On Wed, 25 Nov 2015 11:07:09 -0800
Daniel Cashman [off-list ref] wrote:

On 11/24/2015 04:39 PM, Andrew Morton wrote:

quoted

mips, powerpc and s390 also implement arch_mmap_rnd().  Are there any
special considerations here, or it just a matter of maintainers wiring
it up and testing it?

I had not yet looked at those at all, as I had no way to do even a
rudimentary "does it boot" test and opted to post v3 first.  Upon first
glance, it should just be a matter of wiring it up:

Mips is divided into 12/16 bits for 32/64 bit (assume baseline 4k page)
w/COMPAT kconfig,  powerpc is 11/18 w/COMPAT, s390 is 11/11 w/COMPAT.
s390 is a bit strange as COMPAT is for a 31-bit address space, although
is_32bit_task() is used to determine which mask to use, and the mask
itself for 64-bit only introduces 11 bits of entropy, but while still
affecting larger chunks of the address space (mask is 0x3ff80, resulting
in an effective 0x7ff shift of PAGE_SIZE + 7 bits).

s390 uses a mmap randomization of 11 bits but applies it to different
bits dependent if the task is a compat task or not. From the machine
perspective we would like to always use the randomization bits for
normal, non-compat tasks. But as the 2GB address space for compat tasks
is really limited the randomization is applied in bits 2^12..2^22 for
compat tasks vs 2^19..2^29 for normal tasks at the cost of performance.
This has to do with the cache aliasing on z13.

By the way we will replace is_32bit_task with() is_compat_task(), I have
a patch from Heiko pending for that.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.