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

From: Stanislav Fomichev <sdf@fomichev.me>
To: John Fastabend <john.fastabend@gmail.com>
Cc: Stanislav Fomichev <sdf@google.com>,
	netdev@vger.kernel.org, bpf@vger.kernel.org, davem@davemloft.net,
	ast@kernel.org, daniel@iogearbox.net,
	Andrii Nakryiko <andriin@fb.com>
Subject: Re: [PATCH bpf v2] bpf: support pre-2.25-binutils objcopy for vmlinux BTF
Date: Tue, 26 Nov 2019 15:45:23 -0800	[thread overview]
Message-ID: <20191126234523.GF3145429@mini-arch.hsd1.ca.comcast.net> (raw)
In-Reply-To: <5dddb7059b13e_13b82abee0d625bc2d@john-XPS-13-9370.notmuch>

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.