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

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Message-ID: <892F3434-8DB2-438B-8A1A-314F39A2B4BD@fb.com>
Date:   Fri, 8 Jul 2022 04:53:00 +0000
From:   Song Liu <songliubraving@...com>
To:     Pu Lehui <pulehui@...wei.com>
CC:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, Martin Lau <kafai@...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 Jul 7, 2022, at 8:12 PM, Pu Lehui <pulehui@...wei.com> wrote:
> 
> 
> 
> On 2022/7/8 10:46, Pu Lehui wrote:
>> On 2022/7/8 3:12, Song Liu wrote:
>>> 
>>> 
>>>> On Jul 7, 2022, at 7:08 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/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
>>> 
>>> Why do we need the -include here?
>>> 
>> HOSTLD is defined in tools/scripts/Makefile.include, we need to add it.
>> And for -include, mainly to resolve some conflicts:
>> 1. If workdir is kernel_src, then 'include tools/scripts/Makefile.include' is fine when 'make M=samples/bpf'.
>> 2. Since the trick in samples/bpf/Makefile:
>> # Trick to allow make to be run from this directory
>> all:
>> $(MAKE) -C ../../ M=$(CURDIR) BPF_SAMPLES_PATH=$(CURDIR)
>> If workdir is samples/bpf, the compile process will first load the Makefile in samples/bpf, then change workdir to kernel_src and load the kernel_src's Makefile. So if we just add 'include tools/scripts/Makefile.include', then the first load will occur error for not found the file, so we add -include to skip the first load.
> 
> sorry, correct the reply, so we add -include to skip the 'tools/scripts/Makefile.include' file on the fisrt load.


Thanks for the explanation. 

Acked-by: Song Liu <song@...nel.org>

> 
>>> Thanks,
>>> Song
>>> 
>>>> 
>>>> 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
>>>> 
>>> 
>>> .
>>> 
>> .