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SubjectRE: [PATCH] rsi: fix integer overflow warning
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From: Joe Perches
> Sent: 05 October 2017 13:19
> 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_constant_p((__u16)(x)) ? \
> ___constant_swab16((__u16)(x)) : \
> __fswab16((__u16)(x)))

You probably don't want the cast in the call to __fswab16() since
that is likely to generate an explicit and with 0xffff.
You will likely also get one if the argument is _u16 (not unsigned int).

David


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