Message-ID: <6a15aa00-5649-42e5-1c97-2e2985891607@isovalent.com>
Date:   Tue, 26 Jan 2021 11:24:51 +0000
From:   Quentin Monnet <quentin@...valent.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Brendan Higgins <brendanhiggins@...gle.com>,
        David Gow <davidgow@...gle.com>
Subject: Re: [PATCH] bpf: fix build for BPF preload when $(O) points to a
 relative path

2021-01-25 16:32 UTC-0800 ~ Andrii Nakryiko <andrii.nakryiko@...il.com>
> On Mon, Jan 25, 2021 at 7:49 AM Quentin Monnet <quentin@...valent.com> wrote:
>>
>> Building the kernel with CONFIG_BPF_PRELOAD, and by providing a relative
>> path for the output directory, may fail with the following error:
>>
>>   $ make O=build bindeb-pkg
>>   ...
>>   /.../linux/tools/scripts/Makefile.include:5: *** O=build does not exist.  Stop.
>>   make[7]: *** [/.../linux/kernel/bpf/preload/Makefile:9: kernel/bpf/preload/libbpf.a] Error 2
>>   make[6]: *** [/.../linux/scripts/Makefile.build:500: kernel/bpf/preload] Error 2
>>   make[5]: *** [/.../linux/scripts/Makefile.build:500: kernel/bpf] Error 2
>>   make[4]: *** [/.../linux/Makefile:1799: kernel] Error 2
>>   make[4]: *** Waiting for unfinished jobs....
>>
>> In the case above, for the "bindeb-pkg" target, the error is produced by
>> the "dummy" check in Makefile.include, called from libbpf's Makefile.
>> This check changes directory to $(PWD) before checking for the existence
>> of $(O). But at this step we have $(PWD) pointing to "/.../linux/build",
>> and $(O) pointing to "build". So the Makefile.include tries in fact to
>> assert the existence of a directory named "/.../linux/build/build",
>> which does not exist.
>>
>> By contrast, other tools called from the main Linux Makefile get the
>> variable set to $(abspath $(objtree)), where $(objtree) is ".". We can
>> update the Makefile for kernel/bpf/preload to set $(O) to the same
>> value, to permit compiling with a relative path for output. Note that
>> apart from the Makefile.include, the variable $(O) is not used in
>> libbpf's build system.
>>
>> Note that the error does not occur for all make targets and
>> architectures combinations.
>>
>> - On x86, "make O=build vmlinux" appears to work fine.
>>   $(PWD) points to "/.../linux/tools", but $(O) points to the absolute
>>   path "/.../linux/build" and the test succeeds.
>> - On UML, it has been reported to fail with a message similar to the
>>   above (see [0]).
>> - On x86, "make O=build bindeb-pkg" fails, as described above.
>>
>> It is unsure where the different values for $(O) and $(PWD) come from
>> (likely some recursive make with different arguments at some point), and
>> because several targets are broken, it feels safer to fix the $(O) value
>> passed to libbpf rather than to hunt down all changes to the variable.
>>
>> David Gow previously posted a slightly different version of this patch
>> as a RFC [0], two months ago or so.
>>
>> [0] https://lore.kernel.org/bpf/20201119085022.3606135-1-davidgow@google.com/t/#u
>>
>> Cc: Andrii Nakryiko <andrii.nakryiko@...il.com>
>> Cc: Brendan Higgins <brendanhiggins@...gle.com>
>> Cc: David Gow <davidgow@...gle.com>
>> Reported-by: David Gow <davidgow@...gle.com>
>> Signed-off-by: Quentin Monnet <quentin@...valent.com>
>> ---
> 
> I still think it would benefit everyone to figure out where this is
> breaking (given Linux Makefile explicitly tries to handle such
> relative path situation for O=, I believe), but this is trivial
> enough, so:
> 
> Acked-by: Andrii Nakryiko <andrii@...nel.org>

Agreed, I'll try to spend a bit more time on this when I can. But it
would be nice to have the fix in the meantime. Thanks for the review and
ack.

> 
> BTW, you haven't specified which tree you intended it for.

Oops! I _knew_ I was missing something, sorry. This build issue is here
since eBPF preload was introduced, so I meant to send to the *bpf* tree.

Because it does not concern the major build targets, I was not sure if a
"Fixes:" tag would be appropriate. If we want one, it should be for
d71fa5c9763c ("bpf: Add kernel module with user mode driver that
populates bpffs.")

> 
>>  kernel/bpf/preload/Makefile | 5 ++++-
>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/bpf/preload/Makefile b/kernel/bpf/preload/Makefile
>> index 23ee310b6eb4..11b9896424c0 100644
>> --- a/kernel/bpf/preload/Makefile
>> +++ b/kernel/bpf/preload/Makefile
>> @@ -4,8 +4,11 @@ LIBBPF_SRCS = $(srctree)/tools/lib/bpf/
>>  LIBBPF_A = $(obj)/libbpf.a
>>  LIBBPF_OUT = $(abspath $(obj))
>>
>> +# Set $(O) so that the "dummy" test in tools/scripts/Makefile.include, called
>> +# by libbpf's Makefile, succeeds when building the kernel with $(O) pointing to
>> +# a relative path, as in "make O=build bindeb-pkg".
>>  $(LIBBPF_A):
>> -       $(Q)$(MAKE) -C $(LIBBPF_SRCS) OUTPUT=$(LIBBPF_OUT)/ $(LIBBPF_OUT)/libbpf.a
>> +       $(Q)$(MAKE) -C $(LIBBPF_SRCS) O=$(abspath .) OUTPUT=$(LIBBPF_OUT)/ $(LIBBPF_OUT)/libbpf.a
> 
> why not O=$(LIBBPF_OUT), btw?

$(LIBBPF_OUT) points to /.../linux/ZZ_BUILD/build/kernel/bpf/preload.
This is an absolute path so the "dummy" check should work, too. I
preferred to align the value on the root Makefile, which has
"O=$(abspath $(objtree))" for target "tools/%", but no strong opinion
here. David would simply empty the variable in his patch. I'm fine with
any of the three versions. Would you prefer me to resend with $(LIBBPF_OUT)?

Thanks,
Quentin

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