On 2020-03-16, Andrii Nakryiko wrote:
On Mon, Mar 16, 2020 at 8:37 PM Fangrui Song <maskray@xxxxxxxxxx> wrote:On 2020-03-16, Andrii Nakryiko wrote: >On Mon, Mar 16, 2020 at 6:17 PM Fangrui Song <maskray@xxxxxxxxxx> wrote: >> >> 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 <sdf@xxxxxxxxxx> >> Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> >> Tested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> >> Reported-by: Nathan Chancellor <natechancellor@xxxxxxxxx> >> Link: https://github.com/ClangBuiltLinux/linux/issues/871 >> Signed-off-by: Fangrui Song <maskray@xxxxxxxxxx> >> --- >> 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... Sorry, dropped the whole line:/ I don't know how to test .BTF . I can only check readelf -S... Attached the new patch. From 02afb9417d4f0f8d2175c94fc3797a94a95cc248 Mon Sep 17 00:00:00 2001 From: Fangrui Song <maskray@xxxxxxxxxx> Date: Mon, 16 Mar 2020 18:02:31 -0700 Subject: [PATCH bpf v2] bpf: Support llvm-objcopy and llvm-objdump for vmlinux BTF Simplify gen_btf logic to make it work with llvm-objcopy and llvm-objdump. 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 can be accepted by lld. Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux") Cc: Stanislav Fomichev <sdf@xxxxxxxxxx> Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> Reported-by: Nathan Chancellor <natechancellor@xxxxxxxxx> Link: https://github.com/ClangBuiltLinux/linux/issues/871 Signed-off-by: Fangrui Song <maskray@xxxxxxxxxx> --- scripts/link-vmlinux.sh | 14 +++----------- 1 file changed, 3 insertions(+), 11 deletions(-) diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh index dd484e92752e..b23313944c89 100755 --- a/scripts/link-vmlinux.sh +++ b/scripts/link-vmlinux.sh @@ -120,18 +120,10 @@ gen_btf() info "BTF" ${2} vmlinux_link ${1} - LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1} + ${PAHOLE} -J ${1}I'm not sure why you are touching this line at all. LLVM_OBJCOPY part is necessary, pahole assumes llvm-objcopy by default, but that can (and should for objcopy) be overridden with LLVM_OBJCOPY.
Why is LLVM_OBJCOPY assumed? What if llvm-objcopy is not available? This is confusing that one tool assumes llvm-objcopy while the block below immediately uses GNU objcopy (without this patch). e83b9f55448afce3fe1abcd1d10db9584f8042a6 "kbuild: add ability to generate BTF type info for vmlinux" does not say why LLVM_OBJCOPY is set.
- # dump .BTF section into raw binary file to link with final vmlinux - bin_arch=$(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
