[PATCH bpf v3] bpf: support pre-2.25-binutils objcopy for vmlinux BTF

[PATCH bpf v3] bpf: support pre-2.25-binutils objcopy for vmlinux BTF

[Stanislav Fomichev]

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.

v2:
* exit and print an error if gen_btf fails (John Fastabend)

v3:
* resend with Andrii's Acked-by/Tested-by tags

Cc: Andrii Nakryiko <andriin@fb.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Fixes: 341dfcf8d78ea ("btf: expose BTF info through sysfs")
Acked-by: Andrii Nakryiko <andriin@fb.com>
Tested-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
---
 scripts/link-vmlinux.sh | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index 06495379fcd8..2998ddb323e3 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 ${2}
 }
@@ -253,6 +254,10 @@ btf_vmlinux_bin_o=""
 if [ -n "${CONFIG_DEBUG_INFO_BTF}" ]; then
 	if gen_btf .tmp_vmlinux.btf .btf.vmlinux.bin.o ; then
 		btf_vmlinux_bin_o=.btf.vmlinux.bin.o
+	else
+		echo >&2 "Failed to generate BTF for vmlinux"
+		echo >&2 "Try to disable CONFIG_DEBUG_INFO_BTF"
+		exit 1
 	fi
 fi
 
-- 
2.24.0.432.g9d3f5f5b63-goog

Re: [PATCH bpf v3] bpf: support pre-2.25-binutils objcopy for vmlinux BTF

[Alexei Starovoitov]

On Wed, Nov 27, 2019 at 8:14 AM Stanislav Fomichev <sdf@google.com> wrote:
>
> 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.
>
> v2:
> * exit and print an error if gen_btf fails (John Fastabend)
>
> v3:
> * resend with Andrii's Acked-by/Tested-by tags
>
> Cc: Andrii Nakryiko <andriin@fb.com>
> Cc: John Fastabend <john.fastabend@gmail.com>
> Fixes: 341dfcf8d78ea ("btf: expose BTF info through sysfs")
> Acked-by: Andrii Nakryiko <andriin@fb.com>
> Tested-by: Andrii Nakryiko <andriin@fb.com>
> Signed-off-by: Stanislav Fomichev <sdf@google.com>

Applied. Thanks