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Date: Mon, 16 Mar 2020 16:57:25 -0700 From: Nathan Chancellor <natechancellor@...il.com> To: Stanislav Fomichev <sdf@...gle.com> Cc: netdev@...r.kernel.org, bpf@...r.kernel.org, davem@...emloft.net, ast@...nel.org, daniel@...earbox.net, Nick Desaulniers <ndesaulniers@...gle.com> Subject: Re: [PATCH bpf] bpf: Support llvm-objcopy for vmlinux BTF On Mon, Mar 16, 2020 at 03:25:18PM -0700, Stanislav Fomichev wrote: > Commit da5fb18225b4 ("bpf: Support pre-2.25-binutils objcopy for vmlinux > BTF") switched from --dump-section to > --only-section/--change-section-address for BTF export assuming > those ("legacy") options should cover all objcopy versions. > > Turns out llvm-objcopy doesn't implement --change-section-address [1], > but it does support --dump-section. Let's partially roll back and > try to use --dump-section first and fall back to > --only-section/--change-section-address for the older binutils. > > 1. https://bugs.llvm.org/show_bug.cgi?id=45217 > > Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux") > Tested-by: Nick Desaulniers <ndesaulniers@...gle.com> > Reported-by: Nathan Chancellor <natechancellor@...il.com> > Link: https://github.com/ClangBuiltLinux/linux/issues/871 > Signed-off-by: Stanislav Fomichev <sdf@...gle.com> > --- > scripts/link-vmlinux.sh | 10 ++++++++++ > 1 file changed, 10 insertions(+) > > diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh > index dd484e92752e..8ddf57cbc439 100755 > --- a/scripts/link-vmlinux.sh > +++ b/scripts/link-vmlinux.sh > @@ -127,6 +127,16 @@ gen_btf() > cut -d, -f1 | cut -d' ' -f2) > bin_format=$(LANG=C ${OBJDUMP} -f ${1} | grep 'file format' | \ > awk '{print $4}') > + > + # Compatibility issues: > + # - pre-2.25 binutils objcopy doesn't support --dump-section > + # - llvm-objcopy doesn't support --change-section-address, but > + # does support --dump-section > + # > + # Try to use --dump-section which should cover both recent > + # binutils and llvm-objcopy and fall back to --only-section > + # for pre-2.25 binutils. > + ${OBJCOPY} --dump-section .BTF=$bin_file ${1} 2>/dev/null || \ > ${OBJCOPY} --change-section-address .BTF=0 \ > --set-section-flags .BTF=alloc -O binary \ > --only-section=.BTF ${1} .btf.vmlinux.bin > -- > 2.25.1.481.gfbce0eb801-goog > Hi Stanislav, Thank you for the patch! This is targeting the bpf tree but it uses the bin_file variable that comes from commit af73d78bd384 ("kbuild: Remove debug info from kallsyms linking") in the bpf-next tree. In this form, when applied to mainline, the first command fails because $bin_file doesn't exist and falls back to the second command, which results in no net change for llvm-objcopy. When manually applied to next-20200316 or applied to mainline with $bin_file replaced with .btf.vmlinux.bin, x86_64 and aarch64 successfully generate BTF with tip of tree llvm-objcopy. Tested-by: Nathan Chancellor <natechancellor@...il.com>
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