[PATCH v4 1/4] mm: mmap: Add new /proc tunable for mmap_base ASLR.

[Andrew Morton]

On Mon, 30 Nov 2015 15:54:12 -0800 Andrew Morton <akpm at linux-foundation.org> wrote:

> On Thu, 26 Nov 2015 14:59:42 -0800 Daniel Cashman <dcashman at android.com> wrote:
> 
> > ASLR  only uses as few as 8 bits to generate the random offset for the
> > mmap base address on 32 bit architectures. This value was chosen to
> > prevent a poorly chosen value from dividing the address space in such
> > a way as to prevent large allocations. This may not be an issue on all
> > platforms. Allow the specification of a minimum number of bits so that
> > platforms desiring greater ASLR protection may determine where to place
> > the trade-off.
> > 
> > --- a/kernel/sysctl.c
> > +++ b/kernel/sysctl.c
> > @@ -1568,6 +1568,28 @@ static struct ctl_table vm_table[] = {
> >  		.mode		= 0644,
> >  		.proc_handler	= proc_doulongvec_minmax,
> >  	},
> > +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
> > +	{
> > +		.procname	= "mmap_rnd_bits",
> > +		.data		= &mmap_rnd_bits,
> > +		.maxlen		= sizeof(mmap_rnd_bits),
> > +		.mode		= 0600,
> > +		.proc_handler	= proc_dointvec_minmax,
> > +		.extra1		= (void *) &mmap_rnd_bits_min,
> > +		.extra2		= (void *) &mmap_rnd_bits_max,
> 
> hm, why the typecasts?  They're unneeded and are omitted everywhere(?)
> else in kernel/sysctl.c.
> 

Oh.  Casting away constness.

What's the thinking here?  They can change at any time so they aren't
const so we shouldn't declare them to be const?