[PATCH 1/2] mm: mmap: Add new /proc tunable for mmap_base ASLR.

[Daniel Cashman]

On 10/28/2015 08:41 PM, Eric W. Biederman wrote:
> Dan Cashman <dcashman at android.com> writes:
> 
>>>> This all would be much cleaner if the arm architecture code were just to
>>>> register the sysctl itself.
>>>>
>>>> As it sits this looks like a patchset that does not meaninfully bisect,
>>>> and would result in code that is hard to trace and understand.
>>>
>>> I believe the intent is to follow up with more architecture specific
>>> patches to allow each architecture to define the number of bits to use
>>
>> Yes.  I included these patches together because they provide mutual
>> context, but each has a different outcome and they could be taken
>> separately.
> 
> They can not.  The first patch is incomplete by itself.

Could you be more specific in what makes the first patch incomplete?  Is
it because it is essentially a no-op without additional architecture
changes (e.g. the second patch) or is it specifically because it
introduces and uses the three "mmap_rnd_bits*" variables without
defining them?  If the former, I'd like to avoid combining the general
procfs change with any architecture-specific one(s).  If the latter, I
hope the proposal below addresses that.

>> The arm architecture-specific portion allows the changing
>> of the number of bits used for mmap ASLR, useful even without the
>> sysctl.  The sysctl patch (patch 1) provides another way of setting
>> this value, and the hope is that this will be adopted across multiple
>> architectures, with the arm changes (patch 2) providing an example.  I
>> hope to follow this with changes to arm64 and x86, for example.
> 
> If you want to make the code generic.  Please maximize the sharing.
> That is please define the variables in a generic location, as well
> as the Kconfig variables (if possible).
> 
> As it is you have an architecture specific piece of code that can not be
> reused without duplicating code, and that is just begging for problems.

I think it would make sense to move the variable definitions into
mm/mmap.c, included conditionally based on the presence of
CONFIG_ARCH_MMAP_RND_BITS.

As for the Kconfigs, I am open to suggestions.  I considered declaring
and documenting ARCH_MMAP_RND_BITS in arch/Kconfig, but I would like it
to be bounded in range by the _MIN and _MAX values, which necessarily
must be defined in the arch-specific Kconfigs.  Thus, we'd have
ARCH_MMAP_RND_BITS declared in arch/Kconfig as it currently is in
arch/arm/Kconfig defaulting to _MIN, and would declare both the _MIN and
_MAX in arch/Kconfig, while specifying default values in
arch/${ARCH}/Kconfig.

Would these changes be more acceptable?

Thank You,
Dan