Kernel: Forked children should inherit their RangeAllocator by copy.

Otherwise we'll start handing out addresses that are very likely already in
use by existing ranges.

Kernel/Process.cpp

@@ -557,7 +557,7 @@ Process::Process(String&& name, uid_t uid, gid_t gid, pid_t ppid, RingLevel ring
 {
     dbgprintf("Process: New process PID=%u with name=%s\n", m_pid, m_name.characters());
 
-    m_page_directory = PageDirectory::create_for_userspace();
+    m_page_directory = PageDirectory::create_for_userspace(fork_parent ? &fork_parent->page_directory().range_allocator() : nullptr);
 #ifdef MM_DEBUG
     dbgprintf("Process %u ctor: PD=%x created\n", pid(), m_page_directory.ptr());
 #endif

Kernel/VM/PageDirectory.cpp

@@ -12,8 +12,8 @@ PageDirectory::PageDirectory(PhysicalAddress paddr)
     m_directory_page = PhysicalPage::create_eternal(paddr, true);
 }
 
-PageDirectory::PageDirectory()
+PageDirectory::PageDirectory(const RangeAllocator* parent_range_allocator)
-    : m_range_allocator(LinearAddress(userspace_range_base), kernelspace_range_base - userspace_range_base)
+    : m_range_allocator(parent_range_allocator ? RangeAllocator(*parent_range_allocator) : RangeAllocator(LinearAddress(userspace_range_base), kernelspace_range_base - userspace_range_base))
 {
     MM.populate_page_directory(*this);
 }

Kernel/VM/PageDirectory.h

@@ -9,7 +9,7 @@
 class PageDirectory : public Retainable<PageDirectory> {
     friend class MemoryManager;
 public:
-    static Retained<PageDirectory> create_for_userspace() { return adopt(*new PageDirectory); }
+    static Retained<PageDirectory> create_for_userspace(const RangeAllocator* parent_range_allocator = nullptr) { return adopt(*new PageDirectory(parent_range_allocator)); }
     static Retained<PageDirectory> create_at_fixed_address(PhysicalAddress paddr) { return adopt(*new PageDirectory(paddr)); }
     ~PageDirectory();
 
@@ -21,7 +21,7 @@ public:
     RangeAllocator& range_allocator() { return m_range_allocator; }
 
 private:
-    PageDirectory();
+    explicit PageDirectory(const RangeAllocator* parent_range_allocator);
     explicit PageDirectory(PhysicalAddress);
 
     RangeAllocator m_range_allocator;

Kernel/VM/RangeAllocator.cpp

@@ -12,6 +12,11 @@ RangeAllocator::RangeAllocator(LinearAddress base, size_t size)
 #endif
 }
 
+RangeAllocator::RangeAllocator(const RangeAllocator& parent_allocator)
+    : m_available_ranges(parent_allocator.m_available_ranges)
+{
+}
+
 RangeAllocator::~RangeAllocator()
 {
 }

Kernel/VM/RangeAllocator.h

@@ -46,6 +46,7 @@ private:
 class RangeAllocator {
 public:
     RangeAllocator(LinearAddress, size_t);
+    RangeAllocator(const RangeAllocator&);
     ~RangeAllocator();
 
     Range allocate_anywhere(size_t);