Re: [patch 0/4] MAP_NOZERO v2 - VM_NOZERO/MAP_NOZERO early summer madness

[Rik van Riel <riel@redhat.com>]

Kyle Moffett wrote:
> On Jun 28, 2007, at 14:49:24, Davide Libenzi wrote:
> > I was using oprofile to sample some userspace code I am working on, 
> > and I was continuosly noticing  clear_page  in the top three entries 
> > of the oprofile logs.
> >
> > Also, a simple kernel build, in my Dual Opteron with 8GB of RAM, 
> > shows  clear_page  as the first kernel entry, second only to the 
> > userspace the  cc1  and  as. Most of the userspace code uses malloc() 
> > (and anonymous  mappings) in such a way that the memory returned via 
> > kernel->glibc is immediately written soon after. The POSIX malloc() 
> > definition itself also, does not require the returned memory to be 
> > zeroed (as calloc() does).
> >
> > So I implemented a rather quick hack that introduces a new mmap() flag 
> > MAP_NOZERO (only valid for anonymous mappings) and the  vma  
> > counter-part VM_NOZERO. Also, a new sys_brk2() has been introduced to 
> > accept a new flags  parameter. A brief description of the patches 
> > follows in the next emails.
>
> Hmm, sounds like this would also need a "MAP_NOREUSE" flag of some kind 
> for security sensitive applications.

That wants MAP_PRIVATE so that the kernel can also decide to not
swap these pages out to an unencrypted swap area.

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