On Wed, Jun 24, 2020 at 12:12 PM Arnaldo Carvalho de Melo <arnaldo.melo@xxxxxxxxx> wrote: > > Em Wed, Jun 24, 2020 at 12:06:24PM -0700, Andrii Nakryiko escreveu: > > On Wed, Jun 24, 2020 at 11:57 AM Arnaldo Carvalho de Melo > > <arnaldo.melo@xxxxxxxxx> wrote: > > > > > > Em Wed, Jun 24, 2020 at 11:40:21AM -0700, Andrii Nakryiko escreveu: > > > > On Wed, Jun 24, 2020 at 10:57 AM Arnaldo Carvalho de Melo > > > > <arnaldo.melo@xxxxxxxxx> wrote: > > > > > > > > > > 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. > > > > > > > > I disagree. Those who insist probably have some wrong conceptual > > > > understanding and it's better to fix that (understanding), rather than > > > > lose focus and bend tool to do what it's not supposed to do and > > > > ultimately cause more confusion. > > > > > > So we can instead notice the presence of .BTF.ext when the user calls > > > 'pahole -J' on a target BPF object file and bail out, only allowing it > > > to convert from DWARF to BTF and thus encode the .BTF elf section when > > > .BTF.ext isn't present, as we can't easily figure out if the present of > > > just .BTF section was done by clang or pahole on a BTF target object > > > file built without -g. > > > > Can't we check ELF's target machine and reject if it's a BPF one? > > I think there is value in allowing pahole to convert DWARF to BTF even > for a BPF target object file, say in some case people may think clang is > not generating correct BTF so one may want to see what pahole generates > and compare. sure, and will the warning for wrong architecture would give a hint that it's not the right thing to do, probably. Or we could have more specific message for BPF target. I don't care all that much. > > > Someday we might also support "cross-compilation" to be able to dedup > > arm ELF from x86 machine. It's sort of ok today for little-endian > > ARMs, so maybe not outright reject if architecture is not the same as > > the local one? > > I think outright reject if arch is not t he same it not necessary. > > We may warn the user that using -g in clang is the preferred method for > generating BTF, wdyt? > sounds reasonable > - Arnaldo > > > > > > > - Arnaldo > > > > > > > pahole's BTF conversion is really driven towards kernel use-case > > > > (e.g., with global variables, etc). I wouldn't distract ourselves with > > > > supporting de-duplicating BPF object files. Single .o's BTF is already > > > > deduplicated as produced by Clang. Once we add static linking of > > > > multiple BPF .o's (which I hope to start working on very soon), that > > > > de-duplication will be handled automatically by libbpf (and hopefully > > > > integrated into lld as well), among many other things that need to > > > > happen to make static linking work. > > > > > > > > > > > > > > - 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 > > > > > > -- > > > > > > - Arnaldo > > -- > > - Arnaldo
