[PATCH bpf] bpf: support pre-2.25-binutils objcopy for vmlinux BTF

[Stanislav Fomichev <sdf@google.com>]

If vmlinux BTF generation fails, but CONFIG_DEBUG_INFO_BTF is set,
.BTF section of vmlinux is empty and kernel will prohibit
BPF loading and return "in-kernel BTF is malformed".

--dump-section argument to binutils' objcopy was added in version 2.25.
When using pre-2.25 binutils, BTF generation silently fails. Convert
to --only-section which is present on pre-2.25 binutils.

Documentation/process/changes.rst states that binutils 2.21+
is supported, not sure those standards apply to BPF subsystem.

Fixes: 341dfcf8d78ea ("btf: expose BTF info through sysfs")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
---
 scripts/link-vmlinux.sh | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index 06495379fcd8..c56ba91f52b0 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -127,7 +127,8 @@ gen_btf()
 		cut -d, -f1 | cut -d' ' -f2)
 	bin_format=$(LANG=C ${OBJDUMP} -f ${1} | grep 'file format' | \
 		awk '{print $4}')
-	${OBJCOPY} --dump-section .BTF=.btf.vmlinux.bin ${1} 2>/dev/null
+	${OBJCOPY} --set-section-flags .BTF=alloc -O binary \
+		--only-section=.BTF ${1} .btf.vmlinux.bin 2>/dev/null
 	${OBJCOPY} -I binary -O ${bin_format} -B ${bin_arch} \
 		--rename-section .data=.BTF .btf.vmlinux.bin ${2}
 }
-- 
2.24.0.432.g9d3f5f5b63-goog