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

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Date:   Thu, 15 Feb 2018 10:06:03 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: objtool warnings on 4.14-stable/gcc-7.3.0

On Thu, Feb 15, 2018 at 7:59 AM, Arnd Bergmann <arnd@...db.de> wrote:
> 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.

Uuhhh... I don't see the NULL deref, and even if it was eliminating
later stuff, I'd still expect a pr_info() ...

void lkdtm_CORRUPT_LIST_ADD(void)
{
        /*
         * Initially, an empty list via LIST_HEAD:
         *      test_head.next = &test_head
         *      test_head.prev = &test_head
         */
        LIST_HEAD(test_head);
        struct lkdtm_list good, bad;
        void *target[2] = { };
        void *redirection = &target;

        pr_info("attempting good list addition\n");
...

>>> >  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.

I'll see if I can reproduce this...

-Kees

-- 
Kees Cook
Pixel Security