/* *  'fork.c' contains the help-routines for the 'fork' system call * (see also system_call.s), and some misc functions ('verify_area'). * Fork is rather simple, once you get the hang of it, but the memory * management can be a bitch. See 'mm/mm.c': 'copy_page_tables()' */#include <errno.h>#include <linux/sched.h>#include <linux/kernel.h>#include <asm/segment.h>#include <asm/system.h>extern void write_verify(unsigned long address);long last_pid=0;void verify_area(void * addr,int size){    unsigned long start;    start = (unsigned long) addr;    size += start & 0xfff;    start &= 0xfffff000;    start += get_base(current->ldt[2]);		//current->ldt[2]指向數(shù)據(jù)段    while (size>0) {        size -= 4096;        write_verify(start);				//在memory.c中定義        start += 4096;    }}int copy_mem(int nr,struct task_struct * p){    unsigned long old_data_base,new_data_base,data_limit;    unsigned long old_code_base,new_code_base,code_limit;    code_limit=get_limit(0x0f);				//0X0F=代碼段選擇器，8M    data_limit=get_limit(0x17);				//0x17=數(shù)據(jù)段選擇器，8m    old_code_base = get_base(current->ldt[1]);    old_data_base = get_base(current->ldt[2]);    if (old_data_base != old_code_base)        panic("We don't support separate I&D");    if (data_limit < code_limit)        panic("Bad data_limit");    new_data_base = new_code_base = nr * 0x4000000;	//0x4000000=64M    set_base(p->ldt[1],new_code_base);			//重新設(shè)置本任務(wù)的LDT描述符    set_base(p->ldt[2],new_data_base);    if (copy_page_tables(old_data_base,new_data_base,data_limit)) {        free_page_tables(new_data_base,data_limit);	//如果不能copy_page_tables，那么恢復(fù)以前的狀態(tài)        return -ENOMEM;    }    return 0;}/* *  Ok, this is the main fork-routine. It copies the system process * information (task[nr]) and sets up the necessary registers. It * also copies the data segment in it's entirety. */int copy_process(int nr,long ebp,long edi,long esi,long gs,long none,                 long ebx,long ecx,long edx,                 long fs,long es,long ds,                 long eip,long cs,long eflags,long esp,long ss){    struct task_struct *p;    int i;    struct file *f;    p = (struct task_struct *) get_free_page();    if (!p)        return -EAGAIN;    *p = *current;	/* NOTE! this doesn't copy the supervisor stack */    p->state = TASK_RUNNING;    p->pid = last_pid;    p->father = current->pid;    p->counter = p->priority;    p->signal = 0;    p->alarm = 0;    p->leader = 0;		/* process leadership doesn't inherit */    p->utime = p->stime = 0;    p->cutime = p->cstime = 0;    p->start_time = jiffies;    p->tss.back_link = 0;    p->tss.esp0 = PAGE_SIZE + (long) p;    p->tss.ss0 = 0x10;    p->tss.eip = eip;    p->tss.eflags = eflags;    p->tss.eax = 0;    p->tss.ecx = ecx;    p->tss.edx = edx;    p->tss.ebx = ebx;    p->tss.esp = esp;    p->tss.ebp = ebp;    p->tss.esi = esi;    p->tss.edi = edi;    p->tss.es = es & 0xffff;    p->tss.cs = cs & 0xffff;    p->tss.ss = ss & 0xffff;    p->tss.ds = ds & 0xffff;    p->tss.fs = fs & 0xffff;    p->tss.gs = gs & 0xffff;    p->tss.ldt = _LDT(nr);    p->tss.trace_bitmap = 0x80000000;		//0X80000000=2048M    if (last_task_used_math == current)        __asm__("fnsave %0"::"m" (p->tss.i387));		//保存浮點(diǎn)參數(shù)    if (copy_mem(nr,p)) {			//為此任務(wù)設(shè)置LDT，并且拷貝父進(jìn)程的頁(yè)表作為自己的頁(yè)表        free_page((long) p);        return -EAGAIN;    }    for (i=0; i<NR_OPEN;i++)        if (f=p->filp[i])			//系統(tǒng)init_task設(shè)置filp[NR_OPEN]為空            f->f_count++;    if (current->pwd)				//系統(tǒng)init_task設(shè)置PWD為空        current->pwd->i_count++;    if (current->root)        current->root->i_count++;    //下面二行是在GDT中安裝本任務(wù)的TSS和LDT    set_tss_desc(gdt+(nr<<1)+FIRST_TSS_ENTRY,&(p->tss));	//gdt+(nr<<1)是因?yàn)槊總€(gè)任務(wù)在GDT中占兩個(gè)單位    set_ldt_desc(gdt+(nr<<1)+FIRST_LDT_ENTRY,&(p->ldt));    task[nr] = p;	/* do this last, just in case */		//將此任務(wù)登記入任務(wù)組    return last_pid;}int find_empty_process(void)			//在初始化過(guò)的64個(gè)任務(wù)中尋找最近的一個(gè)空任務(wù){    int i;repeat:    if ((++last_pid)<0) last_pid=1;    for(i=0 ; i<NR_TASKS ; i++)        if (task[i] && task[i]->pid == last_pid) goto repeat;    for(i=1 ; i<NR_TASKS ; i++)        if (!task[i])            return i;    return -EAGAIN;}