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From:   Brendan Higgins <brendanhiggins@google.com>
Date:   Thu, 19 Nov 2020 13:14:02 -0800
Message-ID: <CAFd5g45MO-F-Jw9oRzdvOHJ0+19OQWDC-HwgRDwz2iPrkXJAww@mail.gmail.com>
Subject: Re: [RFC PATCH] bpf: preload: Fix build error when O= is set
To:     David Gow <davidgow@google.com>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        =?UTF-8?B?VG9rZSBIw7hpbGFuZC1Kw7hyZ2Vuc2Vu?= <toke@redhat.com>,
        netdev@vger.kernel.org, bpf@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Thu, Nov 19, 2020 at 12:50 AM David Gow <davidgow@google.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.
>
> Signed-off-by: David Gow <davidgow@google.com>

Seems sensible to me. I have no strong feeling as to whether we just
turn this off on UML or whether we do the fix you proposed here
though. Nevertheless, I would like to see *some* fix go in before
v5.10 is released.

Reviewed-by: Brendan Higgins <brendanhiggins@google.com>