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

[Pu Lehui <pulehui@huawei.com>]

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@huawei.com>
>> Suggested-by: Andrii Nakryiko <andrii@kernel.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?

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