Subject: Re: [PATCH] rsi: fix integer overflow warning

From: Joe Perches <[email-masked]>
Date: Thu, 05 Oct 2017 05:19:07 -0700

On Thu, 2017-10-05 at 14:05 +0200, Arnd Bergmann wrote:
> gcc produces a harmless warning about a recently introduced
> signed integer overflow:
> 
> drivers/net/wireless/rsi/rsi_91x_hal.c: In function 'rsi_prepare_mgmt_desc':
> include/uapi/linux/swab.h:13:15: error: integer overflow in expression [-Werror=overflow]
>   (((__u16)(x) & (__u16)0x00ffU) << 8) |   \
>    ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~
> include/uapi/linux/swab.h:104:2: note: in expansion of macro '___constant_swab16'
>   ___constant_swab16(x) :   \
>   ^~~~~~~~~~~~~~~~~~
> include/uapi/linux/byteorder/big_endian.h:34:43: note: in expansion of macro '__swab16'
>  #define __cpu_to_le16(x) ((__force __le16)__swab16((x)))

[]

> The problem is that the 'mask' value is a signed integer that gets
> turned into a negative number when truncated to 16 bits. Making it
> an unsigned constant avoids this.

I would expect there are more of these.

Perhaps this define in include/uapi/linux/swab.h:

#define __swab16(x)				\
	(__builtin_constant_p((__u16)(x)) ?	\
	___constant_swab16(x) :			\
	__fswab16(x))

should be

#define __swab16(x)				\
	(__builtin_c
onstant_p((__u16)(x)) ?	\
	___constant_swab16((__u16)(x)) :
		\
	__fswab16((__u16)(x)))