On Mon, 1 Aug 2016, David Miller wrote:
> From: Mikulas Patocka <mpatocka@xxxxxxxxxx>
> Date: Sun, 31 Jul 2016 19:50:57 -0400 (EDT)
>
> > @@ -18,9 +25,9 @@
> > * of the cases, just fix things up simply here.
> > */
> >
> > -static unsigned long compute_size(unsigned long start, unsigned long size, unsigned long *offset)
> > +static unsigned long compute_size(unsigned long start, unsigned long size, unsigned long *offset, unsigned long prefetch)
> > {
> > - unsigned long fault_addr = current_thread_info()->fault_address;
> > + unsigned long fault_addr = current_thread_info()->fault_address - prefetch;
> > unsigned long end = start + size;
> >
> > if (fault_addr < start || fault_addr >= end) {
> > @@ -36,7 +43,7 @@ unsigned long copy_from_user_fixup(void
> > {
> > unsigned long offset;
> >
> > - size = compute_size((unsigned long) from, size, &offset);
> > + size = compute_size((unsigned long) from, size, &offset, COPY_FROM_USER_PREFETCH);
> > if (likely(size))
> > memset(to + offset, 0, size);
> >
>
> I think this might cause a problem. Assume we are not in one of those
> prefetching loops and are just doing a byte at a time, and therefore
> hit the fault exactly at the beginning of the missing page.
>
> You will rewind 0x100 bytes and the caller will restart the copy at
> "faulting address - 0x100".
>
> If someone is using atomic user copies, and using the returned length
> to determine which page in userspace needs to be faulted in, and
> then restart the copy, then we will loop forever.
This isn't guaranteed on x86 neither.
__copy_user_intel reads and writes 64 bytes in one loop iteration (and it
prefetches the data for the next iteration with "movl 64(%4), %%eax". If
it fails, it reports the amount of remaining data at the start of the loop
iteration. The reported value may be 67 bytes lower than the fault
location.
Mikulas
> We must, therefore, find some way to calculate this length _precisely_.
> It must be exactly at the furthest byte successfully copied to the
> destination.
>
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