netdev - Re: [PATCH bpf-next v3 05/10] bpf: preload: install libbpf headers when building

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Date:   Wed, 6 Oct 2021 11:21:59 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Quentin Monnet <quentin@...valent.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next v3 05/10] bpf: preload: install libbpf headers
 when building

On Sun, Oct 3, 2021 at 12:22 PM Quentin Monnet <quentin@...valent.com> wrote:
>
> API headers from libbpf should not be accessed directly from the
> library's source directory. Instead, they should be exported with "make
> install_headers". Let's make sure that bpf/preload/Makefile installs the
> headers properly when building.
>
> Note that we declare an additional dependency for iterators/iterators.o:
> having $(LIBBPF_A) as a dependency to "$(obj)/bpf_preload_umd" is not
> sufficient, as it makes it required only at the linking step. But we
> need libbpf to be compiled, and in particular its headers to be
> exported, before we attempt to compile iterators.o. The issue would not
> occur before this commit, because libbpf's headers were not exported and
> were always available under tools/lib/bpf.
>
> Signed-off-by: Quentin Monnet <quentin@...valent.com>
> ---
>  kernel/bpf/preload/Makefile | 25 ++++++++++++++++++++-----
>  1 file changed, 20 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/bpf/preload/Makefile b/kernel/bpf/preload/Makefile
> index 1951332dd15f..efccf857f7ed 100644
> --- a/kernel/bpf/preload/Makefile
> +++ b/kernel/bpf/preload/Makefile
> @@ -1,21 +1,36 @@
>  # SPDX-License-Identifier: GPL-2.0
>
>  LIBBPF_SRCS = $(srctree)/tools/lib/bpf/
> -LIBBPF_A = $(obj)/libbpf.a
> -LIBBPF_OUT = $(abspath $(obj))
> +LIBBPF_OUT = $(abspath $(obj))/libbpf
> +LIBBPF_A = $(LIBBPF_OUT)/libbpf.a
> +LIBBPF_DESTDIR = $(LIBBPF_OUT)
> +LIBBPF_INCLUDE = $(LIBBPF_DESTDIR)/include
>
>  # Although not in use by libbpf's Makefile, set $(O) so that the "dummy" test
>  # in tools/scripts/Makefile.include always succeeds when building the kernel
>  # with $(O) pointing to a relative path, as in "make O=build bindeb-pkg".
> -$(LIBBPF_A):
> -       $(Q)$(MAKE) -C $(LIBBPF_SRCS) O=$(LIBBPF_OUT)/ OUTPUT=$(LIBBPF_OUT)/ $(LIBBPF_OUT)/libbpf.a
> +$(LIBBPF_A): | $(LIBBPF_OUT)
> +       $(Q)$(MAKE) -C $(LIBBPF_SRCS) O=$(LIBBPF_OUT)/ OUTPUT=$(LIBBPF_OUT)/   \
> +               DESTDIR=$(LIBBPF_DESTDIR) prefix=                              \
> +               $(LIBBPF_OUT)/libbpf.a install_headers
> +
> +libbpf_hdrs: $(LIBBPF_A)
> +
> +.PHONY: libbpf_hdrs
> +
> +$(LIBBPF_OUT):
> +       $(call msg,MKDIR,$@)
> +       $(Q)mkdir -p $@
>
>  userccflags += -I $(srctree)/tools/include/ -I $(srctree)/tools/include/uapi \
> -       -I $(srctree)/tools/lib/ -Wno-unused-result
> +       -I $(LIBBPF_INCLUDE) -Wno-unused-result
>
>  userprogs := bpf_preload_umd
>
>  clean-files := $(userprogs) bpf_helper_defs.h FEATURE-DUMP.libbpf staticobjs/ feature/
> +clean-files += $(LIBBPF_OUT) $(LIBBPF_DESTDIR)
> +
> +$(obj)/iterators/iterators.o: libbpf_hdrs

this should probably be order-only dependency as well

>
>  bpf_preload_umd-objs := iterators/iterators.o
>  bpf_preload_umd-userldlibs := $(LIBBPF_A) -lelf -lz
> --
> 2.30.2
>