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

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Message-ID: <d44f8faf-06df-6fdf-0adf-2abbdf9c9a49@huawei.com>
Date:   Fri, 8 Jul 2022 11:12:33 +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 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,
>> 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
>>>
>>
>> .
>>
> .