Re: [PATCHv3 5/8] x86/uaccess: Provide untagged_addr() and remove tags before address check

[Peter Zijlstra]

On Fri, Jun 17, 2022 at 07:22:57AM -0700, H.J. Lu wrote:
> On Fri, Jun 17, 2022 at 4:36 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > On Thu, Jun 16, 2022 at 07:44:40PM +0300, Kirill A. Shutemov wrote:
> > > Any objection against this variant (was posted in the thread):
> > >
> > >               #define untagged_addr(mm, addr) ({                                      \
> > >                       u64 __addr = (__force u64)(addr);                               \
> > >                       s64 sign = (s64)__addr >> 63;                                   \
> > >                       __addr &= (mm)->context.untag_mask | sign;                      \
> > >                       (__force __typeof__(addr))__addr;                               \
> > >               })
> > >
> > > ?
> >
> > Yeah, I suppose that should work fine.
> 
> Won't the sign bit be put at the wrong place?

sign is either 0 or ~0, by or-ing ~0 into the mask, the masking becomes
no-op.