netdev - Re: samples/bpf not working?

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Message-ID: <CAEf4BzbCoP6R0UbbW4HU6jqK8T3F-0SCTj_5ex8brnfCJLeBAA@mail.gmail.com>
Date:   Sat, 5 Oct 2019 11:27:01 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     "Daniel T. Lee" <danieltimlee@...il.com>
Cc:     bpf <bpf@...r.kernel.org>, xdp-newbies@...r.kernel.org,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        netdev <netdev@...r.kernel.org>
Subject: Re: samples/bpf not working?

On Fri, Oct 4, 2019 at 3:28 PM Daniel T. Lee <danieltimlee@...il.com> wrote:
>
> Currently, building the bpf samples isn't working.
> Running make from the directory 'samples/bpf' will just shows following
> result without compiling any samples.
>
> $ make
> make -C ../../ /git/linux/samples/bpf/ BPF_SAMPLES_PATH=/git/linux/samples/bpf
> make[1]: Entering directory '/git/linux'
>   CALL    scripts/checksyscalls.sh
>   CALL    scripts/atomic/check-atomics.sh
>   DESCEND  objtool
> make[1]: Leaving directory '/git/linux'
> $ ls *kern.o
> ls: cannot access '*kern.o': No such file or directory
>
> By using 'git bisect', found the problem is derived from below commit.
> commit 394053f4a4b3 ("kbuild: make single targets work more correctly")
>
> > Currently, the single target build directly descends into the directory
> > of the target. For example,
> >
> >     $ make foo/bar/baz.o
> >
> > ... directly descends into foo/bar/.
> >
> > On the other hand, the normal build usually descends one directory at
> > a time, i.e. descends into foo/, and then foo/bar/.
> >
> > This difference causes some problems.
> >
> > [...]
> >
> > This commit fixes those problems by making the single target build
> > descend in the same way as the normal build does.
>
> Not familiar with kbuild, so I'm not sure why this led to build failure.
> My humble guess is, samples/bpf/Makefile tries to run make from current
> directory, 'sample/bpf', but somehow upper commit changed the way it works.
>
> samples/bpf/Makefile:232
> # Trick to allow make to be run from this directory
> all:
>         $(MAKE) -C ../../ $(CURDIR)/ BPF_SAMPLES_PATH=$(CURDIR)
>
> I've tried to figure out the problem with 'make --trace', but not sure why
> it's not working. Just a hunch with build directory.
>
> Any ideas to fix this problem?

Yes, it's now a known problem. You need to run it as `make
M=samples/bpf` from root directory, as well as have all the recent
fixes both from bpf and bpf-next trees (:(, this will be a bit better
once bpf is merged into bpf-next).