From: Mikulas Patocka <mpatocka@xxxxxxxxxx>
Date: Tue, 2 Aug 2016 08:20:15 -0400 (EDT)
> 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.
That's very interesting, let me do some research into this, as I was
pretty sure something like futexes or similar had some requirement in
this area.
Thanks.
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