[RFC PATCH v4] ARM: uprobes xol write directly to userspace

[David Miller]

From: Oleg Nesterov <oleg at redhat.com>
Date: Wed, 16 Apr 2014 18:43:10 +0200

> But did you really mean get_unmapped_area(pgoff => page_to_pfn(area->page)) ?

Yes.

> I simply can't understand how this can work, arm (and x86) really use it as
> "pgoff << PAGE_SHIFT" align_offset accounted in unmapped_area() ?

When a platform has D-cache aliasing issues, it must make sure that
every shared page is mapped to the same D-cache alias in all such
shared mappings.

The way this is done is to map each pgoff to a page in the D-cache.
For example, pgoff 0 would be given a virtual address that maps to the
first page in the D-cache, pgoff 1 to the second, and so forth.

What we're doing with this get_unmapped_area() call is to make it so
that userspace's virtual address will land at the same virtual alias
as the kernel one does.

So that stores on the kernel side will be seen properly, without any
cache flushing, by the user side mapping.

>> So we end up with all of the benefits of storing directly to
>> userspace, along with what you're trying to achieve.
> 
> And in this case we can avoid copy_to_user(), right ?

Yes, that's the whole idea, you can forget about the VM_WRITE etc.
that caused your concerns.

It'd be just memcpy to kernel side mapping of the page + i-cache flush
where necessary.