Re: [PATCH v5 4/4] sparc64: Add support for ADI (Application Data Integrity)

[Khalid Aziz]

On 01/25/2017 03:00 PM, Rob Gardner wrote:
> On 01/25/2017 12:57 PM, Khalid Aziz wrote:
> > @@ -157,6 +158,24 @@ int __get_user_pages_fast(unsigned long start,
> > int nr_pages, int write,
> >       pgd_t *pgdp;
> >       int nr = 0;
> >   +#ifdef CONFIG_SPARC64
> > +    if (adi_capable()) {
> > +        long addr = start;
> > +
> > +        /* If userspace has passed a versioned address, kernel
> > +         * will not find it in the VMAs since it does not store
> > +         * the version tags in the list of VMAs. Storing version
> > +         * tags in list of VMAs is impractical since they can be
> > +         * changed any time from userspace without dropping into
> > +         * kernel. Any address search in VMAs will be done with
> > +         * non-versioned addresses. Ensure the ADI version bits
> > +         * are dropped here by sign extending the last bit before
> > +         * ADI bits. IOMMU does not implement version tags.
> > +         */
> > +        addr = (addr << (long)adi_nbits()) >> (long)adi_nbits();
>
>
> So you are depending on the sign extension to clear the ADI bits... but
> this only happens if there is a zero in that "last bit before ADI bits".
> If the last bit is a 1, then the ADI bits will be set instead of
> cleared.  That seems like an unintended consequence given the comment. I
> am aware of the value of adi_nbits() and of the number of valid bits in
> a virtual address on the M7 processor, but wouldn't using 'unsigned
> long' for everything here guarantee the ADI bits get cleared regardless
> of the state of the last non-adi bit?

Sign extension is the right thing to do. MMU considers values of 0 and 
15 for bits 63-60 to be untagged addresses and expects bit 59 to be 
sign-extended for untagged virtual addresses. The code I added is 
explicitly meant to sign-extend, not zero out the top 4 bits.

--
Khalid