netdev - Re: [PATCH bpf] tools/bpf: fix bpftool build with OUTPUT set

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Date:   Fri, 19 Jul 2019 11:17:16 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Ilya Leoshkevich <iii@...ux.ibm.com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org, lmb@...udflare.com,
        gor@...ux.ibm.com, heiko.carstens@...ibm.com,
        Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [PATCH bpf] tools/bpf: fix bpftool build with OUTPUT set

On Fri, 19 Jul 2019 15:12:24 +0200, Ilya Leoshkevich wrote:
> > Am 18.07.2019 um 20:51 schrieb Jakub Kicinski <jakub.kicinski@...ronome.com>:
> > 
> > We should probably make a script with all the ways of calling make
> > should work. Otherwise we can lose track too easily.  
> 
> Thanks for the script!
> 
> I’m trying to make it all pass now, and hitting a weird issue in the
> Kbuild case. The build prints "No rule to make target
> 'scripts/Makefile.ubsan.o'" and proceeds with an empty BPFTOOL_VERSION,
> which causes problems later on.

Does it only break with UBSAN enabled?

> I've found that this is caused by sub_make_done=1 environment variable,
> and unsetting it indeed fixes the problem, since the root Makefile no
> longer uses the implicit %.o rule.
> 
> However, I wonder if that would be acceptable in the final version of
> the patch, and whether there is a cleaner way to achieve the same
> effect?

I'm not sure to be honest. Did you check how perf deals with that?

My goal was primarily to make sure we don't regress, so maybe if some
corner cases don't work that's not the end of the world. I think a good
rule of the thumb would be "if it works for perf it should work for
bpftool" ;) Perf gets a lot more build testing.