path: root/scripts/
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authorFangrui Song <>2020-03-18 15:27:46 -0700
committerDaniel Borkmann <>2020-03-19 12:32:38 +0100
commit90ceddcb495008ac8ba7a3dce297841efcd7d584 (patch)
tree16ba978c36201f3c8f02f4643243e5e6dc693c35 /scripts/
parent483d7a30f538e2f8addd32aa9a3d2e94ae55fa65 (diff)
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/ 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: Link:
Diffstat (limited to 'scripts/')
1 files changed, 10 insertions, 14 deletions
diff --git a/scripts/ b/scripts/
index ac569e197bfa..d09ab4afbda4 100755
--- a/scripts/
+++ b/scripts/
@@ -113,9 +113,6 @@ vmlinux_link()
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" ${2}
- # dump .BTF section into raw binary file to link with final vmlinux
- bin_arch=$(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