On Fri, Dec 11, 2015 at 09:19:17PM +0000, Luck, Tony wrote:
> > I still don't get the BIT(63) thing. Can you explain it?
>
> It will be more obvious when I get around to writing copy_from_user().
>
> Then we will have a function that can take page faults if there are pages
> that are not present. If the page faults can't be fixed we have a -EFAULT
> condition. We can also take machine checks if we reads from a location with an
> uncorrected error.
>
> We need to distinguish these two cases because the action we take is
> different. For the unresolved page fault we already have the ABI that the
> copy_to/from_user() functions return zero for success, and a non-zero
> return is the number of not-copied bytes.
>
> So for my new case I'm setting bit63 ... this is never going to be set for
> a failed page fault.
Isn't 63 NX?
>
> copy_from_user() conceptually will look like this:
>
> int copy_from_user(void *to, void *from, unsigned long n)
> {
> u64 ret = mcsafe_memcpy(to, from, n);
>
> if (COPY_HAD_MCHECK(r)) {
> if (memory_failure(COPY_MCHECK_PADDR(ret) >> PAGE_SIZE, ...))
> force_sig(SIGBUS, current);
> return something;
> } else
> return ret;
> }
>
> -Tony
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