linux-kernel - Re: [PATCH] locking/atomics: don't alias ___

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Date:   Wed, 28 Jun 2017 13:21:43 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dmitry Vyukov <dvyukov@...gle.com>
cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        "x86@...nel.org" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] locking/atomics: don't alias ____ptr

On Wed, 28 Jun 2017, Dmitry Vyukov wrote:
> On Wed, Jun 28, 2017 at 1:10 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> >> In my case I ended up with something like:
> >>
> >> __typeof__(foo) __ptr = __ptr;
> >>
> >> which compiler decided to turn into 0.
> >>
> >> Thank you, macros.
> >>
> >> We can add more underscores, but the problem can happen again. Should
> >> we prefix current function/macro name to all local vars?..
> >
> > Actually we can void that ___ptr dance completely.
> >
> > Thanks,
> >
> >         tglx
> >
> > 8<--------------------
> >
> > --- a/include/asm-generic/atomic-instrumented.h
> > +++ b/include/asm-generic/atomic-instrumented.h
> > @@ -359,37 +359,32 @@ static __always_inline bool atomic64_add
> >
> >  #define cmpxchg(ptr, old, new)                         \
> >  ({                                                     \
> > -       __typeof__(ptr) ___ptr = (ptr);                 \
> > -       kasan_check_write(___ptr, sizeof(*___ptr));     \
> > +       kasan_check_write((ptr), sizeof(*(ptr)));       \
> >         arch_cmpxchg((ptr), (old), (new));              \
> >  })
> >
> >  #define sync_cmpxchg(ptr, old, new)                    \
> >  ({                                                     \
> > -       __typeof__(ptr) ___ptr = (ptr);                 \
> > -       kasan_check_write(___ptr, sizeof(*___ptr));     \
> > -       arch_sync_cmpxchg(___ptr, (old), (new));        \
> > +       kasan_check_write((ptr), sizeof(*(ptr)));       \
> > +       arch_sync_cmpxchg((ptr), (old), (new));         \
> >  })
> >
> >  #define cmpxchg_local(ptr, old, new)                   \
> >  ({                                                     \
> > -       __typeof__(ptr) ____ptr = (ptr);                \
> > -       kasan_check_write(____ptr, sizeof(*____ptr));   \
> > -       arch_cmpxchg_local(____ptr, (old), (new));      \
> > +       kasan_check_write((ptr), sizeof(*(ptr)));       \
> > +       arch_cmpxchg_local((ptr), (old), (new));        \
> 
> 
> /\/\/\/\/\/\/\/\/\/\/\/\
> 
> These are macros.
> If ptr is foo(), then we will call foo() twice.

Sigh, is that actually used?

That's all insane. The whole crap gets worse because:

      cmpxchg() can be used on u8, u16, u32 ....

There is a reason why type safety matters.

Thanks,

	tglx