[PATCH v4] aarch64: enforce >=64K guard size

[Siddhesh Poyarekar]

On 07/16/2018 04:09 PM, Szabolcs Nagy wrote:
> on aarch64 safe behaviour is only guaranteed if the
> guard is >=64k, a smaller guard does not guard against
> stack overflow, so what's the point of a smaller guard?
> 
> user code cannot reason about stack usage in any sensible
> way so it cannot be smart about the guard size, yet there
> is lot of existing code setting the guard size explicitly
> to a single page.  the only effect of granting those
> settings is to make code unsafe on systems with smaller
> than 64k page size.

To elaborate, aarch64 has two configurations in the wild; RHEL does 64K 
pages and Debian does 4K pages but the probe interval seems to be fixed 
at 64K.  RHEL won't be affected because any stack allocation requests < 
64K will simply be rounded to 64K but Debian systems with 4K sizes will 
need this hack.

This should however also include a note in the manual to warn users 
about this behaviour.

Siddhesh