[PATCH 2/2] ARM: topdown mmap support

[Nicolas Pitre]

On Mon, 14 Nov 2011, Rob Herring wrote:

> On 11/14/2011 11:41 AM, Nicolas Pitre wrote:
> > On Mon, 7 Nov 2011, Rob Herring wrote:
> > 
> >> From: Rob Herring <rob.herring at calxeda.com>
> >>
> >> Similar to other architectures, this adds topdown mmap support in user
> >> process address space allocation policy. This allows mmap sizes greater
> >> than 2GB. This support is largely copied from MIPS and the generic
> >> implementations.
> >>
> >> The address space randomization is moved into arch_pick_mmap_layout.
> > 
> > This is a problem by effectively weakening the randomization greatly.  
> > Now you get a random starting point but all mmaps are otherwise fixed 
> > relative to each other, whereas you had random distances between each 
> > mmaps before.
> > 
> 
> You mean within a single process the mmap's are not randomized? 

With regard to each other: not anymore with your patch.

> Couldn't that end up wasting a lot of virtual space if you have a lot 
> of mmaps?

That is what we have now without this patch.  The Potential for wasted 
space is 8 bits on a page level, i.e. 1MB max or 512 KB on average, per 
mmap.  I don't think this is that bad.

> It is aligned with other arch's and the generic implementation. The
> generic implementation doesn't even do randomization for legacy layouts.

Only x86 does complete ASLR besides ARM.


Nicolas