From 2ff03f8f3c86d19866ab6280c3b2f3aac0754063 Mon Sep 17 00:00:00 2001
From: hinus <hinus@163.com>
Date: Thu, 17 Jun 2021 18:30:29 +0800
Subject: [PATCH] Title: Move init_task to privilege 3. Issue:
 https://gitee.com/hinus/linux_kernel_011/issues/I3VOQT Description: Intialize
 first task and move it to lower privilege.

---
 include/asm/system.h  | 34 ++++++++++++++++++++++++++
 include/linux/sched.h | 56 +++++++++++++++++++++++++++++++++++++++++++
 kernel/main.c         |  3 +++
 kernel/sched.c        | 15 +++++++++++-
 4 files changed, 107 insertions(+), 1 deletion(-)

diff --git a/include/asm/system.h b/include/asm/system.h
index 001b687..b44e6b6 100644
--- a/include/asm/system.h
+++ b/include/asm/system.h
@@ -1,6 +1,22 @@
 #ifndef _SYSTEM_H
 #define _SYSTEM_H
 
+#define move_to_user_mode() \
+__asm__("movl %%esp, %%eax\n\t" \
+        "pushl $0x17\n\t"       \
+        "pushl %%eax\n\t"       \
+        "pushfl\n\t"            \
+        "pushl $0x0f\n\t"       \
+        "pushl $1f\n\t"         \
+        "iret\n\t"              \
+        "1:\tmovl $0x17, %%eax\n\t" \
+        "movw %%ax, %%ds\n\t"   \
+        "movw %%ax, %%es\n\t"   \
+        "movw %%ax, %%fs\n\t"   \
+        "movw %%ax, %%gs\n\t"   \
+        :::"ax")
+
+
 #define sti()   __asm__("sti"::)
 #define cli()   __asm__("cli"::)
 #define nop()   __asm__("nop"::)
@@ -21,4 +37,22 @@ __asm__("movw %%dx, %%ax\n\t"   \
 #define set_intr_gate(n, addr) \
     _set_gate(&idt[n], 14, 0, addr)
 
+#define set_system_gate(n, addr) \
+    _set_gate(&idt[n], 15, 3, addr)
+
+#define _set_tssldt_desc(n, addr, type) \
+__asm__("movw $104, %1\n\t"             \
+        "movw %%ax, %2\n\t"             \
+        "rorl $16, %%eax\n\t"           \
+        "movb %%al, %3\n\t"             \
+        "movb $" type ", %4\n\t"        \
+        "movb $0x00, %5\n\t"            \
+        "movb %%ah, %6\n\t"             \
+        "rorl $16, %%eax\n\t"           \
+        ::"a"(addr), "m"(*(n)), "m"(*(n+2)), "m"(*(n+4)), \
+        "m"(*(n+5)), "m"(*(n+6)), "m"(*(n+7)) \
+       )
+
+#define set_tss_desc(n, addr) _set_tssldt_desc(((char*)(n)), addr, "0x89")
+#define set_ldt_desc(n, addr) _set_tssldt_desc(((char*)(n)), addr, "0x82")
 #endif
diff --git a/include/linux/sched.h b/include/linux/sched.h
index e7f1a39..dce7118 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -6,4 +6,60 @@
 
 void sched_init();
 
+struct tss_struct {
+    long back_link;
+    long esp0;
+    long ss0;
+    long esp1;
+    long ss1;
+    long esp2;
+    long ss2;
+    long cr3;
+    long eip;
+    long eflags;
+    long eax, ecx, edx, ebx;
+    long esp;
+    long ebp;
+    long esi;
+    long edi;
+    long es;
+    long cs;
+    long ss;
+    long ds;
+    long fs;
+    long gs;
+    long ldt;
+    long trace_bitmap;
+};
+
+struct task_struct {
+    struct desc_struct ldt[3];
+    struct tss_struct tss;
+};
+
+#define INIT_TASK \
+{                   \
+    {               \
+        {0, 0},     \
+        {0x9f, 0xc0fa00},   \
+        {0x9f, 0xc0f200},   \
+    },              \
+    {0, PAGE_SIZE + (long)&init_task, 0x10, 0, 0, 0, 0, (long)&pg_dir, \
+        0, 0, 0, 0, 0, 0, 0, 0, \
+        0, 0, 0x17, 0x17, 0x17, 0x17, 0x17, 0x17,   \
+        _LDT(0), 0x80000000,    \
+    },              \
+}
+
+/*
+ * In linux is 4, because we add video selector,
+ * so, it is 5 here.
+ * */
+#define FIRST_TSS_ENTRY 5
+#define FIRST_LDT_ENTRY (FIRST_TSS_ENTRY + 1)
+#define _TSS(n) ((((unsigned long)n) << 4) + (FIRST_TSS_ENTRY << 3))
+#define _LDT(n) ((((unsigned long)n) << 4) + (FIRST_LDT_ENTRY << 3))
+#define ltr(n) __asm__("ltr %%ax"::"a"(_TSS(n)))
+#define lldt(n) __asm__("lldt %%ax"::"a"(_LDT(n)))
+
 #endif
diff --git a/kernel/main.c b/kernel/main.c
index 6caab5f..b68535b 100644
--- a/kernel/main.c
+++ b/kernel/main.c
@@ -1,5 +1,7 @@
 #define __LIBRARY__
 
+#include <asm/system.h>
+
 #include <linux/tty.h>
 #include <linux/kernel.h>
 #include <linux/sched.h>
@@ -41,6 +43,7 @@ void main(void)
 
     tty_init();
     printk("%d %d\n\r", 12, 34);
+    move_to_user_mode();
 
     __asm__ __volatile__(
             "int $0x80\n\r"
diff --git a/kernel/sched.c b/kernel/sched.c
index 7be08e4..b7a5043 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -6,6 +6,13 @@
 
 extern int system_call();
 
+union task_union {
+    struct task_struct task;
+    char stack[PAGE_SIZE];
+};
+
+static union task_union init_task = {INIT_TASK, };
+
 long user_stack[PAGE_SIZE >> 2];
 
 struct
@@ -15,6 +22,12 @@ struct
 } stack_start = {&user_stack[PAGE_SIZE >> 2], 0x10};
 
 void sched_init() {
-    set_intr_gate(0x80, &system_call);
+    struct desc_struct* p;
+    set_tss_desc(gdt + FIRST_TSS_ENTRY, &(init_task.task.tss));
+    set_ldt_desc(gdt + FIRST_LDT_ENTRY, &(init_task.task.ldt));
+    __asm__("pushfl; andl $0xffffbfff, (%esp); popfl");
+    ltr(0);
+    lldt(0);
+    set_system_gate(0x80, &system_call);
 }
 
-- 
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