linux-kernel - Re: READ_ONCE() + STACKPROTECTOR_STRONG == :/ (was Re: [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops))

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Message-Id: <fb4bba85-1f83-435e-b84e-ee29770d7090@de.ibm.com>
Date:   Wed, 18 Dec 2019 11:22:05 +0100
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Will Deacon <will@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Michael Ellerman <mpe@...erman.id.au>, dja@...ens.net,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linuxppc-dev@...ts.ozlabs.org,
        Christophe Leroy <christophe.leroy@....fr>,
        linux-arch <linux-arch@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: READ_ONCE() + STACKPROTECTOR_STRONG == :/ (was Re: [GIT PULL]
 Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops))

On 12.12.19 21:49, Linus Torvalds wrote:
> On Thu, Dec 12, 2019 at 11:34 AM Will Deacon <will@...nel.org> wrote:
>>
>> The root of my concern in all of this, and what started me looking at it in
>> the first place, is the interaction with 'typeof()'. Inheriting 'volatile'
>> for a pointer means that local variables in macros declared using typeof()
>> suddenly start generating *hideous* code, particularly when pointless stack
>> spills get stackprotector all excited.
> 
> Yeah, removing volatile can be a bit annoying.
> 
> For the particular case of the bitops, though, it's not an issue.
> Since you know the type there, you can just cast it.
> 
> And if we had the rule that READ_ONCE() was an arithmetic type, you could do
> 
>     typeof(0+(*p)) __var;
> 
> since you might as well get the integer promotion anyway (on the
> non-volatile result).
> 
> But that doesn't work with structures or unions, of course.

We do have a READ_ONCE on the following union in s390 code.

union ipte_control {
        unsigned long val;
        struct {
                unsigned long k  : 1;
                unsigned long kh : 31;
                unsigned long kg : 32;
        }; 
};


In fact this one was the original failure case why we change ACCESS_ONCE.

see arch/s390/kvm/gaccess.c