linux-kernel - Re: [PATCH 00/18] uaccess: fix sparse warning on get

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Date:	Mon, 15 Dec 2014 12:17:03 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	"LF.Tan" <lftan.linux@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>,
	Linux-Arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH 00/18] uaccess: fix sparse warning on get_user for
 bitwise types

On Mon, Dec 15, 2014 at 06:03:10PM +0800, LF.Tan wrote:
> On Mon, Dec 15, 2014 at 12:51 AM, Michael S. Tsirkin <mst@...hat.com> wrote:
> > At the moment, if p and x are both tagged as bitwise types,
> > get_user(x, p) produces a sparse warning on many architectures.
> > This is because *p on these architectures is loaded into long
> > (typically using asm), then cast back to typeof(*p).
> >
> > When typeof(*p) is a bitwise type (which is uncommon), such a cast needs
> > __force, otherwise sparse produces a warning.
> >
> > Some architectures already have the __force tag, add it
> > where it's missing.
> >
> > Specificlly, vhost 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.
> >
> > For now, I worked around this by converting vhost code to access_ok +
> > __get_user, but that's not robust, I'd like to switch to get_user for 3.19.
> >
> > I tested this on x86 only so far. Since it's just adding __force, should be
> > trivially safe everywhere?
> >
> > Please review, and consider for 3.19.
> >
> > Who can merge this? Arnd?
> Hi Michael
> 
> FYI, there is new arch (arch/nios2) in 3.19. Is your patchset include
> the fix for it?
> Thanks.
> 
> Regards
> Ley Foon

Thanks for the heads up.
No - it will probably need a similar fix for __get_user and get_user.
It's just a parse warning so it's not a blocker.
Can you fix it yourself or prefer me to?

-- 
MST
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