Re: [PATCH] flexmmap: optimise mmap_base gap for hard limited stack

[Utz Lehmann <lkml@de.tecosim.com>]

Andrea Arcangeli [andrea@novell.com] wrote:
> I developed a sysctl several years ago in all my 2.2 and 2.4 kernels
> including all 2.2 and 2.4 SUSE kernels that major software vendors
> requires for safety of their apps. IIRC I tried to merge it once but I
> failed (got not applied to mainline). Now I'v just got another bugzilla
> open about the lack of the sysctl and the major app is now again not
> foolproof. A fixed number won't work, so I have to drop such a fixed GAP
> anyways and rewrite it by forward porting my patch.
> 
> The sysctl in question is /proc/sys/vm/heap-stack-gap, so I recommend to
> drop all those fixed GAP sizes and implement this instead:
> 
> 	http://www.us.kernel.org/pub/linux/kernel/people/andrea/kernels/v2.4/2.4.23aa3/00_silent-stack-overflow-20
> 
> If you reinvet the wheel and you prefer not to share the above code to
> make a sysctl, at least make sure to use the name "heap-stack-gap" to
> avoid any pointless incompatibility.

I dont know if i understand your patch correctly. It looks that there is a
gap wandering below the actual stack. If this is the case than i think it's
not suited for the flexmmap layout. With flexmmap the mappings are top down
from mmap_base.