Kernel+LibC: Add MAP_RANDOMIZED flag for sys$mmap()

This can be used to request random VM placement instead of the highly
predictable regular mmap(nullptr, ...) VM allocation strategy.

It will soon be used to implement ASLR in the dynamic loader. :^)

Kernel/VM/RangeAllocator.cpp

@@ -106,6 +106,31 @@ void RangeAllocator::carve_at_index(int index, const Range& range)
         m_available_ranges.insert(index + 1, move(remaining_parts[1]));
 }
 
+Optional<Range> RangeAllocator::allocate_randomized(size_t size, size_t alignment)
+{
+    if (!size)
+        return {};
+
+    ASSERT((size % PAGE_SIZE) == 0);
+    ASSERT((alignment % PAGE_SIZE) == 0);
+
+    // FIXME: I'm sure there's a smarter way to do this.
+    static constexpr size_t maximum_randomization_attempts = 1000;
+    for (size_t i = 0; i < maximum_randomization_attempts; ++i) {
+        VirtualAddress random_address { get_good_random<FlatPtr>() };
+        random_address.mask(PAGE_MASK);
+
+        if (!m_total_range.contains(random_address))
+            continue;
+
+        auto range = allocate_specific(random_address, size);
+        if (range.has_value())
+            return range;
+    }
+
+    return allocate_anywhere(size, alignment);
+}
+
 Optional<Range> RangeAllocator::allocate_anywhere(size_t size, size_t alignment)
 {
     if (!size)