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Subject: Re: [RFC] kasan stack overflow warnings again: READ_ONCE(),
 typecheck()
To: Arnd Bergmann <arnd@arndb.de>, Dmitry Vyukov <dvyukov@google.com>
References: <1745339.Zp71hEDpdI@wuerfel>
Cc: "kasan-dev@googlegroups.com" <kasan-dev@googlegroups.com>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Alexander Potapenko <glider@google.com>, linux-kernel@vger.kernel.org
From: Christian Borntraeger <borntraeger@de.ibm.com>
Date: Wed, 15 Feb 2017 17:18:50 +0100
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On 02/15/2017 12:03 AM, Arnd Bergmann wrote:
> 
> -#define WRITE_ONCE(x, val) \
> -({							\
> -	union { typeof(x) __val; char __c[1]; } __u =	\
> -		{ .__val = (__force typeof(x)) (val) }; \
> -	__write_once_size(&(x), __u.__c, sizeof(x));	\
> -	__u.__val;					\
> -})
> +#define WRITE_ONCE(x, val)								\
> +(											\
> +	__builtin_choose_expr(sizeof(x) == 1, *(volatile typeof(&(x)))&(x) = (val),	\
> +	__builtin_choose_expr(sizeof(x) == 2, *(volatile typeof(&(x)))&(x) = (val),	\
> +	__builtin_choose_expr(sizeof(x) == 4, *(volatile typeof(&(x)))&(x) = (val),	\
> +	__builtin_choose_expr(sizeof(x) == sizeof(long), *(volatile typeof(&(x)))&(x) = (val),	\

Have you run sparse on those changes?
IIRC we had to add the __force to get rid of address space annotations
in that macro above. Cannot tell if we need something like that here.