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

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Message-Id: <06890ADF-E310-41D1-9A4A-26755296F525@linux.ibm.com>
Date:   Tue, 23 Jul 2019 17:14:23 +0200
From:   Ilya Leoshkevich <iii@...ux.ibm.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.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

> Am 19.07.2019 um 20:17 schrieb Jakub Kicinski <jakub.kicinski@...ronome.com>:
> 
> 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?

No, all the time. I think this is a coincidence - make happens to scan
scripts/Makefile.ubsan first.

> 
>> 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?

perf obtains the version using "git describe". However, if we are
building it from a tarball, it falls back to "make kernelversion" and
fails in a similar way:

linux-5.3-rc1$ make defconfig
linux-5.3-rc1$ make tools/perf
<snip>
make[6]: Circular scripts/Makefile.ubsan.mod <- scripts/Makefile.ubsan.o dependency dropped.
make[6]: m2c: Command not found
make[6]: *** [<builtin>: scripts/Makefile.ubsan.o] Error 127
make[5]: *** [Makefile:1765: scripts/Makefile.ubsan.o] Error 2
<snip>

The same trick helps:

--- tools/perf/util/PERF-VERSION-GEN.orig	2019-07-23 17:12:07.621123187 +0200
+++ tools/perf/util/PERF-VERSION-GEN	2019-07-23 17:12:33.441133619 +0200
@@ -26,7 +26,7 @@
 fi
 if test -z "$TAG"
 then
-	TAG=$(MAKEFLAGS= make -sC ../.. kernelversion)
+	TAG=$(MAKEFLAGS= sub_make_done= make -sC ../.. kernelversion)
 fi
 VN="$TAG$CID"
 if test -n "$CID"