linux-kernel - Re: [GIT PULL] x86/build changes for v4.17

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Date:   Wed, 4 Apr 2018 14:32:47 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        James Y Knight <jyknight@...gle.com>,
        Chandler Carruth <chandlerc@...gle.com>,
        Stephen Hines <srhines@...gle.com>,
        Kees Cook <keescook@...gle.com>, groeck@...omium.org,
        Greg Hackmann <ghackmann@...gle.com>
Subject: Re: [GIT PULL] x86/build changes for v4.17

On Wed, Apr 04, 2018 at 04:53:52PM +0000, Nick Desaulniers wrote:
> (re-sending as plain text)
> 
> On Wed, Apr 4, 2018 at 2:38 AM Greg KH <gregkh@...uxfoundation.org> wrote:
> > There are known-bugs with building a kernel with clang right now (I
> > pointed one out a few days ago about NULL checks being deleted from the
> > clang output for no good reason, which really is scary for obvious
> > reasons).
> 
> Is this the thread you are referring to?
> https://lkml.org/lkml/2018/3/27/1286
> 
> It's definitely something curious that I'll need to sit down and
> investigate more.  If there are other known instances, it would be good to
> let me know.

As Matthias mentioned elsewhere, it sounds like they're planning to
implement -fno-delete-null-pointer-checks, which would presumably fix
the above issue.

Aside from the above-mentioned debugfs NULL checks, there was also
another (seemingly valid) objtool warning:

  arch/x86/mm/pti.o: warning: objtool: pti_init() falls through to next function pti_user_pagetable_walk_pmd()

I don't know if that one was also caused by removed NULL checks, but
it's worth investigating.

More details (and the .o file) here:

  https://lkml.kernel.org/r/20180319232255.GF37438@google.com

-- 
Josh