Re: [PATCH repost 09/16] metag/uaccess: fix sparse errors

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Jan 02, 2015 at 03:41:02PM +0000, James Hogan wrote:
> Hi,
> 
> On 25/12/14 09:29, 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.
> 
> I still see these sparse warnings with metag even with your patch:
> 
>   CHECK   drivers/vhost/vhost.c
> drivers/vhost/vhost.c:1004:13: warning: cast from restricted __virtio16
> drivers/vhost/vhost.c:1004:13: warning: cast from restricted __virtio16
> drivers/vhost/vhost.c:1004:13: warning: cast from restricted __virtio16
> drivers/vhost/vhost.c:1004:13: warning: cast from restricted __virtio16
> drivers/vhost/vhost.c:1022:13: warning: cast from restricted __virtio16
> drivers/vhost/vhost.c:1022:13: warning: cast from restricted __virtio16
> drivers/vhost/vhost.c:1022:13: warning: cast from restricted __virtio16
> drivers/vhost/vhost.c:1022:13: warning: cast from restricted __virtio16
> drivers/vhost/vhost.c:1373:21: warning: cast from restricted __virtio32
> drivers/vhost/vhost.c:1373:21: warning: cast from restricted __virtio32
> drivers/vhost/vhost.c:1373:21: warning: cast from restricted __virtio32
> drivers/vhost/vhost.c:1373:21: warning: cast from restricted __virtio32
> drivers/vhost/vhost.c:1377:21: warning: cast from restricted __virtio32
> drivers/vhost/vhost.c:1377:21: warning: cast from restricted __virtio32
> drivers/vhost/vhost.c:1377:21: warning: cast from restricted __virtio32
> drivers/vhost/vhost.c:1377:21: warning: cast from restricted __virtio32
> drivers/vhost/vhost.c:1425:13: warning: cast from restricted __virtio16
> drivers/vhost/vhost.c:1425:13: warning: cast from restricted __virtio16
> drivers/vhost/vhost.c:1425:13: warning: cast from restricted __virtio16
> drivers/vhost/vhost.c:1425:13: warning: cast from restricted __virtio16
> 
> Which something like the following hunk fixes in a similar way to yours:
> 
> diff --git a/arch/metag/include/asm/uaccess.h b/arch/metag/include/asm/uaccess.h
> index 0748b0a97986..594497053748 100644
> --- a/arch/metag/include/asm/uaccess.h
> +++ b/arch/metag/include/asm/uaccess.h
> @@ -112,13 +112,17 @@ do {                                                            \
>  	retval = 0;                                             \
>  	switch (size) {                                         \
>  	case 1:								\
> -		retval = __put_user_asm_b((unsigned int)x, ptr); break;	\
> +		retval = __put_user_asm_b((__force unsigned int)x, ptr); \
> +		break;							\
>  	case 2:								\
> -		retval = __put_user_asm_w((unsigned int)x, ptr); break;	\
> +		retval = __put_user_asm_w((__force unsigned int)x, ptr); \
> +		break;							\
>  	case 4:								\
> -		retval = __put_user_asm_d((unsigned int)x, ptr); break;	\
> +		retval = __put_user_asm_d((__force unsigned int)x, ptr); \
> +		break;							\
>  	case 8:								\
> -		retval = __put_user_asm_l((unsigned long long)x, ptr); break; \
> +		retval = __put_user_asm_l((__force unsigned long long)x, ptr); \
> +		break;							\
>  	default:							\
>  		__put_user_bad();					\
>  	}	
> 
> As far as I understand it, using __force on the value (as opposed to the
> pointer) is safe here, in the sense of not masking any genuine defects.
> Do you agree? Do you want to apply that hunk with your patch too?

what about this code:
             u16 v = 0;
             int rc = get_user(v, (__force __le16 __user *)p);

it should trigger a warning, does it?



> Note, this change also suppresses warnings for writing a pointer to
> userland due to the casts to unsigned int / unsigned long long, such as
> the following (each 4 times due to 4 cases above):
> 
> kernel/signal.c:2740:25: warning: cast removes address space of expression
> kernel/signal.c:2747:24: warning: cast removes address space of expression
> kernel/signal.c:2760:24: warning: cast removes address space of expression
> kernel/signal.c:2761:24: warning: cast removes address space of expression
> kernel/signal.c:2775:24: warning: cast removes address space of expression
> kernel/signal.c:2779:24: warning: cast removes address space of expression
> kernel/signal.c:3202:25: warning: cast removes address space of expression
> kernel/signal.c:3225:17: warning: cast removes address space of expression
> kernel/futex.c:2769:16: warning: cast removes address space of expression
> 
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> >  arch/metag/include/asm/uaccess.h | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/metag/include/asm/uaccess.h b/arch/metag/include/asm/uaccess.h
> > index 0748b0a..c314b45 100644
> > --- a/arch/metag/include/asm/uaccess.h
> > +++ b/arch/metag/include/asm/uaccess.h
> > @@ -135,7 +135,7 @@ extern long __get_user_bad(void);
> >  ({                                                              \
> >  	long __gu_err, __gu_val;                                \
> >  	__get_user_size(__gu_val, (ptr), (size), __gu_err);	\
> > -	(x) = (__typeof__(*(ptr)))__gu_val;                     \
> > +	(x) = (__force __typeof__(*(ptr)))__gu_val;                     \
> 
> Can you adjust the position of the \ to line up please
> 
> >  	__gu_err;                                               \
> >  })
> >  
> > @@ -145,7 +145,7 @@ extern long __get_user_bad(void);
> >  	const __typeof__(*(ptr)) __user *__gu_addr = (ptr);		\
> >  	if (access_ok(VERIFY_READ, __gu_addr, size))			\
> >  		__get_user_size(__gu_val, __gu_addr, (size), __gu_err);	\
> > -	(x) = (__typeof__(*(ptr)))__gu_val;                             \
> > +	(x) = (__force __typeof__(*(ptr)))__gu_val;                             \
> 
> same here (this one causes a checkpatch error due to 80 column limit)
> 
> >  	__gu_err;                                                       \
> >  })
> >  
> > 
> 
> With those changes,
> Acked-by: James Hogan <james.hogan@imgtec.com>
> 
> Cheers
> James
>