[PATCH] MXS: Fix mxs_defconfig MMAP_MIN_ADDR

[Russell King - ARM Linux]

On Fri, Aug 03, 2012 at 08:46:27PM +0200, Marek Vasut wrote:
> Dear Russell King - ARM Linux,
> 
> [...]
> 
> > > > No need to set it to 32768.
> > > > 
> > > > If you just remove the 'CONFIG_DEFAULT_MMAP_MIN_ADDR=65536' then this
> > > > symbol will be 32768.
> > > 
> > > Sorry, ignore what I said. I just realized that the default is 4096.
> > 
> > 4096 is also fine for ARM too.  There's not much point in having
> > defconfigs change it - that would just be pure noise in the config
> > files.
> 
> Wasn't there a security concern being the reason for setting this higher?

I don't believe there is.  The only requirement is that the first page
on older CPUs isn't stomped on (and we preserve that for later CPUs so
that NULL pointer derefs get caught.)

The higher it is the better though, because it means NULL pointer + offset
deref with larger offsets also gets caught.

> Also, 
> I still don't completely understand why ARM has to set it lower than
> others, is that an ABI thing?

Yes, we have always loaded user programs at 0x8000 by default, which are
generally not relocatable.  So if you set it to 64K, you prevent non-root
user programs being mapped in, which is why stuff gets killed as soon as
UID != 0.