linux-kernel - Re: objtool warnings on 4.14-stable/gcc-7.3.0

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Message-ID: <CAK8P3a1WVDLW=m0wUtsoiGjYHqvD9PpEea7n8tfbEy1A=8Ougw@mail.gmail.com>
Date:   Thu, 15 Feb 2018 16:59:50 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: objtool warnings on 4.14-stable/gcc-7.3.0

On Thu, Feb 15, 2018 at 4:25 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> On Thu, Feb 15, 2018 at 04:01:57PM +0100, Arnd Bergmann wrote:
>> On Wed, Feb 14, 2018 at 11:45 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>> > On Wed, Feb 14, 2018 at 04:24:12PM -0600, Josh Poimboeuf wrote:
>> >> On Wed, Feb 14, 2018 at 04:11:15PM +0100, Arnd Bergmann wrote:

> Ok, I expected something like that.  GCC "undefined behavior" strikes
> again.
>
> Kees, I suppose you'll need to obfuscate the code to stay one step ahead
> of GCC.
>
> While this may be an objtool bug, I might not fix it because it served a
> useful purpose here in finding GCC crap.
>
>> I would have expected an actual NULL pointer dereference to remain
>> in the function though, or at least another trapping instruction.
>>
>> >  Can you share the config for this one?
>>
>> https://pastebin.com/qFV6SPWP
>
> Would be interesting to analyze that config to understand what options
> are causing GCC to do that.  I don't see this "optimization" with my
> config.

This seems like a very rare combination, the flags I need to reproduce are
"gcc -O2 -mno-red-zone  -mpreferred-stack-boundary=3 -march=nocona",
however I do see the same behavior with every gcc version since 4.8!

Aside from -march=nocona, also bonnell, atom, silvermont, slm, and knl
show this, but none of the modern microarchitectures do.

        Arnd