Kernel: Use the pre-image kernel memory range introduced by KASLR

This ensures we don't just waste the memory range between the default
base load address and the actual load address that was shifted by the
KASLR offset.

Kernel/BootInfo.h

@@ -18,6 +18,7 @@ extern "C" PhysicalAddress start_of_prekernel_image;
 extern "C" PhysicalAddress end_of_prekernel_image;
 extern "C" size_t physical_to_virtual_offset;
 extern "C" FlatPtr kernel_mapping_base;
+extern "C" FlatPtr default_kernel_load_base;
 extern "C" FlatPtr kernel_load_base;
 #if ARCH(X86_64)
 extern "C" u32 gdt64ptr;

Kernel/Memory/PageDirectory.cpp

@@ -13,6 +13,7 @@
 #include <Kernel/Random.h>
 #include <Kernel/Sections.h>
 
+extern u8 start_of_kernel_image[];
 extern u8 end_of_kernel_image[];
 
 namespace Kernel::Memory {
@@ -35,9 +36,11 @@ UNMAP_AFTER_INIT NonnullRefPtr<PageDirectory> PageDirectory::must_create_kernel_
 {
     auto directory = adopt_ref_if_nonnull(new (nothrow) PageDirectory).release_nonnull();
 
-    // make sure this starts in a new page directory to make MemoryManager::initialize_physical_pages() happy
-    FlatPtr start_of_range = ((FlatPtr)end_of_kernel_image & ~(FlatPtr)0x1fffff) + 0x200000;
-    MUST(directory->m_range_allocator.initialize_with_range(VirtualAddress(start_of_range), KERNEL_PD_END - start_of_range));
+    MUST(directory->m_range_allocator.initialize_with_range(VirtualAddress(default_kernel_load_base), KERNEL_PD_END - default_kernel_load_base));
+    // Carve out the whole page directory covering the kernel image to make MemoryManager::initialize_physical_pages() happy
+    FlatPtr start_of_range = ((FlatPtr)start_of_kernel_image & ~(FlatPtr)0x1fffff);
+    FlatPtr end_of_range = ((FlatPtr)end_of_kernel_image & ~(FlatPtr)0x1fffff) + 0x200000;
+    MUST(directory->m_range_allocator.try_allocate_specific(VirtualAddress(start_of_range), end_of_range - start_of_range));
 
     return directory;
 }

Kernel/Prekernel/Prekernel.h

@@ -22,6 +22,7 @@ struct [[gnu::packed]] BootInfo {
     u32 end_of_prekernel_image;
     u64 physical_to_virtual_offset;
     u64 kernel_mapping_base;
+    u64 default_kernel_load_base;
     u64 kernel_load_base;
 #    if ARCH(X86_64)
     u32 gdt64ptr;

Kernel/Prekernel/init.cpp

@@ -184,6 +184,7 @@ extern "C" [[noreturn]] void init()
     info.end_of_prekernel_image = (PhysicalPtr)end_of_prekernel_image;
     info.physical_to_virtual_offset = kernel_load_base - kernel_physical_base;
     info.kernel_mapping_base = kernel_mapping_base;
+    info.default_kernel_load_base = default_kernel_load_base;
     info.kernel_load_base = kernel_load_base;
 #if ARCH(X86_64)
     info.gdt64ptr = (PhysicalPtr)gdt64ptr;

Kernel/init.cpp

@@ -116,6 +116,7 @@ READONLY_AFTER_INIT PhysicalAddress start_of_prekernel_image;
 READONLY_AFTER_INIT PhysicalAddress end_of_prekernel_image;
 READONLY_AFTER_INIT size_t physical_to_virtual_offset;
 READONLY_AFTER_INIT FlatPtr kernel_mapping_base;
+READONLY_AFTER_INIT FlatPtr default_kernel_load_base;
 READONLY_AFTER_INIT FlatPtr kernel_load_base;
 #if ARCH(X86_64)
 READONLY_AFTER_INIT PhysicalAddress boot_pml4t;
@@ -148,6 +149,7 @@ extern "C" [[noreturn]] UNMAP_AFTER_INIT void init(BootInfo const& boot_info)
     end_of_prekernel_image = PhysicalAddress { boot_info.end_of_prekernel_image };
     physical_to_virtual_offset = boot_info.physical_to_virtual_offset;
     kernel_mapping_base = boot_info.kernel_mapping_base;
+    default_kernel_load_base = boot_info.default_kernel_load_base;
     kernel_load_base = boot_info.kernel_load_base;
 #if ARCH(X86_64)
     gdt64ptr = boot_info.gdt64ptr;