netdev - [PATCH bpf] bpf: Support llvm-objcopy and llvm-objdump for vmlinux BTF

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Date:   Mon, 16 Mar 2020 18:16:54 -0700
From:   Fangrui Song <maskray@...gle.com>
To:     bpf@...r.kernel.org, netdev@...r.kernel.org
Cc:     daniel@...earbox.net, Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        clang-built-linux@...glegroups.com,
        Stanislav Fomichev <sdf@...gle.com>, davem@...emloft.net,
        ast@...nel.org
Subject: [PATCH bpf] 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 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@...gle.com>
Cc: Nick Desaulniers <ndesaulniers@...gle.com>
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: Fangrui Song <maskray@...gle.com>
---
 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}
 
-	# 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