linux-kernel - Re: [Regression 4.15] Can't kill CONFIG_UNWINDER

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Message-ID: <20181216214202.GQ11670@windriver.com>
Date:   Sun, 16 Dec 2018 16:42:02 -0500
From:   Paul Gortmaker <paul.gortmaker@...driver.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
CC:     Randy Dunlap <rdunlap@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        <linux-kbuild@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [Regression 4.15] Can't kill CONFIG_UNWINDER_ORC with fire or
 plague.

[Re: [Regression 4.15] Can't kill CONFIG_UNWINDER_ORC with fire or plague.] On 29/12/2017 (Fri 13:18) Paul Gortmaker wrote:

> [Re: [Regression 4.15] Can't kill CONFIG_UNWINDER_ORC with fire or plague.] On 29/12/2017 (Fri 10:47) Josh Poimboeuf wrote:
> 
> > This seems to be related to a kconfig quirk where only silentoldconfig
> > updates the include/config/auto.conf file.  The other config targets
> > (oldconfig, defconfig, etc) don't touch it.  It seems intentional, but I
> > have no idea why.
> > 
> > That causes the Makefile to get stale data for 'CONFIG_*' variables when
> > it includes auto.conf.  So I don't think this is specific to the ORC
> > check.  It seems like it could also cause bugs elsewhere.
> 
> OK, good - you agree with my initial diagnosis of stale auto.conf then.
> Not sure what Randy was testing when he said he couldn't reproduce it.
> 
> > The below (ugly) patch fixes it, though I'm not sure this is the best
> > way to do it.  We probably need Masahiro or Michal to chime in here.
> 
> Yep, that is why I intentionally put the kbuild folks on the To/Cc of
> the original report (and ran away screaming at the prospect of debugging
> Makefiles on xmas day).  But with holidays and all, it might not be
> until early January before they have a chance to reply.

It is nearly a year later and we still have this false positive.

paul@sm:~/git/linux-head$ make -j12 > /dev/null
Makefile:966: *** "Cannot generate ORC metadata for CONFIG_UNWINDER_ORC=y,
 please install libelf-dev, libelf-devel or elfutils-libelf-devel".  Stop.
paul@sm:~/git/linux-head$ grep UNWINDER_ORC .config
# CONFIG_UNWINDER_ORC is not set

We do know a bit more now -- the auto.conf issue has been independently
confirmed and "fixed" for other subsystems/issues since, like RETPOLINE:

 ---------------------
  commit 25896d073d8a0403b07e6dec56f58e6c33678207
  Author: Masahiro Yamada <yamada.masahiro@...ionext.com>
  Date:   Wed Dec 5 15:27:19 2018 +0900
   
    x86/build: Fix compiler support check for CONFIG_RETPOLINE

    It is troublesome to add a diagnostic like this to the Makefile
    parse stage because the top-level Makefile could be parsed with
    a stale include/config/auto.conf.

    Once you are hit by the error about non-retpoline compiler, the
    compilation still breaks even after disabling CONFIG_RETPOLINE.
 ---------------------

I'm not sure if we want to treat this on a per config option each time
again and again, or undertake a more global kbuild approach, but it does
warrant a mention and a re-examination before we "solve" this again.

Paul.
--