[PATCH 0/4] Fix PROT_NONE page permissions when !CPU_USE_DOMAINS

[Benjamin Herrenschmidt]

On Thu, 2012-09-20 at 23:12 +0100, Russell King - ARM Linux wrote:

 .../...

> That'll make the set_pte assembly more horrid, but I guess that's the
> price you pay for removing useful features which architecture folk don't
> like from CPUs... and it gets more horrid because you can't encode some
> of those bit patterns with the standard 8-bit and shift opcode
> representation.
> 
> Really fun bug, but it needs more thought about how to solve it.

Yeah, fun isn't it ? :-)

The x86 way to solve it is simple, though ugly ... They use a SW bit
called "PROTNONE". A PROT_NONE page gets that instead of the HW valid
bit and pte_present() test if any of them is set.

Cheers,
Ben.