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

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Date:   Wed, 28 Jun 2017 13:10:14 +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 12:02 PM, Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:
> > @@ -359,16 +359,16 @@ static __always_inline bool atomic64_add_negative(s64 i, atomic64_t *v)
> >
> >  #define cmpxchg64(ptr, old, new)                       \
> >  ({                                                     \
> > -       __typeof__(ptr) ____ptr = (ptr);                \
> > -       kasan_check_write(____ptr, sizeof(*____ptr));   \
> > -       arch_cmpxchg64(____ptr, (old), (new));          \
> > +       __typeof__(ptr) ____ptr64 = (ptr);              \
> > +       kasan_check_write(____ptr64, sizeof(*____ptr64));\
> > +       arch_cmpxchg64(____ptr64, (old), (new));        \
> >  })
> >
> >  #define cmpxchg64_local(ptr, old, new)                 \
> >  ({                                                     \
> > -       __typeof__(ptr) ____ptr = (ptr);                \
> > -       kasan_check_write(____ptr, sizeof(*____ptr));   \
> > -       arch_cmpxchg64_local(____ptr, (old), (new));    \
> > +       __typeof__(ptr) ____ptr64 = (ptr);              \
> > +       kasan_check_write(____ptr64, sizeof(*____ptr64));\
> > +       arch_cmpxchg64_local(____ptr64, (old), (new));  \
> >  })
> >
> >  #define cmpxchg_double(p1, p2, o1, o2, n1, n2)                         \
> 
> 
> Doh! Thanks for fixing this. I think I've a similar crash in a
> different place when I developed the patch.
> The problem is that when we do:
> 
>        __typeof__(ptr) ____ptr = (ptr);                \
>        arch_cmpxchg64_local(____ptr, (old), (new));    \
> 
> We don't necessary pass value of our just declared ____ptr to
> arch_cmpxchg64_local(). We just pass a symbolic identifier. So if
> arch_cmpxchg64_local() declares own ____ptr and then tries to use what
> we passed ("____ptr") it will actually refer to own variable declared
> rather than to what we wanted to pass in.
> 
> 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));	\
 })
 
 #define cmpxchg64(ptr, old, new)			\
 ({							\
-	__typeof__(ptr) ____ptr = (ptr);		\
-	kasan_check_write(____ptr, sizeof(*____ptr));	\
-	arch_cmpxchg64(____ptr, (old), (new));		\
+	kasan_check_write((ptr), sizeof(*(ptr)));	\
+	arch_cmpxchg64((ptr), (old), (new));		\
 })
 
 #define cmpxchg64_local(ptr, old, new)			\
 ({							\
-	__typeof__(ptr) ____ptr = (ptr);		\
-	kasan_check_write(____ptr, sizeof(*____ptr));	\
-	arch_cmpxchg64_local(____ptr, (old), (new));	\
+	kasan_check_write((ptr), sizeof(*(ptr)));	\
+	arch_cmpxchg64_local((ptr), (old), (new));	\
 })
 
 /*