On Mon, Apr 24, 2017 at 1:32 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > On Fri, Apr 21, 2017 at 03:09:39PM -0700, Kees Cook wrote: >> This patch ports the x86-specific atomic overflow handling from PaX's >> PAX_REFCOUNT to the upstream refcount_t API. This is an updated version >> from PaX that eliminates the saturation race condition by resetting the >> atomic counter back to the INT_MAX saturation value on both overflow and >> underflow. To win a race, a system would have to have INT_MAX threads >> simultaneously overflow before the saturation handler runs. > > And is this impossible? Highly unlikely I'll grant you, but absolutely > impossible? I'll adjust the language. "Highly unlikely" is still better than "trivially doable with a single thread". :) > Also, you forgot nr_cpus in your bound. Afaict the worst case here is > O(nr_tasks + 3*nr_cpus). > > Because PaX does it, is not a correctness argument. And this really > wants one. Sure, I didn't mean to imply anything other than a demonstration of what PaX is doing (and that it's better than not having it). If we can improve it, that's great. -Kees -- Kees Cook Pixel Security
