linux-kernel - Re: [PATCH bpf-next 1/3] samples: bpf: Fix cross-compiling error by using bootstrap bpftool

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Date:   Wed, 13 Jul 2022 11:50:30 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Pu Lehui <pulehui@...wei.com>
Cc:     Quentin Monnet <quentin@...valent.com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>
Subject: Re: [PATCH bpf-next 1/3] samples: bpf: Fix cross-compiling error by
 using bootstrap bpftool

On Tue, Jul 12, 2022 at 4:32 AM Pu Lehui <pulehui@...wei.com> wrote:
>
>
>
> On 2022/7/12 18:11, Quentin Monnet wrote:
> > On 12/07/2022 04:08, Pu Lehui wrote:
> >> Currently, when cross compiling bpf samples, the host side cannot
> >> use arch-specific bpftool to generate vmlinux.h or skeleton. Since
> >> samples/bpf use bpftool for vmlinux.h, skeleton, and static linking
> >> only, we can use lightweight bootstrap version of bpftool to handle
> >> these, and it's always host-native.
> >>
> >> Signed-off-by: Pu Lehui <pulehui@...wei.com>
> >> Suggested-by: Andrii Nakryiko <andrii@...nel.org>
> >> ---
> >>   samples/bpf/Makefile | 16 +++++++++++-----
> >>   1 file changed, 11 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
> >> index 5002a5b9a7da..57012b8259d2 100644
> >> --- a/samples/bpf/Makefile
> >> +++ b/samples/bpf/Makefile
> >> @@ -282,12 +282,18 @@ $(LIBBPF): $(wildcard $(LIBBPF_SRC)/*.[ch] $(LIBBPF_SRC)/Makefile) | $(LIBBPF_OU
> >>
> >>   BPFTOOLDIR := $(TOOLS_PATH)/bpf/bpftool
> >>   BPFTOOL_OUTPUT := $(abspath $(BPF_SAMPLES_PATH))/bpftool
> >> -BPFTOOL := $(BPFTOOL_OUTPUT)/bpftool
> >> +BPFTOOL := $(BPFTOOL_OUTPUT)/bootstrap/bpftool
> >> +ifeq ($(CROSS_COMPILE),)
> >>   $(BPFTOOL): $(LIBBPF) $(wildcard $(BPFTOOLDIR)/*.[ch] $(BPFTOOLDIR)/Makefile) | $(BPFTOOL_OUTPUT)
> >> -        $(MAKE) -C $(BPFTOOLDIR) srctree=$(BPF_SAMPLES_PATH)/../../ \
> >> -            OUTPUT=$(BPFTOOL_OUTPUT)/ \
> >> -            LIBBPF_OUTPUT=$(LIBBPF_OUTPUT)/ \
> >> -            LIBBPF_DESTDIR=$(LIBBPF_DESTDIR)/
> >> +    $(MAKE) -C $(BPFTOOLDIR) srctree=$(BPF_SAMPLES_PATH)/../../             \
> >> +            OUTPUT=$(BPFTOOL_OUTPUT)/                                       \
> >> +            LIBBPF_BOOTSTRAP_OUTPUT=$(LIBBPF_OUTPUT)/                       \
> >> +            LIBBPF_BOOTSTRAP_DESTDIR=$(LIBBPF_DESTDIR)/ bootstrap
> >> +else
> >> +$(BPFTOOL): $(wildcard $(BPFTOOLDIR)/*.[ch] $(BPFTOOLDIR)/Makefile) | $(BPFTOOL_OUTPUT)
> >
> > Thanks for this! Just trying to fully understand the details here. When
> > cross-compiling, you leave aside the dependency on target-arch-libbpf,
> > so that "make -C <bpftool-dir> bootstrap" rebuilds its own host-arch
> > libbpf, is this correct?
> >
>
> You're right. libbpf may does get out-of-sync. So the best way is to
> compile both arch-specific libbpf simultaneously, and then attach to
> bpftool. But it will make this job more complicated. Could we just add
> back $(LIBBPF) to handle this?
>

Maybe let's keep it simple and let bpftool's Makefile deal with
cross-compile issue and building its own libbpf? So just request
bootstrap, but not try to share libbpf between samples/bpf and
bpftool? Especially that is this "samples", such complexity in
Makefile seems like a micro-optimization.

> >> +    $(MAKE) -C $(BPFTOOLDIR) srctree=$(BPF_SAMPLES_PATH)/../../             \
> >> +            OUTPUT=$(BPFTOOL_OUTPUT)/ bootstrap
> >> +endif
> >>
> >>   $(LIBBPF_OUTPUT) $(BPFTOOL_OUTPUT):
> >>      $(call msg,MKDIR,$@)
> >
> > .
> >