From 036a7423fe3d756010561f860e50b9de81043c47 Mon Sep 17 00:00:00 2001
From: Furkan Mudanyali <fmudanyali@icloud.com>
Date: Tue, 19 Jul 2022 11:47:14 +0300
Subject: [PATCH] Improve GDT

---
 kernel/arch/i386/boot/gdt.c | 60 +++++++++++++++----------------------
 kernel/arch/i386/boot/idt.c |  1 -
 kernel/include/kernel/gdt.h | 31 ++++---------------
 3 files changed, 30 insertions(+), 62 deletions(-)

diff --git a/kernel/arch/i386/boot/gdt.c b/kernel/arch/i386/boot/gdt.c
index 7b8ee61..9a3df80 100644
--- a/kernel/arch/i386/boot/gdt.c
+++ b/kernel/arch/i386/boot/gdt.c
@@ -1,51 +1,39 @@
-/**
- *  Referenced from https://github.com/tendstofortytwo/clay
- *  kernel/kernel.c
- */
-
 #include <kernel/gdt.h>
 #include <stdio.h>
 #include <stdint.h>
 
-uint32_t TSS[32];
-gdt_entry GDT[GDT_ENTRIES];
-gdt_ptr gp;
+static uint32_t tss[32];
+static gdt_entry_t gdt[GDT_ENTRIES];
+static gdtr_t gdtptr;
 
-void gdt_fill(gdt_entry* target, gdt* source) {
-    if(source->limit > 0xFFFFF) {
-        printf("Invalid GDT limit %d\n", source->limit);
-        for(;;);
-    }
+void gdt_set_descriptor(uint8_t vector, uint32_t base, uint32_t limit, uint8_t access, uint8_t flags) {
+    gdt[vector].base_low = base & 0xFFFF;
+    gdt[vector].base_mid = (base >> 16) & 0xFF;
+    gdt[vector].base_high = (base >> 24) & 0xFF;
 
-    target->limit_lo = source->limit & 0xFFFF;
-    target->limit_hi_flags = (source->limit >> 16) & 0x0F;
-    target->base_lo = source->base & 0xFFFF;
-    target->base_mid = (source->base >> 16) & 0xFF;
-    target->base_hi = (source->base >> 24) & 0xFF;
-    target->access = source->access;
-    target->limit_hi_flags |= (source->flags << 4);
+    gdt[vector].limit_low = limit & 0xFFFF;
+    gdt[vector].limit_hi_flags = (limit >> 16) & 0x0F;
+    gdt[vector].limit_hi_flags |= flags << 4;
+
+    gdt[vector].access = access;
 }
 
 void gdt_init(void) {
-    uint32_t tssRef = (uint32_t) TSS;
-    gdt rows[GDT_ENTRIES] = {
-        {0, 0x00000, 0x00, 0x0}, // 0x0000 Null Descriptor
-        {0, 0xFFFFF, 0x9A, 0xC}, // 0x0008 Kernel Mode Code Segment
-        {0, 0xFFFFF, 0x92, 0xC}, // 0x0010 Kernel Mode Data Segment
-        {0, 0xFFFFF, 0xFA, 0xC}, // 0x0018 User Mode Code Segment
-        {0, 0xFFFFF, 0xF2, 0xC}, // 0x0020 User Mode Data Segment
-        {tssRef, sizeof(TSS), 0x89, 0x0}, // 0x0028 Task State Segment
-    };
+    gdt_set_descriptor(0, 0, 0, 0, 0);              // 0x0000 Null Descriptor
+    gdt_set_descriptor(1, 0, 0xFFFFF, 0x9A, 0xC);   // 0x0008 Kernel Mode Code Segment
+    gdt_set_descriptor(2, 0, 0xFFFFF, 0x92, 0xC);   // 0x0010 Kernel Mode Data Segment
+    gdt_set_descriptor(3, 0, 0xFFFFF, 0xFA, 0xC);   // 0x0018 User Mode Code Segment
+    gdt_set_descriptor(4, 0, 0xFFFFF, 0xF2, 0xC);   // 0x0020 User Mode Data Segment
+    gdt_set_descriptor(5, (uintptr_t)&tss[0], sizeof(tss), 0x89, 0); // 0x0028 Task State Segment
 
-    for(uint32_t i = 0; i < GDT_ENTRIES; ++i) {
-        gdt_fill(GDT+i, rows+i);
-    }
+    gdtptr.base = (uintptr_t)&gdt[0];
+    gdtptr.limit = sizeof(gdt_entry_t) * GDT_ENTRIES - 1;
 
-    gp.base = (uintptr_t)&GDT[0];
-    gp.limit = sizeof(gdt_entry) * GDT_ENTRIES - 1;
-
-    __asm__ volatile("lgdt %0" : : "m"(gp)); // Load new GDT
+    __asm__ volatile("lgdt %0" : : "m"(gdtptr)); // Load new GDT
     __asm__ volatile(
+        "mov %cr0, %eax\n"
+        "or $1, %al\n" // set PE (Protection Enable) bit in CR0 (Control Register 0)
+        "mov %eax, %cr0\n"
         "jmp $0x0008, $fix_cs\n"
         "fix_cs:\n"
         "mov $0x0010, %ax\n"
diff --git a/kernel/arch/i386/boot/idt.c b/kernel/arch/i386/boot/idt.c
index 688ecc0..5c3e280 100644
--- a/kernel/arch/i386/boot/idt.c
+++ b/kernel/arch/i386/boot/idt.c
@@ -141,7 +141,6 @@ void idt_init(void) {
     idtptr.base = (uintptr_t)&idt[0];
     idtptr.limit = (uint16_t)sizeof(idt_entry_t) * IDT_MAX_DESCRIPTORS - 1;
 
-    idt_set_descriptor(0, int0_wrapper, 0x8E);
     idt_set_descriptor(0, int0_wrapper, 0x8E);
     idt_set_descriptor(1, int1_wrapper, 0x8E);
     idt_set_descriptor(2, int2_wrapper, 0x8E);
diff --git a/kernel/include/kernel/gdt.h b/kernel/include/kernel/gdt.h
index a1b111d..3c7ebd6 100644
--- a/kernel/include/kernel/gdt.h
+++ b/kernel/include/kernel/gdt.h
@@ -1,8 +1,3 @@
-/**
- * Referenced from https://github.com/tendstofortytwo/clay
- * kernel/kernel.c
- */
-
 #ifndef _KERNEL_GDT_H
 #define _KERNEL_GDT_H
 
@@ -13,29 +8,15 @@
 typedef struct {
     uint16_t limit;
     uint32_t base;
-} __attribute__((packed)) gdt_ptr;
+} __attribute__((packed)) gdtr_t;
 
 typedef struct {
-    uint16_t limit_lo;
-    uint16_t base_lo;
+    uint16_t limit_low;
+    uint16_t base_low;
     uint8_t base_mid;
     uint8_t access;
     uint8_t limit_hi_flags;
-    uint8_t base_hi;
-} __attribute__((packed)) gdt_entry;
+    uint8_t base_high;
+} __attribute__((packed)) gdt_entry_t;
 
-typedef struct {
-    uint32_t base;
-    uint32_t limit;
-    uint8_t access;
-    uint8_t flags;
-} __attribute__((packed)) gdt;
-
-void gdt_fill(gdt_entry*, gdt*);
-void gdt_init(void);
-
-extern uint32_t TSS[32];
-extern gdt_entry GDT[GDT_ENTRIES];
-extern gdt_ptr gp;
-
-#endif
+#endif
\ No newline at end of file