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

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Message-Id: <20200316222518.191601-1-sdf@google.com>
Date:   Mon, 16 Mar 2020 15:25:18 -0700
From:   Stanislav Fomichev <sdf@...gle.com>
To:     netdev@...r.kernel.org, bpf@...r.kernel.org
Cc:     davem@...emloft.net, ast@...nel.org, daniel@...earbox.net,
        Stanislav Fomichev <sdf@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>
Subject: [PATCH bpf] bpf: Support llvm-objcopy for vmlinux BTF

Commit da5fb18225b4 ("bpf: Support pre-2.25-binutils objcopy for vmlinux
BTF") switched from --dump-section to
--only-section/--change-section-address for BTF export assuming
those ("legacy") options should cover all objcopy versions.

Turns out llvm-objcopy doesn't implement --change-section-address [1],
but it does support --dump-section. Let's partially roll back and
try to use --dump-section first and fall back to
--only-section/--change-section-address for the older binutils.

1. https://bugs.llvm.org/show_bug.cgi?id=45217

Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux")
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: Stanislav Fomichev <sdf@...gle.com>
---
 scripts/link-vmlinux.sh | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index dd484e92752e..8ddf57cbc439 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -127,6 +127,16 @@ gen_btf()
 		cut -d, -f1 | cut -d' ' -f2)
 	bin_format=$(LANG=C ${OBJDUMP} -f ${1} | grep 'file format' | \
 		awk '{print $4}')
+
+	# Compatibility issues:
+	# - pre-2.25 binutils objcopy doesn't support --dump-section
+	# - llvm-objcopy doesn't support --change-section-address, but
+	#   does support --dump-section
+	#
+	# Try to use --dump-section which should cover both recent
+	# binutils and llvm-objcopy and fall back to --only-section
+	# for pre-2.25 binutils.
+	${OBJCOPY} --dump-section .BTF=$bin_file ${1} 2>/dev/null || \
 	${OBJCOPY} --change-section-address .BTF=0 \
 		--set-section-flags .BTF=alloc -O binary \
 		--only-section=.BTF ${1} .btf.vmlinux.bin
-- 
2.25.1.481.gfbce0eb801-goog