linux-kernel - Re: [PATCH bpf-next] samples: bpf: Fix cross-compiling error about bpftool

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Message-ID: <CAEf4Bzb_re+o2zALCA+Rf_cJS-31350PjhzRg42bgW0mO-GVbg@mail.gmail.com>
Date:   Fri, 8 Jul 2022 15:42:54 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Pu Lehui <pulehui@...wei.com>,
        Quentin Monnet <quentin@...valent.com>
Cc:     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>
Subject: Re: [PATCH bpf-next] samples: bpf: Fix cross-compiling error about bpftool

On Thu, Jul 7, 2022 at 6:37 AM Pu Lehui <pulehui@...wei.com> wrote:
>
> Currently, when cross compiling bpf samples, the host side
> cannot use arch-specific bpftool to generate vmlinux.h or
> skeleton. We need to compile the bpftool with the host
> compiler.
>
> Signed-off-by: Pu Lehui <pulehui@...wei.com>
> ---

samples/bpf use bpftool for vmlinux.h, skeleton, and static linking
only. All that is supported by lightweight "bootstrap" bpftool
version, so we can build just that. It will be faster, and bootstrap
version should be always host-native even during cross compilation.
See [0] for what I did in libbpf-bootstrap.

Also please cc Quention for bpftool-related changes. Thanks!

   [0] https://github.com/libbpf/libbpf-bootstrap/commit/fc28424eb3f0e39cfb5959296b070389b9a8bd8f

>  samples/bpf/Makefile | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
> index 5002a5b9a7da..fe54a8c8f312 100644
> --- a/samples/bpf/Makefile
> +++ b/samples/bpf/Makefile
> @@ -1,4 +1,5 @@
>  # SPDX-License-Identifier: GPL-2.0
> +-include tools/scripts/Makefile.include
>
>  BPF_SAMPLES_PATH ?= $(abspath $(srctree)/$(src))
>  TOOLS_PATH := $(BPF_SAMPLES_PATH)/../../tools
> @@ -283,11 +284,10 @@ $(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): $(LIBBPF) $(wildcard $(BPFTOOLDIR)/*.[ch] $(BPFTOOLDIR)/Makefile) | $(BPFTOOL_OUTPUT)
> +$(BPFTOOL): $(wildcard $(BPFTOOLDIR)/*.[ch] $(BPFTOOLDIR)/Makefile) | $(BPFTOOL_OUTPUT)
>             $(MAKE) -C $(BPFTOOLDIR) srctree=$(BPF_SAMPLES_PATH)/../../ \
> -               OUTPUT=$(BPFTOOL_OUTPUT)/ \
> -               LIBBPF_OUTPUT=$(LIBBPF_OUTPUT)/ \
> -               LIBBPF_DESTDIR=$(LIBBPF_DESTDIR)/
> +               ARCH= CROSS_COMPILE= CC=$(HOSTCC) LD=$(HOSTLD) \
> +               OUTPUT=$(BPFTOOL_OUTPUT)/
>
>  $(LIBBPF_OUTPUT) $(BPFTOOL_OUTPUT):
>         $(call msg,MKDIR,$@)
> --
> 2.25.1
>