fork.c

linux的内核源码

/*
 *  '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]指向数据段
	while (size>0) {
		size -= 4096;
		write_verify(start);				//在memory.c中定义
		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=代码段选择器，8M
	data_limit=get_limit(0x17);				//0x17=数据段选择器，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);			//重新设置本任务的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，那么恢复以前的状态
		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));		//保存浮点参数
	if (copy_mem(nr,p)) {			//为此任务设置LDT，并且拷贝父进程的页表作为自己的页表
		free_page((long) p);
		return -EAGAIN;
	}
	for (i=0; i<NR_OPEN;i++)
		if (f=p->filp[i])			//系统init_task设置filp[NR_OPEN]为空
			f->f_count++;
	if (current->pwd)				//系统init_task设置PWD为空
		current->pwd->i_count++;
	if (current->root)
		current->root->i_count++;
//下面二行是在GDT中安装本任务的TSS和LDT
	set_tss_desc(gdt+(nr<<1)+FIRST_TSS_ENTRY,&(p->tss));	//gdt+(nr<<1)是因为每个任务在GDT中占两个单位
	set_ldt_desc(gdt+(nr<<1)+FIRST_LDT_ENTRY,&(p->ldt));
	task[nr] = p;	/* do this last, just in case */		//将此任务登记入任务组
	return last_pid;
}

int find_empty_process(void)			//在初始化过的64个任务中寻找最近的一个空任务
{
	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;
}