On Mon, Apr 24, 2017 at 10:32 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? > > 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. >From include/linux/threads.h: /* * A maximum of 4 million PIDs should be enough for a while. * [NOTE: PID/TIDs are limited to 2^29 ~= 500+ million, see futex.h.] */ #define PID_MAX_LIMIT (CONFIG_BASE_SMALL ? PAGE_SIZE * 8 : \ (sizeof(long) > 4 ? 4 * 1024 * 1024 : PID_MAX_DEFAULT)) AFAICS that means you can only have up to 2^22 running tasks at a time, since every running task requires a PID in the init pid namespace.
