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/vmlinux.lds.h
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 <natechancellor@gmail.com>
Signed-off-by: Fangrui Song <maskray@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Stanislav Fomichev <sdf@google.com>
Tested-by: Andrii Nakryiko <andriin@fb.com>
Reviewed-by: Stanislav Fomichev <sdf@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Link: https://github.com/ClangBuiltLinux/linux/issues/871
Link: https://lore.kernel.org/bpf/20200318222746.173648-1-maskray@google.com

1 files changed, 10 insertions, 14 deletions

diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index ac569e197bfa..d09ab4afbda4 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -113,9 +113,6 @@ vmlinux_link()
 gen_btf()
 {
 	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" ${2}
 	LLVM_OBJCOPY=${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}')
-	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