linux-kernel - Re: [RFC] kasan stack overflow warnings again: READ

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Message-Id: <30a6c19f-c854-786d-8b36-1825528b64cf@de.ibm.com>
Date:   Wed, 15 Feb 2017 17:18:50 +0100
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Arnd Bergmann <arnd@...db.de>, Dmitry Vyukov <dvyukov@...gle.com>
Cc:     "kasan-dev@...glegroups.com" <kasan-dev@...glegroups.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC] kasan stack overflow warnings again: READ_ONCE(),
 typecheck()

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.