netdev - Re: [PATCH bpf-next v2 6/9] bpf: iterators: install libbpf headers when building

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Date:   Mon, 4 Oct 2021 12:10:57 -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 v2 6/9] bpf: iterators: install libbpf headers
 when building

On Sat, Oct 2, 2021 at 3:12 PM Quentin Monnet <quentin@...valent.com> wrote:
>
> On Sat, 2 Oct 2021 at 21:27, Quentin Monnet <quentin@...valent.com> wrote:
> >
> > On Sat, 2 Oct 2021 at 00:20, Andrii Nakryiko <andrii.nakryiko@...il.com> wrote:
> > >
> > > On Fri, Oct 1, 2021 at 4:09 AM 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/iterators/Makefile
> > > > installs the headers properly when building.
> >
> > > >
> > > > -$(BPFOBJ): $(wildcard $(LIBBPF_SRC)/*.[ch] $(LIBBPF_SRC)/Makefile) | $(OUTPUT)
> > > > +$(BPFOBJ): $(wildcard $(LIBBPF_SRC)/*.[ch] $(LIBBPF_SRC)/Makefile)            \
> > > > +          | $(LIBBPF_OUTPUT) $(LIBBPF_INCLUDE)
> > >
> > > Would it make sense for libbpf's Makefile to create include and output
> > > directories on its own? We wouldn't need to have these order-only
> > > dependencies everywhere, right?
> >
> > Good point, I'll have a look at it.
> > Quentin
>
> So libbpf already creates the include (and parent $(DESTDIR))
> directory, so I can get rid of the related dependencies. But I don't
> see an easy solution for the output directory for the object files.
> The issue is that libbpf's Makefile includes
> tools/scripts/Makefile.include, which checks $(OUTPUT) and errors out

Did you check what benefits the use of tools/scripts/Makefile.include
brings? Last time I had to deal with some non-trivial Makefile
problem, this extra dance with tools/scripts/Makefile.include and some
related complexities didn't seem very justified. So unless there are
some very big benefits to having tool's Makefile.include included, I'd
rather simplify libbpf's in-kernel Makefile and make it more
straightforward. We have a completely independent separate Makefile
for libbpf in Github, and I think it's more straightforward. Doesn't
have to be done in this change, of course, but I was curious to hear
your thoughts given you seem to have spent tons of time on this
already.

> if the directory does not exist. This prevents us from creating the
> directory as part of the regular targets. We could create it
> unconditionally before running any target, but it's ugly; and I don't
> see any simple workaround.
>
> So I'll remove the deps on $(LIBBPF_INCLUDE) and keep the ones on
> $(LIBBPF_OUTPUT).