linux-kernel - Re: [PATCH 3/3] tools build: Correct bpf fixdep dependencies

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Message-ID: <ZoNTuvhq3tSNpXT4@google.com>
Date: Mon, 1 Jul 2024 18:11:22 -0700
From: Brian Norris <briannorris@...omium.org>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Arnaldo Carvalho de Melo <acme@...hat.com>,
	Namhyung Kim <namhyung@...nel.org>, Ian Rogers <irogers@...gle.com>,
	Thomas Richter <tmricht@...ux.ibm.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org,
	bpf@...r.kernel.org, linux-kbuild@...r.kernel.org,
	Masahiro Yamada <masahiroy@...nel.org>
Subject: Re: [PATCH 3/3] tools build: Correct bpf fixdep dependencies

On Mon, Jul 01, 2024 at 05:50:35PM -0700, Andrii Nakryiko wrote:
> On Mon, Jul 1, 2024 at 5:32 PM Brian Norris <briannorris@...omium.org> wrote:
> > --- a/tools/lib/bpf/Makefile
> > +++ b/tools/lib/bpf/Makefile
> > @@ -153,7 +153,11 @@ $(BPF_IN_SHARED): force $(BPF_GENERATED)
> >         echo "Warning: Kernel ABI header at 'tools/include/uapi/linux/if_xdp.h' differs from latest version at 'include/uapi/linux/if_xdp.h'" >&2 )) || true
> >         $(Q)$(MAKE) $(build)=libbpf OUTPUT=$(SHARED_OBJDIR) CFLAGS="$(CFLAGS) $(SHLIB_FLAGS)"
> >
> > -$(BPF_IN_STATIC): force $(BPF_GENERATED)
> > +$(STATIC_OBJDIR):
> > +       $(Q)mkdir -p $@
> > +
> > +$(BPF_IN_STATIC): force $(BPF_GENERATED) | $(STATIC_OBJDIR)
> 
> wouldn't $(BPF_IN_SHARED) target need a similar treatment?

Hmm, probably. I'll admit, I only debugged errors that show up in the
top-level kernel build. The only tools/bpf stuff tied to the top-level
build is resolve_btfids, which uses the static library, not the shared.

And now that I poke around at some other build targets, this highlights
that my patch introduced some problems with the independent libbpf
build. Particularly, $(OUTPUT) is a relative path in some cases, and
that relative path gets interpreted differently in recursive make (when
we change cwd). So please don't accept this patch as-is.

Brian

> > +       $(SILENT_MAKE) -C $(srctree)/tools/build CFLAGS= LDFLAGS= OUTPUT=$(STATIC_OBJDIR) $(STATIC_OBJDIR)fixdep
> >         $(Q)$(MAKE) $(build)=libbpf OUTPUT=$(STATIC_OBJDIR)
> >
> >  $(BPF_HELPER_DEFS): $(srctree)/tools/include/uapi/linux/bpf.h