Re: [PATCH bpf v2] bpf: support pre-2.25-binutils objcopy for vmlinux BTF

[Stanislav Fomichev <sdf@fomichev.me>]

On 11/26, Andrii Nakryiko wrote:
> On 11/26/19 3:45 PM, Stanislav Fomichev wrote:
> > On 11/26, John Fastabend wrote:
> >> Stanislav Fomichev 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)
> >>>
> >>> Cc: Andrii Nakryiko <andriin@fb.com>
> >>> Cc: John Fastabend <john.fastabend@gmail.com>
> >>> Fixes: 341dfcf8d78ea ("btf: expose BTF info through sysfs")
> >>> 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"
> >>
> >> I think we should encourage upgrading binutils first? Maybe
> >>
> >> "binutils 2.25+ required for BTF please upgrade or disable CONFIG_DEBUG_INFO_BTF"
> >>
> >> otherwise I guess its going to be a bit mystical why it works in
> >> cases and not others to folks unfamiliar with the details.
> > With the conversion from --dump-section to --only-section that I
> > did in this patch, binutils 2.25+ is no longer a requirement.
> > 2.21 (minimal version from Documentation/process/changes.rst) should work
> > just fine.
> 
> Yeah, instead it's better to mention that pahole v1.13+ is required.
We already have most of the messages about missing pahole or wrong version:
https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/scripts/link-vmlinux.sh#n111
https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/scripts/link-vmlinux.sh#n117

They are 'info' though, but it seems logical to drop -s from make arguments
if someone wants to debug further.