From 90ceddcb495008ac8ba7a3dce297841efcd7d584 Mon Sep 17 00:00:00 2001 From: Fangrui Song Date: Wed, 18 Mar 2020 15:27:46 -0700 Subject: bpf: Support llvm-objcopy for vmlinux BTF Simplify gen_btf logic to make it work with llvm-objcopy. The existing 'file format' and 'architecture' parsing logic is brittle and does not work with llvm-objcopy/llvm-objdump. 'file format' output of llvm-objdump>=11 will match GNU objdump, but 'architecture' (bfdarch) may not. .BTF in .tmp_vmlinux.btf is non-SHF_ALLOC. Add the SHF_ALLOC flag because it is part of vmlinux image used for introspection. C code can reference the section via linker script defined __start_BTF and __stop_BTF. This fixes a small problem that previous .BTF had the SHF_WRITE flag (objcopy -I binary -O elf* synthesized .data). Additionally, `objcopy -I binary` synthesized symbols _binary__btf_vmlinux_bin_start and _binary__btf_vmlinux_bin_stop (not used elsewhere) are replaced with more commonplace __start_BTF and __stop_BTF. Add 2>/dev/null because GNU objcopy (but not llvm-objcopy) warns "empty loadable segment detected at vaddr=0xffffffff81000000, is this intentional?" We use a dd command to change the e_type field in the ELF header from ET_EXEC to ET_REL so that lld will accept .btf.vmlinux.bin.o. Accepting ET_EXEC as an input file is an extremely rare GNU ld feature that lld does not intend to support, because this is error-prone. The output section description .BTF in include/asm-generic/vmlinux.lds.h avoids potential subtle orphan section placement issues and suppresses --orphan-handling=warn warnings. Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux") Fixes: cb0cc635c7a9 ("powerpc: Include .BTF section") Reported-by: Nathan Chancellor Signed-off-by: Fangrui Song Signed-off-by: Daniel Borkmann Tested-by: Stanislav Fomichev Tested-by: Andrii Nakryiko Reviewed-by: Stanislav Fomichev Reviewed-by: Kees Cook Acked-by: Andrii Nakryiko Acked-by: Michael Ellerman (powerpc) Link: https://github.com/ClangBuiltLinux/linux/issues/871 Link: https://lore.kernel.org/bpf/20200318222746.173648-1-maskray@google.com --- scripts/link-vmlinux.sh | 24 ++++++++++-------------- 1 file changed, 10 insertions(+), 14 deletions(-) (limited to 'scripts/link-vmlinux.sh') diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh index ac569e197bfa..d09ab4afbda4 100755 --- a/scripts/link-vmlinux.sh +++ b/scripts/link-vmlinux.sh @@ -113,9 +113,6 @@ vmlinux_link() gen_btf() { local pahole_ver - local bin_arch - local bin_format - local bin_file if ! [ -x "$(command -v ${PAHOLE})" ]; then echo >&2 "BTF: ${1}: pahole (${PAHOLE}) is not available" @@ -133,17 +130,16 @@ gen_btf() info "BTF" ${2} LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1} - # dump .BTF section into raw binary file to link with final vmlinux - bin_arch=$(LANG=C ${OBJDUMP} -f ${1} | grep architecture | \ - cut -d, -f1 | cut -d' ' -f2) - bin_format=$(LANG=C ${OBJDUMP} -f ${1} | grep 'file format' | \ - awk '{print $4}') - bin_file=.btf.vmlinux.bin - ${OBJCOPY} --change-section-address .BTF=0 \ - --set-section-flags .BTF=alloc -O binary \ - --only-section=.BTF ${1} $bin_file - ${OBJCOPY} -I binary -O ${bin_format} -B ${bin_arch} \ - --rename-section .data=.BTF $bin_file ${2} + # Create ${2} which contains just .BTF section but no symbols. Add + # SHF_ALLOC because .BTF will be part of the vmlinux image. --strip-all + # deletes all symbols including __start_BTF and __stop_BTF, which will + # be redefined in the linker script. Add 2>/dev/null to suppress GNU + # objcopy warnings: "empty loadable segment detected at ..." + ${OBJCOPY} --only-section=.BTF --set-section-flags .BTF=alloc,readonly \ + --strip-all ${1} ${2} 2>/dev/null + # Change e_type to ET_REL so that it can be used to link final vmlinux. + # Unlike GNU ld, lld does not allow an ET_EXEC input. + printf '\1' | dd of=${2} conv=notrunc bs=1 seek=16 status=none } # Create ${2} .o file with all symbols from the ${1} object file -- cgit