POWER: Unexpected fault when writing to brk-allocated memory

[Nicholas Piggin]

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. If
not, then behaviour changes. And you didn't catch or prevent any bugs.

Thanks,
Nick