// N.B. since we used a custom stack with makecontext,
	// callbackUpdateSystemStack had to guess the bounds. Its guess assumes
	// a 32KiB stack.
	char *prev_stack_lo = stack2 + STACK_SIZE - (32*1024);

	// New SP is just barely in bounds, but if we don't update the bounds
	// we'll almost certainly overflow. The SP that
	// callbackUpdateSystemStack sees already has some data pushed, so it