linux-kernel - RE: [PATCH v1 1/1] KVM: s390: Don't cast parameter in bit operations

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Date:   Thu, 24 Feb 2022 23:15:01 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Andy Shevchenko' <andy.shevchenko@...il.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>
CC:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        "open list:VFIO DRIVER" <kvm@...r.kernel.org>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        Janosch Frank <frankja@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>
Subject: RE: [PATCH v1 1/1] KVM: s390: Don't cast parameter in bit operations

From: Andy Shevchenko
> Sent: 24 February 2022 19:51
> 
> On Thu, Feb 24, 2022 at 2:51 PM Claudio Imbrenda <imbrenda@...ux.ibm.com> wrote:
> >
> > On Wed, 23 Feb 2022 18:44:20 +0200
> > Andy Shevchenko <andriy.shevchenko@...ux.intel.com> wrote:
> >
> > > While in this particular case it would not be a (critical) issue,
> > > the pattern itself is bad and error prone in case somebody blindly
> > > copies to their code.
> > >
> > > Don't cast parameter to unsigned long pointer in the bit operations.
> > > Instead copy to a local variable on stack of a proper type and use.
> 
> ...
> 
> > > +             struct { /* as a 256-bit bitmap */
> > > +                     DECLARE_BITMAP(b, 256);
> > > +             } bitmap;
> > > +             struct { /* as a set of 64-bit words */
> > >                       u64 word[4];
> > >               } u64;
> 
> > > -     set_bit_inv(IPM_BIT_OFFSET + gisc, (unsigned long *) gisa);
> > > +     set_bit_inv(IPM_BIT_OFFSET + gisc, gisa->bitmap.b);
> >
> > wouldn't it be enough to pass gisa->u64.word here?
> > then no cast would be necessary
> 
> No, it will have the same hidden bugs. As I stated in the commit
> message, the pattern is quite bad even if in particular code it would
> work.
> 
> Thanks, Michael, for pointing out other places. They all need to be fixed.

It may even be worth writing some alternate bitmap functions
that use u64[] and unlocked operations?

Although I think I'd still want to encapsulate the actual array
(somehow) so that what is defined has to be the bitmap type.

	David

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