, but it still happens. I think it's reasonable to expect to get valid BPF ELFs out of this process. > > 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. connect4_prog.o is BPF: $ readelf -h connect4_prog.o | grep BPF Machine: Linux BPF Maybe I misunderstand what you're trying to say. Best Lorenz -- Lorenz Bauer | Systems Engineer 6th Floor, County Hall/The Riverside Building, SE1 7PB, UK www.cloudflare.com
