From c3c905aa6c36d63b55736d361ebecb857ecfd605 Mon Sep 17 00:00:00 2001
From: supercomputer7 <liavalb@gmail.com>
Date: Fri, 8 Nov 2019 16:37:33 +0200
Subject: [PATCH] Kernel: Removing hardcoded offsets from Memory Manager

Now the kernel page directory and the page tables are located at a
safe address, to prevent from paging data colliding with garbage.
---
 Kernel/Arch/i386/Boot/boot.S |  7 +++++++
 Kernel/VM/MemoryManager.cpp  | 14 ++++++--------
 Kernel/VM/MemoryManager.h    |  4 ++--
 Kernel/init.cpp              |  4 ++--
 Kernel/linker.ld             |  1 +
 5 files changed, 18 insertions(+), 12 deletions(-)

diff --git a/Kernel/Arch/i386/Boot/boot.S b/Kernel/Arch/i386/Boot/boot.S
index baab5ec2e8..535f13d848 100644
--- a/Kernel/Arch/i386/Boot/boot.S
+++ b/Kernel/Arch/i386/Boot/boot.S
@@ -31,6 +31,11 @@ stack_bottom:
 .skip 32768
 stack_top:
 
+.section .page_tables
+.align 4096
+page_tables_start:
+.skip 4096*3
+
 .section .text
 
 .global start
@@ -52,7 +57,9 @@ start:
 
     mov %ebx, multiboot_info_ptr
 
+    pushl $page_tables_start
     call init
+    add $4, %esp
 
     pushl $exit_message
     call kprintf
diff --git a/Kernel/VM/MemoryManager.cpp b/Kernel/VM/MemoryManager.cpp
index 475d9015bc..39fa0ad3c6 100644
--- a/Kernel/VM/MemoryManager.cpp
+++ b/Kernel/VM/MemoryManager.cpp
@@ -20,13 +20,11 @@ MemoryManager& MM
     return *s_the;
 }
 
-MemoryManager::MemoryManager()
+MemoryManager::MemoryManager(u32 physical_address_for_kernel_page_tables)
 {
-    // FIXME: Hard-coding these is stupid. Find a better way.
-    m_kernel_page_directory = PageDirectory::create_at_fixed_address(PhysicalAddress(0x4000));
-    m_page_table_zero = (PageTableEntry*)0x6000;
-    m_page_table_one = (PageTableEntry*)0x7000;
-
+    m_kernel_page_directory = PageDirectory::create_at_fixed_address(PhysicalAddress(physical_address_for_kernel_page_tables));
+    m_page_table_zero = (PageTableEntry*)(physical_address_for_kernel_page_tables + PAGE_SIZE);
+    m_page_table_one = (PageTableEntry*)(physical_address_for_kernel_page_tables + PAGE_SIZE * 2);
     initialize_paging();
 
     kprintf("MM initialized.\n");
@@ -262,9 +260,9 @@ void MemoryManager::create_identity_mapping(PageDirectory& page_directory, Virtu
     }
 }
 
-void MemoryManager::initialize()
+void MemoryManager::initialize(u32 physical_address_for_kernel_page_tables)
 {
-    s_the = new MemoryManager;
+    s_the = new MemoryManager(physical_address_for_kernel_page_tables);
 }
 
 Region* MemoryManager::kernel_region_from_vaddr(VirtualAddress vaddr)
diff --git a/Kernel/VM/MemoryManager.h b/Kernel/VM/MemoryManager.h
index 4fad7782c2..9a3ffd1018 100644
--- a/Kernel/VM/MemoryManager.h
+++ b/Kernel/VM/MemoryManager.h
@@ -38,7 +38,7 @@ class MemoryManager {
 public:
     static MemoryManager& the();
 
-    static void initialize();
+    static void initialize(u32 physical_address_for_kernel_page_tables);
 
     PageFaultResponse handle_page_fault(const PageFault&);
 
@@ -79,7 +79,7 @@ public:
     }
 
 private:
-    MemoryManager();
+    MemoryManager(u32 physical_address_for_kernel_page_tables);
     ~MemoryManager();
 
     void register_vmo(VMObject&);
diff --git a/Kernel/init.cpp b/Kernel/init.cpp
index 77f60f1c10..16f03a59e8 100644
--- a/Kernel/init.cpp
+++ b/Kernel/init.cpp
@@ -206,7 +206,7 @@ extern "C" int __cxa_atexit ( void (*)(void *), void *, void *)
     return 0;
 }
 
-extern "C" [[noreturn]] void init()
+extern "C" [[noreturn]] void init(u32 physical_address_for_kernel_page_tables)
 {
     // this is only used one time, directly below here. we can't use this part
     // of libc at this point in the boot process, or we'd just pull strstr in
@@ -268,7 +268,7 @@ extern "C" [[noreturn]] void init()
 
     kprintf("Starting Serenity Operating System...\n");
 
-    MemoryManager::initialize();
+    MemoryManager::initialize(physical_address_for_kernel_page_tables);
 
     if (APIC::init())
         APIC::enable(0);
diff --git a/Kernel/linker.ld b/Kernel/linker.ld
index 169fe43c3a..43d1115caf 100644
--- a/Kernel/linker.ld
+++ b/Kernel/linker.ld
@@ -8,6 +8,7 @@ SECTIONS
 	{
         Arch/i386/Boot/boot.ao
 		*(.multiboot)
+		*(.page_tables)
 		*(.text)
 		*(.text.startup)
 	}