linux-kernel - Re: [PATCH v1] tools/resolve_btfids: Install subcmd headers

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Date:   Mon, 16 Jan 2023 13:15:11 -0800
From:   Ian Rogers <irogers@...gle.com>
To:     Jiri Olsa <olsajiri@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>,
        Connor OBrien <connoro@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] tools/resolve_btfids: Install subcmd headers

On Mon, Jan 16, 2023 at 10:20 AM Jiri Olsa <olsajiri@...il.com> wrote:
>
> On Mon, Jan 16, 2023 at 09:20:39AM -0800, Ian Rogers wrote:
> > On Mon, Jan 16, 2023 at 3:22 AM Jiri Olsa <olsajiri@...il.com> wrote:
> > >
> > > On Wed, Jan 11, 2023 at 04:40:24PM -0800, Ian Rogers wrote:
> > > > Previously tools/lib/subcmd was added to the include path, switch to
> > > > installing the headers and then including from that directory. This
> > > > avoids dependencies on headers internal to tools/lib/subcmd. Add the
> > > > missing subcmd directory to the affected #include.
> > > >
> > > > Signed-off-by: Ian Rogers <irogers@...gle.com>
> > > > ---
> > > >  tools/bpf/resolve_btfids/Makefile | 19 ++++++++++++++-----
> > > >  tools/bpf/resolve_btfids/main.c   |  2 +-
> > > >  2 files changed, 15 insertions(+), 6 deletions(-)
> > >
> > > this depends on changes that went to Arnaldo's tree right?
> > > I can't apply this on bpf-next/master
> >
> > Hmm.. sorry for that. I did the work on the master branch of
> > git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
> >
> > > >
> > > > diff --git a/tools/bpf/resolve_btfids/Makefile b/tools/bpf/resolve_btfids/Makefile
> > > > index 19a3112e271a..de7d29cf43d6 100644
> > > > --- a/tools/bpf/resolve_btfids/Makefile
> > > > +++ b/tools/bpf/resolve_btfids/Makefile
> > > > @@ -35,21 +35,29 @@ SUBCMD_SRC := $(srctree)/tools/lib/subcmd/
> > > >  BPFOBJ     := $(OUTPUT)/libbpf/libbpf.a
> > > >  LIBBPF_OUT := $(abspath $(dir $(BPFOBJ)))/
> > > >  SUBCMDOBJ  := $(OUTPUT)/libsubcmd/libsubcmd.a
> > > > +SUBCMD_OUT := $(abspath $(dir $(SUBCMDOBJ)))/
> > > >
> > > >  LIBBPF_DESTDIR := $(LIBBPF_OUT)
> > > >  LIBBPF_INCLUDE := $(LIBBPF_DESTDIR)include
> > > >
> > > > +SUBCMD_DESTDIR := $(SUBCMD_OUT)
> > > > +SUBCMD_INCLUDE := $(SUBCMD_DESTDIR)include
> > > > +
> > > >  BINARY     := $(OUTPUT)/resolve_btfids
> > > >  BINARY_IN  := $(BINARY)-in.o
> > > >
> > > >  all: $(BINARY)
> > > >
> > > > +prepare: $(SUBCMDOBJ)
> > >
> > > do we need special target for that? we already have BPFOBJ dependency
> > > placed in the BINARY_IN as prereq
> >
> > BPFOBJ is $(OUTPUT)/libbpf/libbpf.a which is needed at link time. The
> > prepare step is one we have elsewhere and it creates things like the
> > header files necessary to compile the C code, so we need it earlier is
> > the answer.
> >
> > > why not place both as BINARY_IN prereq, or is there some other reason
> > > for new 'prepare' target?
> >
> > I was trying to follow the convention elsewhere in the tree of having
> > a prepare target that does things like get the necessary header files
> > ready. Having prepare is useful because if an additional dependency is
> > added, then it just needs to be added to prepare. It could be tedious
> > to list all the dependencies for every C file, although Makefile.build
>
> ok, could we maybe add the BPFOBJ in prepare target as well?

Agreed, added to v2.

> > handles most of that. It isn't clear to me why $(BPFOBJ) is a
> > dependency of $(BINARY_IN) as it is already a dependency of $(BINARY).
>
> I think that if you specify OUTPUT then we need the libbpf headers
> to be created before we go to compile resolve_btfids objects
>
> thanks,
> jirka

Yep. I also noticed that this code is doing the "CC=$(HOSTCC)" thing.
The problem with that is Makefile.include (line 2) will set things
like CFLAGS based on CC and then you change it. I sent out some
objtool build cleanup for this:
https://lore.kernel.org/lkml/20230105090155.357604-4-irogers@google.com/
I'll add a patch for this in v2.

Thanks,
Ian