Re: [PATCH] mm: Enable MAP_UNINITIALIZED for archs with mmu

[Arun Sharma]

On 2/24/12 6:51 AM, Balbir Singh wrote:
> On Fri, Feb 24, 2012 at 8:17 AM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu@xxxxxxxxxxxxxx>  wrote:
> > > They don't have access to each other's VMAs, but if "accidentally" one
> > > of them comes across an uninitialized page with data from another task,
> > > it's not a violation of the security model.
>
> Can you expand more on the single address space model?

I haven't thought this through yet. But I know that just adding

&& (cgroup_task_count() == 1)

to page_needs_clearing() is not going to do it. We'll have to design a 
new mechanism (cgroup_mm_count_all()?) and make sure that it doesn't 
race with fork() and inadvertently expose pages from the new address 
space to the existing one.

A uid based approach such as the one implemented by Davide Libenzi

http://thread.gmane.org/gmane.linux.kernel/548928
http://thread.gmane.org/gmane.linux.kernel/548926

would probably apply the optimization to more use cases - but 
conceptually a bit more complex. If we go with this more relaxed 
approach, we'll have to design a race-free cgroup_uid_count() based 
mechanism.

 -Arun


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