《linux 内核完全剖析》 fork.c 代码分析笔记

本文主要是介绍《linux 内核完全剖析》 fork.c 代码分析笔记，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

fork.c 代码分析笔记

verifiy_area

long last_pid=0; //全局变量，用来记录目前最大的pid数值void verify_area(void * addr,int size) // addr 是虚拟地址 ，size是需要写入的字节大小
{unsigned long start;start = (unsigned long) addr; //把地址强制类型转换之后，赋值给startsize += start & 0xfff; //取addr在当前虚拟地址中4M页面的偏移值，加上size可以得到要求写入虚拟地址的末端start &= 0xfffff000; //取addr所在虚拟地址页面的起始处，即该页在数据段中偏移量start += get_base(current->ldt[2]); //get_base得到当前进程数据段的线性基地址，加上start偏移量，于是实现了虚拟地址转化到线性地址while (size>0) {size -= 4096;write_verify(start);//对线性地址start进行操作，验证该地址处页面是否可写start += 4096;}
}

copy_mem

int copy_mem(int nr,struct task_struct * p) //复制内存页表，把进程p的数据段copy到nr*TASK的线性地址处
{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任务代码段选择符 ,get_limit 得到代码段的段限长data_limit=get_limit(0x17);//0x17任务数据段选择符，get_limit 得到数据段的段限长old_code_base = get_base(current->ldt[1]); //得到代码段的基地址old_data_base = get_base(current->ldt[2]); //得到数据段的基地址if (old_data_base != old_code_base) //linux 0.12 是 I&D的模式panic("We don't support separate I&D");if (data_limit < code_limit) //要求数据段不小于代码段panic("Bad data_limit");new_data_base = new_code_base = nr * TASK_SIZE; //更新代码段的基地址p->start_code = new_code_base; //更新当前进程代码段的基地址set_base(p->ldt[1],new_code_base);//把new_code_base 写入到ldt[1]set_base(p->ldt[2],new_data_base);if (copy_page_tables(old_data_base,new_data_base,data_limit)) {//把old_date_base 线性地址的数据复制到new_data_base处//copy_page_tables 成功返回0，错误返回-1.如果失败。就以页为单位，free掉new_date_base涉及的内存页free_page_tables(new_data_base,data_limit);return -ENOMEM;}return 0;
}

copy_process

/**  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 orig_eax,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(); //申请一页空内存，由p指向它if (!p)return -EAGAIN;task[nr] = p; //把新进程指针copy到task数组里面*p = *current;    /* NOTE! this doesn't copy the supervisor stack */p->state = TASK_UNINTERRUPTIBLE;//copy进程的时候，p进程状态设置为TASK_UNINTERRIPTIBLEp->pid = last_pid;p->counter = p->priority; //运行时间数p->signal = 0; //初始没有接受任何信号，信号图为空p->alarm = 0;p->leader = 0;    //fork出的进程不继承session leader，保证session leader只有一个    /* process leadership doesn't inherit */p->utime = p->stime = 0; //用户态时间和内核态时间p->cutime = p->cstime = 0;//子进程用户态时间和内核态时间p->start_time = jiffies; //进程p开始时间p->tss.back_link = 0;//修改任务状态TSS数据p->tss.esp0 = PAGE_SIZE + (long) p; //任务内核态的栈指针p->tss.ss0 = 0x10;p->tss.eip = eip; //指令代码指针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); //_LDT宏计算出nr进程的LDT描述符的选择符p->tss.trace_bitmap = 0x80000000;if (last_task_used_math == current)__asm__("clts ; fnsave %0 ; frstor %0"::"m" (p->tss.i387));if (copy_mem(nr,p)) { //把 进程p的内容copy到nr*TASK的线性地址处task[nr] = NULL; //失败就把task数组的对应指针置为NULL说明进程创建失败free_page((long) p); //善后，把p相关的内存页释放return -EAGAIN;}for (i=0; i<NR_OPEN;i++) //如果以上copy_mem成功，则把parent 打开的文件也让child继承if (f=p->filp[i])f->f_count++;if (current->pwd)//引用+1current->pwd->i_count++;if (current->root)current->root->i_count++;if (current->executable)current->executable->i_count++;if (current->library)current->library->i_count++;set_tss_desc(gdt+(nr<<1)+FIRST_TSS_ENTRY,&(p->tss));set_ldt_desc(gdt+(nr<<1)+FIRST_LDT_ENTRY,&(p->ldt));p->p_pptr = current;p->p_cptr = 0;p->p_ysptr = 0;p->p_osptr = current->p_cptr;if (p->p_osptr)p->p_osptr->p_ysptr = p;current->p_cptr = p;p->state = TASK_RUNNING;    /* do this last, just in case */return last_pid;
}

find_empty_process

int find_empty_process(void) //为新进程获取不重复的pid
{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) ||(task[i]->pgrp == last_pid))) //如果last_pid存在，那么repeat再测试 ++last_pidgoto repeat;//在已经把lastpid变成所有进程都不同的pid之后，下面继续for(i=1 ; i<NR_TASKS ; i++) //找出一个空闲进程，返回它的索引 iif (!task[i])return i;return -EAGAIN; //如果64个进程都存在，那么报错
}