On Tue 04-07-17 13:59:59, Michal Hocko wrote: > On Tue 04-07-17 12:36:11, Ben Hutchings wrote: > > On Tue, 2017-07-04 at 12:42 +0200, Michal Hocko wrote: > > > On Tue 04-07-17 11:47:28, Willy Tarreau wrote: > > > > On Tue, Jul 04, 2017 at 11:35:38AM +0200, Michal Hocko wrote: > > [...] > > > > But wouldn't this completely disable the check in case such a guard page > > > > is installed, and possibly continue to allow the collision when the > > > > stack > > > > allocation is large enough to skip this guard page ? > > > > > > Yes and but a PROT_NONE would fault and as the changelog says, we _hope_ > > > that userspace does the right thing. > > > > It may well not be large enough, because of the same wrong assumptions > > that resulted in the kernel's guard page not being large enough. We > > should count it as part of the guard gap but not a substitute. > > yes, you are right of course. But isn't this a bug on their side > considering they are managing their _own_ stack gap? Our stack gap > management is a best effort thing and two such approaches competing will > always lead to weird cornercases. That was my assumption when saying > that I am not sure this is really _worth_ it. We should definitely try > to workaround clashes but that's about it. If others think that we > should do everything to prevent even those issues I will not oppose > of course. It just adds more cycles to something that is a weird case > already.
Forgot to mention another point. Currently we do not check other previous vmas if prev->vm_flags & VM_GROWSDOWN. Consider that the stack gap is implemented by mprotect. This wouldn't change the VM_GROWSDOWN flag and we are back to square 1 because the gap might be too small. Do we want/need to handle those cases. Are they too different from MAP_FIXED gaps? I am not so sure but I would be inclined to say no. -- Michal Hocko SUSE Labs