Messages in this thread | | | Date | Thu, 16 Jun 2022 19:48:54 +0300 | From | "Kirill A. Shutemov" <> | Subject | Re: [PATCHv3 5/8] x86/uaccess: Provide untagged_addr() and remove tags before address check |
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On Thu, Jun 16, 2022 at 12:02:16PM +0200, Peter Zijlstra wrote: > On Fri, Jun 10, 2022 at 05:35:24PM +0300, Kirill A. Shutemov wrote: > > +#ifdef CONFIG_X86_64 > > +/* > > + * Mask out tag bits from the address. > > + * > > + * Magic with the 'sign' allows to untag userspace pointer without any branches > > + * while leaving kernel addresses intact. > > + */ > > +#define untagged_addr(mm, addr) ({ \ > > + u64 __addr = (__force u64)(addr); \ > > + s64 sign = (s64)__addr >> 63; \ > > + __addr ^= sign; \ > > + __addr &= (mm)->context.untag_mask; \ > > + __addr ^= sign; \ > > + (__force __typeof__(addr))__addr; \ > > +}) > > Can't we make that mask a constant and *always* unmask U57 irrespective > of LAM being on?
We can do this if we give up on LAM_U48.
It would also needlessly relax canonical check. I'm not sure it is a good idea.
-- Kirill A. Shutemov
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