[PATCH 2/5] [ARM] add address randomization to mmap()

[Nicolas Pitre]

On Thu, 17 Jun 2010, Kyungmin Park wrote:

> Hi,
> 
> On Thu, Jun 17, 2010 at 5:33 AM, Nicolas Pitre <nico at fluxnic.net> wrote:
> > Signed-off-by: Nicolas Pitre <nicolas.pitre at linaro.org>
> > ---
> >  arch/arm/mm/mmap.c |    4 ++++
> >  1 files changed, 4 insertions(+), 0 deletions(-)
> >
> > diff --git a/arch/arm/mm/mmap.c b/arch/arm/mm/mmap.c
> > index f5abc51..4f5b396 100644
> > --- a/arch/arm/mm/mmap.c
> > +++ b/arch/arm/mm/mmap.c
> > @@ -7,6 +7,7 @@
> >  #include <linux/shm.h>
> >  #include <linux/sched.h>
> >  #include <linux/io.h>
> > +#include <linux/random.h>
> >  #include <asm/cputype.h>
> >  #include <asm/system.h>
> >
> > @@ -80,6 +81,9 @@ arch_get_unmapped_area(struct file *filp, unsigned long addr,
> >                start_addr = addr = TASK_UNMAPPED_BASE;
> >                mm->cached_hole_size = 0;
> >        }
> > +       /* 8 bits of randomness in 20 address space bits */
> > +       if (current->flags & PF_RANDOMIZE)
> > +               addr += (get_random_int() % (1 << 8)) << PAGE_SHIFT;
> 
> Doesn't better to use mask operation?
>                    addr += (get_random_int() & ((1 << 8) - 1)) << PAGE_SHIFT;

GCC is smart enough to optimize the modulus into a mask, effectively 
generating the exact same assembly code in both cases.


Nicolas