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Date:	Mon, 5 Jan 2015 15:00:43 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	James Hogan <james.hogan@...tec.com>
Cc:	linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
	linux-arch@...r.kernel.org, linux-metag@...r.kernel.org
Subject: Re: [PATCH repost 09/16] metag/uaccess: fix sparse errors

On Mon, Jan 05, 2015 at 09:44:14AM +0000, James Hogan wrote:
> On 04/01/15 10:52, Michael S. Tsirkin wrote:
> > 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?
> 
> No, but it didn't previously either, and doesn't seem to with x86_64 either.
> 
> I tried __be16 too, since I presume you're referring to a discrepancy
> between the native byte order of v (u16) and the foreign byte order of
> *p (__be16)?
> 
> Cheers
> James

Yes.
It certainly does for me.
Did you define __CHECK_ENDIAN__?

Documentation/sparse.txt says:

To perform endianness
checks, you may define __CHECK_ENDIAN__:

        make C=2 CF="-D__CHECK_ENDIAN__"

These checks are disabled by default as they generate a host of warnings.


Or just try with __virtio16 instead of __le16 - I think these are
enabled unconditionally.


> > 
> > 
> > 
> >> 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@...hat.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@...tec.com>
> >>
> >> Cheers
> >> James
> >>
> > 
> > 
> 


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