linux-kernel - Re: [PATCH net-next v1] samples/bpf: Partially fixes the bpf.o build

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Message-ID: <0a92b999-c65a-0b5d-8e44-872af7003d4a@digikod.net>
Date:   Fri, 26 Jan 2018 09:30:26 +0100
From:   Mickaël Salaün <mic@...ikod.net>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     linux-kernel@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "David S . Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next v1] samples/bpf: Partially fixes the bpf.o build


On 26/01/2018 03:16, Alexei Starovoitov wrote:
> On Fri, Jan 26, 2018 at 01:39:30AM +0100, Mickaël Salaün wrote:
>> Do not build lib/bpf/bpf.o with this Makefile but use the one from the
>> library directory.  This avoid making a buggy bpf.o file (e.g. missing
>> symbols).
> 
> could you provide an example?
> What symbols will be missing?
> I don't think there is an issue with existing Makefile.

You can run this commands:
make -C samples/bpf; nm tools/lib/bpf/bpf.o > a; make -C tools/lib/bpf;
nm tools/lib/bpf/bpf.o > b; diff -u a b

Symbols like bzero and sys_bpf are missing with the samples/bpf
Makefile, which makes the bpf.o shrink from 25K to 7K.

> 
>> This patch is useful if some code (e.g. Landlock tests) needs both the
>> bpf.o (from tools/lib/bpf) and the bpf_load.o (from samples/bpf).
> 
> is that some future patches?

Yes, I'll send them next week.

> 
> we're trying to move everything form samples/bpf/ into selftests/bpf/
> and convert to use libbpf.a instead of obsolete bpf_load.c
> Please use this approach for landlock as well.

Ok, it should be better with this lib.

> 
>> Signed-off-by: Mickaël Salaün <mic@...ikod.net>
>> Cc: Alexei Starovoitov <ast@...nel.org>
>> Cc: Daniel Borkmann <daniel@...earbox.net>
>> ---
>>
>> This is not a complet fix because the call to multi_depend with
>> $(host-cmulti) from scripts/Makefile.host force the build of bpf.o
>> anyway. I'm not sure how to completely avoid this automatic build
>> though.
>> ---
>>  samples/bpf/Makefile | 5 ++++-
>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
>> index 7f61a3d57fa7..64335bb94f9f 100644
>> --- a/samples/bpf/Makefile
>> +++ b/samples/bpf/Makefile
>> @@ -201,13 +201,16 @@ CLANG_ARCH_ARGS = -target $(ARCH)
>>  endif
>>  
>>  # Trick to allow make to be run from this directory
>> -all:
>> +all: $(LIBBPF)
>>  	$(MAKE) -C ../../ $(CURDIR)/
>>  
>>  clean:
>>  	$(MAKE) -C ../../ M=$(CURDIR) clean
>>  	@rm -f *~
>>  
>> +$(LIBBPF): FORCE
>> +	$(MAKE) -C $(dir $@) $(notdir $@)
>> +
>>  $(obj)/syscall_nrs.s:	$(src)/syscall_nrs.c
>>  	$(call if_changed_dep,cc_s_c)
>>  
>> -- 
>> 2.15.1
>>
> 



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