Messages in this thread | | | From | Thomas Gleixner <> | Subject | Re: [RFCv2 05/10] x86/mm: Provide untagged_addr() helper | Date | Thu, 12 May 2022 17:16:11 +0200 |
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On Thu, May 12 2022 at 16:23, Peter Zijlstra wrote: > On Thu, May 12, 2022 at 03:06:38PM +0200, Thomas Gleixner wrote: > >> #define untagged_addr(addr) ({ \ >> u64 __addr = (__force u64)(addr); \ >> \ >> __addr &= current->thread.lam_untag_mask; \ >> (__force __typeof__(addr))__addr; \ >> }) >> >> No conditionals, fast _and_ correct. Setting this untag mask up once >> when LAM is enabled is not rocket science. > > But that goes wrong if someone ever wants to untag a kernel address and > not use the result for access_ok(). > > I'd feel better about something like: > > s64 __addr = (addr); > s64 __sign = __addr; > > __sign >>= 63; > __sign &= lam_untag_mask;
that needs to be
__sign &= ~lam_untag_mask;
> __addr &= lam_untag_mask; > __addr |= __sign; > > __addr; > > Which simply extends bit 63 downwards -- although possibly there's an > easier way to do that, this is pretty gross.
For the price of a conditional:
__addr &= lam_untag_mask; if (__addr & BIT(63)) __addr |= ~lam_untag_mask;
Now you have the choice between gross and ugly.
Thanks,
tglx
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