From: Duje Mihanović <duje.mihanovic@skole.hr>
Date: Tue, 21 Jun 2022 13:30:41 +0000 (+0200)
Subject: Add C code for enabling paging in bootloader
X-Git-Tag: 0.1.1~5
X-Git-Url: https://git.dujemihanovic.xyz/%7B%7B%20.Permalink%20%7D%7D?a=commitdiff_plain;h=485fd9b6752dd8590ac786397e9a0a4487860290;p=nameless-os.git

Add C code for enabling paging in bootloader

Also compile the bootloader as ELF as well for easier debugging.
---

diff --git a/Makefile b/Makefile
index bff97e5..2eedce2 100644
--- a/Makefile
+++ b/Makefile
@@ -8,7 +8,7 @@ CFLAGS = -g -fgnu89-inline -Iinclude/arch/x86 -ffreestanding -DGIT_COMMIT=\"$(GI
 
 KERNEL_OBJ = kernel/entry.o kernel/arch/x86/tty/tty.o kernel/drivers/irq/i8259a.o kernel/arch/x86/irq/idt.o kernel/arch/x86/irq/sample_handler.o kernel/kernel.o
 
-BOOTLOADER_OBJ = boot/x86/mbr boot/x86/vbr-fat32 boot/x86/stage3/LOADER.BIN
+BOOTLOADER_OBJ = boot/x86/mbr boot/x86/vbr-fat32 boot/x86/stage3/LOADER.BIN boot/x86/stage3/loader.elf
 
 default: kernel/kernel.elf bootloader
 
@@ -21,7 +21,8 @@ run: all
 
 boot/x86/mbr: boot/x86/mbr.s
 boot/x86/vbr-fat32: boot/x86/vbr-fat32.s boot/x86/fat32/*.s 
-boot/x86/stage3/LOADER.BIN: boot/x86/stage3/*.s boot/x86/fat32/*.s
+boot/x86/stage3/LOADER.BIN: boot/x86/stage3/*.s boot/x86/stage3/*.c boot/x86/fat32/*.s
+boot/x86/stage3/loader.elf: boot/x86/stage3/*.s boot/x86/stage3/*.c boot/x86/fat32/*.s
 $(BOOTLOADER_OBJ):
 	$(MAKE) -C boot/x86
 
diff --git a/boot/x86/Makefile b/boot/x86/Makefile
index 0e5ac00..202d839 100644
--- a/boot/x86/Makefile
+++ b/boot/x86/Makefile
@@ -1,9 +1,10 @@
-default: mbr vbr-fat32 stage3/LOADER.BIN
+default: mbr vbr-fat32 stage3/LOADER.BIN stage3/loader.elf
 
+CFLAGS=-g -ffreestanding
 LD=i686-elf-gcc
-LDFLAGS=-T stage3/stage3.ld -nostdlib
+LDFLAGS=-T stage3/stage3.ld -nostdlib -g
 
-STAGE3_OBJ=stage3/loader.o
+STAGE3_OBJ=stage3/loader.o stage3/paging.o
 
 mbr: mbr.s
 	$(AS) $(ASFLAGS) -w-zeroing -o $@ $<
@@ -11,8 +12,11 @@ mbr: mbr.s
 vbr-fat32: vbr-fat32.s fat32/*.s
 	$(AS) $(ASFLAGS) -o $@ $<
 
+stage3/loader.elf: $(STAGE3_OBJ)
+	$(CC) $(LDFLAGS) -o $@ $^ -Wl,--oformat=elf32-i386
+
 stage3/LOADER.BIN: $(STAGE3_OBJ)
-	$(LD) $(LDFLAGS) -o $@ $<
+	$(CC) $(LDFLAGS) -o $@ $^
 
 stage3/loader.o: stage3/loader.s stage3/*.s
 	$(AS) $(ASFLAGS) -f elf -g dwarf2 -DGIT_REVISION=\"$(GIT_REV)\" -o $@ $<
diff --git a/boot/x86/stage3/loader.s b/boot/x86/stage3/loader.s
index cdecbaa..9ed7102 100644
--- a/boot/x86/stage3/loader.s
+++ b/boot/x86/stage3/loader.s
@@ -1,6 +1,10 @@
 bits 16
 cpu 686
 
+extern enable_paging
+extern map_range
+extern set_up_page_directory
+
 section .text
 %include "../fat32/fat32-structs.s"
 
@@ -69,6 +73,11 @@ _start:
 	print kernel_loaded
 	
 	call get_e820_map
+	mov ebx, ecx
+	mov eax, 24
+	mul ecx
+	add eax, 20
+	sub sp, ax
 
 	cli
 	lgdt [gdt]
@@ -168,13 +177,43 @@ in_protected:
 	mov fs, ax
 	mov gs, ax
 
-	call load_paging_structs
+	push dword 0
+	push 0xc0003000
+	push 0xc0000000
+	push 0x103000
+	push 0x100000
+	call map_range
+	cmp eax, 0
+	jne .error
+	add esp, 20
+	push dword 2
+	push 0xc0009000
+	push 0xc0003000
+	push 0x109000
+	push 0x103000
+	call map_range
+	cmp eax, 0
+	jne .error
+	add esp, 20
+	push dword 2
+	push 0x100000
+	push dword 0
+	push 0x100000
+	push dword 0
+	call map_range
+	cmp eax, 0
+	jne .error
+	add esp, 20
+
+	call set_up_page_directory
 	call enable_paging
 
-	jmp 0x8:0xc0000000
+	jmp 0xc0000000
 	nop
-
-%include "paging.s"
+.error:
+	mov dword [0xb8000], 0xcf28cf3a
+	hlt
+	jmp $-1
 
 section .rodata
 
diff --git a/boot/x86/stage3/paging.c b/boot/x86/stage3/paging.c
new file mode 100644
index 0000000..0482ded
--- /dev/null
+++ b/boot/x86/stage3/paging.c
@@ -0,0 +1,70 @@
+/* Code for enabling paging */
+
+#include <stdint.h>
+
+#define ADDRESS_NOT_ALIGNED 1
+#define INVL_ADDRESS 2
+#define ADDRESS_ALREADY_MAPPED 3
+#define ADDRESS_RANGE_MISMATCHED 4
+
+static uint32_t page_directory[1024] __attribute__((aligned(4096))) __attribute__((section(".data")));
+static uint32_t page_table_firstmb[1024] __attribute__((aligned(4096))) __attribute__((section(".data")));
+static uint32_t page_table_kernel[1024] __attribute__((aligned(4096))) __attribute__((section(".data")));
+
+void enable_paging()
+{
+	int cr3;
+	asm ("mov %0, %%cr3": : "a" (page_directory));
+	asm ("mov %%cr0, %0": "=a" (cr3));
+	cr3 |= 0x80000000;
+	asm ("mov %0, %%cr0": : "a" (cr3));
+}
+
+int map_address(void *physical, void *virtual, int flags)
+{
+	if ((uint32_t) physical & 0xfff || (uint32_t) virtual & 0xfff)
+		return ADDRESS_NOT_ALIGNED;
+
+	uint32_t pdir_index = (uint32_t) virtual >> 22;
+	if (pdir_index != 0 && pdir_index != 0x300)
+		return INVL_ADDRESS;
+
+	uint32_t ptbl_index = ((uint32_t) virtual >> 12) & 0x3ff;
+	switch (pdir_index) {
+	case 0:
+		if (page_table_firstmb[ptbl_index] & 1 == 1)
+			return ADDRESS_ALREADY_MAPPED;
+		page_table_firstmb[ptbl_index] = ((uint32_t) physical & ~0xfff) | (flags & 0xfff) | 1;
+		break;
+	case 0x300:
+		if (page_table_kernel[ptbl_index] & 1 == 1)
+			return ADDRESS_ALREADY_MAPPED;
+		page_table_kernel[ptbl_index] = ((uint32_t) physical & ~0xfff) | (flags & 0xfff) | 1;
+	}
+
+	return 0;
+}
+
+int map_range(void *phys_start, void *phys_end, void *virt_start, void *virt_end, int flags)
+{
+	if ((uint32_t) phys_start & 0xfff || (uint32_t) phys_end & 0xfff ||
+			(uint32_t) virt_start & 0xfff || (uint32_t) virt_end & 0xfff)
+		return ADDRESS_NOT_ALIGNED;
+
+	if ((uint32_t) phys_end - (uint32_t) phys_start != (uint32_t) virt_end - (uint32_t) virt_start)
+		return ADDRESS_RANGE_MISMATCHED;
+
+	int loops = ((uint32_t) phys_end - (uint32_t) phys_start) / 4096;
+	for (int i=0; i<loops; i++) {
+		int ret = map_address(phys_start + i*4096, virt_start + i*4096, flags);
+		if (ret)
+			return ret;
+	}
+	return 0;
+}
+
+void set_up_page_directory()
+{
+	page_directory[0] = (uint32_t) page_table_firstmb | 3;
+	page_directory[0x300] = (uint32_t) page_table_kernel | 3;
+}
diff --git a/boot/x86/stage3/paging.s b/boot/x86/stage3/paging.s
deleted file mode 100644
index 004da84..0000000
--- a/boot/x86/stage3/paging.s
+++ /dev/null
@@ -1,80 +0,0 @@
-; Code for enabling paging before calling the kernel
-; Identity maps the VGA framebuffer memory and high-half maps the kernel memory
-bits 32
-
-section .text
-
-; The problem with this code is that it assumes that the kernel's various
-; sections occupy a certain number of pages. As of writing it is correct, but as
-; the kernel grows this code may fail to map those pages, which is not good. The
-; solution to this is to use ELF instead of a flat binary. This was not done
-; before because it would require paging, but now that paging works using ELF is
-; a possibility which must be exploited.
-
-enable_paging:
-	push eax
-	mov eax, cr0
-	or eax, 0x80000000
-	mov cr0, eax
-	pop eax
-	ret
-
-load_paging_structs:
-	push eax
-	push ebx
-	xor ebx, ebx
-.pt_low_loop:
-	mov eax, ebx
-	shl eax, 12
-	or eax, 1|2 ; P and R/W flags
-	mov [page_table_low+ebx*4], eax
-	inc ebx
-	cmp ebx, 0x100
-	jl .pt_low_loop
-
-	xor ebx, ebx
-.pt_high_ro_loop:
-	mov eax, ebx
-	add eax, 0x100
-	shl eax, 12
-	or eax, 1 ; P flag
-	mov [page_table_high+ebx*4], eax
-	inc ebx
-	cmp ebx, 0x2
-	jl .pt_high_ro_loop
-
-	mov ebx, 0x2
-.pt_high_rw_loop:
-	mov eax, ebx
-	add eax, 0x100
-	shl eax, 12
-	or eax, 1|2 ; P and R/W flags
-	mov [page_table_high+ebx*4], eax
-	inc ebx
-	cmp ebx, 0x9
-	jl .pt_high_rw_loop
-
-	mov eax, page_table_low
-	and eax, 0xfffff000
-	or eax, 1|2
-	mov [page_directory], eax
-
-	mov eax, page_table_high
-	and eax, 0xfffff000
-	or eax, 1|2
-	mov [page_directory+768*4], eax
-
-	mov eax, page_directory
-	mov cr3, eax
-	pop ebx
-	pop eax
-	ret
-
-section .data
-align 4096
-page_table_low:
-	times 1024 dd 0
-page_table_high:
-	times 1024 dd 0
-page_directory:
-	times 1024 dd 0
diff --git a/kernel/entry.s b/kernel/entry.s
index 5ef71bb..c38ac76 100644
--- a/kernel/entry.s
+++ b/kernel/entry.s
@@ -6,7 +6,7 @@ global _start
 _start:
 	mov ebp, __STACK_BOTTOM__
 	mov esp, ebp
-	push ecx
+	push ebx
 	push edi
 	call kmain
 	hlt