From 586fe3bb8085eff7ac7a8bcf13e8932beaeb9b8b Mon Sep 17 00:00:00 2001
From: ChUrl <christoph.urlacher@protonmail.com>
Date: Mon, 4 Jul 2022 16:04:55 +0200
Subject: [PATCH] fix paging bugs in writeprotection

---
 c_os/kernel/Paging.cc | 10 +++++++---
 c_os/kernel/Paging.h  | 22 ++++++++++++++++++++++
 c_os/main.cc          | 16 +++++++++-------
 3 files changed, 38 insertions(+), 10 deletions(-)

diff --git a/c_os/kernel/Paging.cc b/c_os/kernel/Paging.cc
index a45633f..8cb00f2 100644
--- a/c_os/kernel/Paging.cc
+++ b/c_os/kernel/Paging.cc
@@ -92,7 +92,7 @@ unsigned int* pg_alloc_page() {
         // pruefe ob Page frei
         if (((*p_page) & PAGE_RESERVED) == 0) {
             *p_page = (*p_page | PAGE_RESERVED);
-            return (unsigned int*)(i << 12);
+            return (unsigned int*)(i << 12); // Address without flags (Offset 0)
         }
     }
     return 0;
@@ -108,8 +108,10 @@ void pg_write_protect_page(unsigned int* p_page) {
 
     /* hier muss Code eingefügt werden */
 
+    unsigned int* page = (unsigned int*)PAGE_TABLE + ((unsigned int)p_page >> 12); // Pagetable entry
+
     unsigned int mask = PAGE_WRITEABLE;  // fill to 32bit
-    *p_page = *p_page & ~mask;           // set writable to 0
+    *page = *page & ~mask;           // set writable to 0
 }
 
 /*****************************************************************************
@@ -121,8 +123,10 @@ void pg_notpresent_page(unsigned int* p_page) {
 
     /* hier muss Code eingefügt werden */
 
+    unsigned int* page = (unsigned int*)PAGE_TABLE + ((unsigned int)p_page >> 12); // Pagetable entry
+
     unsigned int mask = PAGE_PRESENT;
-    *p_page = *p_page & ~mask;  // set present to 0
+    *page = *page & ~mask;  // set present to 0
 }
 
 /*****************************************************************************
diff --git a/c_os/kernel/Paging.h b/c_os/kernel/Paging.h
index 3d48591..0ad467f 100644
--- a/c_os/kernel/Paging.h
+++ b/c_os/kernel/Paging.h
@@ -52,6 +52,28 @@
 #ifndef __Paging_include__
 #define __Paging_include__
 
+// Bits fuer Eintraege in der Page-Table
+#define PAGE_PRESENT 0x001
+#define PAGE_WRITEABLE 0x002
+#define PAGE_BIGSIZE 0x080
+#define PAGE_RESERVED 0x800  // Bit 11 ist frei fuer das OS
+
+// Adresse des Page-Directory (benoetigt 4 KB)
+#define PAGE_DIRECTORY 0x200000
+
+// Adresse der Page-Table (benoetigt 4 KB)
+#define PAGE_TABLE 0x201000
+
+// Start- und End-Adresse der 4 KB Seiten die durch die Page-Table adressiert werden
+#define FST_ALLOCABLE_PAGE 0x202000
+#define LST_ALLOCABLE_PAGE 0x2FF000
+
+// Externe Funktionen in startup.asm
+extern "C" {
+    void paging_on(unsigned int* p_pdir);          // Paging einschalten
+    void invalidate_tlb_entry(unsigned int* ptr);  // Page in TLB invalid.
+}
+
 // ativiert paging
 extern void pg_init();
 
diff --git a/c_os/main.cc b/c_os/main.cc
index d213b14..18f9da8 100755
--- a/c_os/main.cc
+++ b/c_os/main.cc
@@ -56,27 +56,29 @@ int main() {
     pg_init();
 
     // Trigger Bluescreen
-    // kout << "Trigger Bluescreen, if you can read this it didn't work" << endl;
+    kout << "Trigger Bluescreen, if you can read this it didn't work" << endl;
 
     // BlueScreen 1
     // asm("int $3");
 
     // BlueScreen 2
-    // unsigned int* page = pg_alloc_page();
-    // *page = 42; // Should work
-    // pg_write_protect_page(page);
-    // *page = 42;  // should trigger BlueScreen
+    unsigned int* page = pg_alloc_page();
+    *page = 42;
+    pg_write_protect_page(page);
+    invalidate_tlb_entry(page); // If we don't invalidate after first access the write protection
+                                // won't work as no lookup is performed (address in tlb)
+    *page = 42; // We map logical to physical 1:1 so no need to do any lookup
+                // If tlb is invalidated this access produces a pagefault
 
     // Demo threads anlegen
     // scheduler.ready(new HelloWorldThread());
-    scheduler.ready(new CoopThreadDemo());
+    // scheduler.ready(new CoopThreadDemo());
     // scheduler.ready(new VBEdemo()); // Switch to VESA graphics mode
 
     // Scheduler starten (schedule() erzeugt den Idle-Thread)
     scheduler.schedule();
 
     // TODO: Use templates for queue so threads don't have to be casted down from chain
-    // TODO: Move scrollback control to thread
 
     // Scheduler doesn't return
     return 0;