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

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Message-ID: <c357fa1a-5160-ed85-19bf-51f3c188d56e@huawei.com>
Date:   Fri, 8 Jul 2022 10:46:21 +0800
From:   Pu Lehui <pulehui@...wei.com>
To:     Song Liu <songliubraving@...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 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.

> 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
>>
> 
> .
>