Re: [PATCH bpf] bpf: Support llvm-objcopy and llvm-objdump for vmlinux BTF
From: Andrii Nakryiko <hidden>
Date: 2020-03-17 03:10:47
Also in:
bpf
On Mon, Mar 16, 2020 at 6:17 PM Fangrui Song [off-list ref] wrote:
quoted hunk ↗ jump to hunk
Simplify gen_btf logic to make it work with llvm-objcopy and llvm-objdump. We just need to retain one section .BTF. To do so, we can use a simple objcopy --only-section=.BTF instead of jumping all the hoops via an architecture-less binary file. We use a dd comment to change the e_type field in the ELF header from ET_EXEC to ET_REL so that .btf.vmlinux.bin.o will be accepted by lld. Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux") Cc: Stanislav Fomichev <redacted> Cc: Nick Desaulniers <ndesaulniers@google.com> Tested-by: Nick Desaulniers <ndesaulniers@google.com> Reported-by: Nathan Chancellor <redacted> Link: https://github.com/ClangBuiltLinux/linux/issues/871 Signed-off-by: Fangrui Song <redacted> --- scripts/link-vmlinux.sh | 13 ++----------- 1 file changed, 2 insertions(+), 11 deletions(-)diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh index dd484e92752e..84be8d7c361d 100755 --- a/scripts/link-vmlinux.sh +++ b/scripts/link-vmlinux.sh@@ -120,18 +120,9 @@ gen_btf() info "BTF" ${2} vmlinux_link ${1} - LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}
Is it really tested? Seems like you just dropped .BTF generation step completely...
- # dump .BTF section into raw binary file to link with final vmlinux
- bin_arch=$(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}')
- ${OBJCOPY} --change-section-address .BTF=0 \
- --set-section-flags .BTF=alloc -O binary \
- --only-section=.BTF ${1} .btf.vmlinux.bin
- ${OBJCOPY} -I binary -O ${bin_format} -B ${bin_arch} \
- --rename-section .data=.BTF .btf.vmlinux.bin ${2}
+ # Extract .BTF section, change e_type to ET_REL, to link with final vmlinux
+ ${OBJCOPY} --only-section=.BTF ${1} ${2} && printf '\1' | dd of=${2} conv=notrunc bs=1 seek=16
}
# Create ${2} .o file with all symbols from the ${1} object file
--
2.25.1.481.gfbce0eb801-goog