linux-kernel - Re: [RFC PATCH] bpf: preload: Fix build error when O= is set

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Date:   Fri, 20 Nov 2020 23:37:54 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     David Gow <davidgow@...gle.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Brendan Higgins <brendanhiggins@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [RFC PATCH] bpf: preload: Fix build error when O= is set

On Thu, Nov 19, 2020 at 12:51 AM David Gow <davidgow@...gle.com> wrote:
>
> If BPF_PRELOAD is enabled, and an out-of-tree build is requested with
> make O=<path>, compilation seems to fail with:
>
> tools/scripts/Makefile.include:4: *** O=.kunit does not exist.  Stop.
> make[4]: *** [../kernel/bpf/preload/Makefile:8: kernel/bpf/preload/libbpf.a] Error 2
> make[3]: *** [../scripts/Makefile.build:500: kernel/bpf/preload] Error 2
> make[2]: *** [../scripts/Makefile.build:500: kernel/bpf] Error 2
> make[2]: *** Waiting for unfinished jobs....
> make[1]: *** [.../Makefile:1799: kernel] Error 2
> make[1]: *** Waiting for unfinished jobs....
> make: *** [Makefile:185: __sub-make] Error 2
>
> By the looks of things, this is because the (relative path) O= passed on
> the command line is being passed to the libbpf Makefile, which then
> can't find the directory. Given OUTPUT= is being passed anyway, we can
> work around this by explicitly setting an empty O=, which will be
> ignored in favour of OUTPUT= in tools/scripts/Makefile.include.

Strange, but I can't repro it. I use make O=<absolute path> all the
time with no issues. I just tried specifically with a make O=.build,
where .build is inside Linux repo, and it still worked fine. See also
be40920fbf10 ("tools: Let O= makes handle a relative path with -C
option") which was supposed to address such an issue. So I'm wondering
what exactly is causing this problem.

>
> Signed-off-by: David Gow <davidgow@...gle.com>
> ---
>
> Hi all,
>
> I'm not 100% sure this is the correct fix here -- it seems to work for
> me, and makes some sense, but let me know if there's a better way.
>
> One other thing worth noting is that I've been hitting this with
> make allyesconfig on ARCH=um, but there's a comment in the Kconfig
> suggesting that, because BPF_PRELOAD depends on !COMPILE_TEST, that
> maybe it shouldn't be being built at all. I figured that it was worth
> trying to fix this anyway.
>
> Cheers,
> -- David
>
>
>  kernel/bpf/preload/Makefile | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/preload/Makefile b/kernel/bpf/preload/Makefile
> index 23ee310b6eb4..39848d296097 100644
> --- a/kernel/bpf/preload/Makefile
> +++ b/kernel/bpf/preload/Makefile
> @@ -5,7 +5,7 @@ LIBBPF_A = $(obj)/libbpf.a
>  LIBBPF_OUT = $(abspath $(obj))
>
>  $(LIBBPF_A):
> -       $(Q)$(MAKE) -C $(LIBBPF_SRCS) OUTPUT=$(LIBBPF_OUT)/ $(LIBBPF_OUT)/libbpf.a
> +       $(Q)$(MAKE) -C $(LIBBPF_SRCS) O= OUTPUT=$(LIBBPF_OUT)/ $(LIBBPF_OUT)/libbpf.a
>
>  userccflags += -I $(srctree)/tools/include/ -I $(srctree)/tools/include/uapi \
>         -I $(srctree)/tools/lib/ -Wno-unused-result
> --
> 2.29.2.454.gaff20da3a2-goog
>