POWER: Unexpected fault when writing to brk-allocated memory

[Aneesh Kumar K.V]

On 11/06/2017 01:55 PM, Nicholas Piggin wrote:
> On Mon, 6 Nov 2017 09:11:37 +0100
> Florian Weimer <fweimer at redhat.com> wrote:
> 
>> On 11/06/2017 07:47 AM, Nicholas Piggin wrote:
>>> "You get < 128TB unless explicitly requested."
>>>
>>> Simple, reasonable, obvious rule. Avoids breaking apps that store
>>> some bits in the top of pointers (provided that memory allocator
>>> userspace libraries also do the right thing).
>>
>> So brk would simplify fail instead of crossing the 128 TiB threshold?
> 
> Yes, that was the intention and that's what x86 seems to do.
> 
>>
>> glibc malloc should cope with that and switch to malloc, but this code
>> path is obviously less well-tested than the regular way.
> 
> Switch to mmap() I guess you meant?
> 
> powerpc has a couple of bugs in corner cases, so those should be fixed
> according to intended policy for stable kernels I think.
> 
> But I question the policy. Just seems like an ugly and ineffective wart.
> Exactly for such cases as this -- behaviour would change from run to run
> depending on your address space randomization for example! In case your
> brk happens to land nicely on 128TB then the next one would succeed.

Why ? It should not change between run to run. We limit the free
area search range based on hint address. So we should get consistent 
results across run. even if we changed the context.addr_limit.

-aneesh