From f60d40f3bf4dfe8ed6f63a27367d323319a4ef97 Mon Sep 17 00:00:00 2001
From: dzwdz
Date: Fri, 25 Jun 2021 16:22:43 +0200
Subject: ring3

---
 Makefile            |  6 +++---
 kernel/gdt.c        | 20 +++++++-------------
 kernel/gdt.h        | 12 ++++++++++++
 kernel/main.c       | 16 ++++++++++++++++
 kernel/util.c       |  9 +++++++++
 kernel/util.h       |  4 ++++
 platform/sysenter.h |  4 ++++
 platform/sysenter.s | 31 +++++++++++++++++++++++++++++++
 8 files changed, 86 insertions(+), 16 deletions(-)
 create mode 100644 kernel/util.c
 create mode 100644 kernel/util.h
 create mode 100644 platform/sysenter.h
 create mode 100644 platform/sysenter.s

diff --git a/Makefile b/Makefile
index a1f9899..605b112 100644
--- a/Makefile
+++ b/Makefile
@@ -3,9 +3,9 @@ CC      = i686-elf-gcc
 CFLAGS  = -std=gnu99 -ffreestanding -O2 -Wall -Wextra
 CFLAGS += -I.
 LFLAGS  = -ffreestanding -O2 -nostdlib -lgcc
-QFLAGS  = -no-reboot -d guest_errors
+QFLAGS  = -no-reboot -d guest_errors,int,pcall,cpu_reset
 
-OBJ  = platform/boot.o
+OBJ  = $(patsubst %.s,%.o,$(wildcard platform/*.s))
 OBJ += $(patsubst %.c,%.o,$(wildcard kernel/*.c))
 
 
@@ -27,7 +27,7 @@ kernel.bin: $(OBJ)
 	$(CC) $(LFLAGS) -T linker.ld $^ -o $@
 	grub-file --is-x86-multiboot $@
 
-platform/boot.o: platform/boot.s
+platform/%.o: platform/%.s
 	$(AS) $^ -o $@
 
 %.o: %.c
diff --git a/kernel/gdt.c b/kernel/gdt.c
index 8d1ac65..8a29032 100644
--- a/kernel/gdt.c
+++ b/kernel/gdt.c
@@ -1,4 +1,6 @@
+#include <kernel/gdt.h>
 #include <kernel/tty.h>
+#include <kernel/util.h>
 #include <stdint.h>
 
 extern void stack_top; // platform/boot.s
@@ -36,16 +38,6 @@ struct lgdt_arg {
 	uint32_t base;
 } __attribute__((packed));
 
-enum {
-	SEG_null,
-	SEG_r0data,
-	SEG_r0code,
-	SEG_r3data,
-	SEG_r3code,
-	SEG_TSS,
-
-	SEG_end    
-};
 static struct gdt_entry GDT[SEG_end];
 static struct tss_entry TSS;
 static struct lgdt_arg lgdt_arg; // probably doesn't need to be global
@@ -84,7 +76,7 @@ static void gdt_prepare() {
 	GDT[SEG_r3data].ring = 3;
 
 	// tss
-	// TODO memset(&TSS, 0, sizeof(TSS));
+	memset(&TSS, 0, sizeof(TSS));
 	TSS.ss0 = SEG_r0data << SEG_r3data; // kernel data segment
 	TSS.esp0 = (uint32_t) &stack_top;
 
@@ -108,13 +100,15 @@ static void gdt_prepare() {
 static void gdt_load() {
 	lgdt_arg.limit = sizeof(GDT) - 1;
 	lgdt_arg.base = (uint32_t) &GDT;
-	asm("lgdt (%0)" : : "b" (&lgdt_arg));
+	asm("lgdt (%0)" : : "r" (&lgdt_arg) : "memory");
+
+	asm("ltr %%ax" : : "a" (SEG_TSS << 3 | 3) : "memory");
 }
 
 static void gdt_check() {
 	tty_write("checking gdt...", 15);
 	asm("mov %0, %%ds;"
-	    : : "r" (SEG_r0data << 3));
+	    : : "r" (SEG_r0data << 3) : "memory");
 	tty_write("ok", 2);
 }
 
diff --git a/kernel/gdt.h b/kernel/gdt.h
index 9ae366c..9e5a6a5 100644
--- a/kernel/gdt.h
+++ b/kernel/gdt.h
@@ -1,3 +1,15 @@
 #pragma once
 
+enum {
+	SEG_null,
+	// order dictated by SYSENTER
+	SEG_r0code,
+	SEG_r0data,
+	SEG_r3code,
+	SEG_r3data,
+	SEG_TSS,
+
+	SEG_end
+};
+
 void gdt_init();
diff --git a/kernel/main.c b/kernel/main.c
index d2f0693..e63ca14 100644
--- a/kernel/main.c
+++ b/kernel/main.c
@@ -1,8 +1,24 @@
 #include <kernel/gdt.h>
 #include <kernel/tty.h>
+#include <platform/sysenter.h>
+
+extern void stack_top;
+
+void r3_test();
 
 void kmain()
 {
 	tty_clear();
 	gdt_init();
+	sysenter_setup();
+
+	tty_write("user...", 7);
+	sysexit(r3_test, &stack_top);
+}
+
+void r3_test() {
+	tty_write("in ring3", 8);
+	asm("cli"); // privileged instruction, should cause a GP
+	tty_write(" oh no", 6); // shouldn't happen
+	for (;;) {}
 }
diff --git a/kernel/util.c b/kernel/util.c
new file mode 100644
index 0000000..da3ac9d
--- /dev/null
+++ b/kernel/util.c
@@ -0,0 +1,9 @@
+#include <kernel/util.h>
+#include <stdint.h>
+
+void *memset(void *s, int c, size_t n) {
+	uint8_t *s2 = s;
+	for (size_t i = 0; i < n; n++)
+		s2[i] = c;
+	return s;
+}
diff --git a/kernel/util.h b/kernel/util.h
new file mode 100644
index 0000000..688ac63
--- /dev/null
+++ b/kernel/util.h
@@ -0,0 +1,4 @@
+#pragma once
+#include <stddef.h>
+
+void *memset(void *s, int c, size_t n);
diff --git a/platform/sysenter.h b/platform/sysenter.h
new file mode 100644
index 0000000..b0495cc
--- /dev/null
+++ b/platform/sysenter.h
@@ -0,0 +1,4 @@
+#pragma once
+
+void sysexit(void (*fun)(), void *stack_top);
+void sysenter_setup();
diff --git a/platform/sysenter.s b/platform/sysenter.s
new file mode 100644
index 0000000..270dc08
--- /dev/null
+++ b/platform/sysenter.s
@@ -0,0 +1,31 @@
+/* kernel/gdt.c */
+.set SEG_r0code, 1
+.set SEG_r3code, 3
+.set SEG_r3data, 4
+
+.set IA32_SYSENTER_CS, 0x174
+
+.section .text
+.global sysexit
+.type sysexit, @function
+sysexit:
+	pop %ecx
+	pop %edx
+
+	mov $(SEG_r3data << 3 | 3), %ax
+	mov %ax, %ds
+	mov %ax, %es
+	mov %ax, %fs
+	mov %ax, %gs
+
+	sysexit
+
+
+.global sysenter_setup
+.type sysenter_setup, @function
+sysenter_setup:
+	xor %edx, %edx
+	mov $(SEG_r0code << 3), %eax
+	mov $IA32_SYSENTER_CS, %ecx
+	wrmsr
+	ret
-- 
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