linux-kernel - Re: [PATCH 03/18] arm64/uaccess: fix sparse errors

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Message-ID: <20141215112310.GG11285@n2100.arm.linux.org.uk>
Date:	Mon, 15 Dec 2014 11:23:11 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Will Deacon <will.deacon@....com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Christopher Covington <cov@...eaurora.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 03/18] arm64/uaccess: fix sparse errors

On Mon, Dec 15, 2014 at 11:17:16AM +0000, Will Deacon wrote:
> On Sun, Dec 14, 2014 at 04:52:09PM +0000, Michael S. Tsirkin wrote:
> > virtio wants to read bitwise types from userspace using get_user.  At the
> > moment this triggers sparse errors, since the value is passed through an
> > integer.
> > 
> > Fix that up using __force.
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> > ---
> >  arch/arm64/include/asm/uaccess.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
> > index 3bf8f4e..8d66bcf 100644
> > --- a/arch/arm64/include/asm/uaccess.h
> > +++ b/arch/arm64/include/asm/uaccess.h
> > @@ -147,7 +147,7 @@ do {									\
> >  	default:							\
> >  		BUILD_BUG();						\
> >  	}								\
> > -	(x) = (__typeof__(*(ptr)))__gu_val;				\
> > +	(x) = (__force __typeof__(*(ptr)))__gu_val;				\
> >  } while (0)
> >  
> >  #define __get_user(x, ptr)						\
> 
> Acked-by: Will Deacon <will.deacon@....com>

This also means you can do stuff like:

	u32 *p;
	__le32 v;

	err = get_user(p, v);

which is not right.  Both the dereferenced pointer type and the destination
type should be compatible, and if one is a bitwise type but the other isn't,
that seems like a valid case to warn.

I don't see any use of get_user() in drivers/virtio in mainline, so I can't
check further.

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