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

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Message-ID: <20180126021636.ghu7p7ds7yvkqato@ast-mbp>
Date:   Thu, 25 Jan 2018 18:16:37 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Mickaël Salaün <mic@...ikod.net>
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 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.

> 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?

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.

> 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
>