20135201 Li Chenxi Description and creation of the sixth week process of Linux kernel analysis

Li Chenxi Original works reproduced please specify the source "Linux kernel Analysis" MOOC course http://mooc.study.163.com/course/USTC-1000029000

I. Description of the process

The three main management functions of the operating system:

• Process management (most important)
• Memory management
• File system

To manage the process, the kernel must have a clear description of each process, and the process descriptor provides the process information that the kernel needs to understand.

Process Control block PCB task_struct:

• Process status
• Files opened by the process
• Process-Priority information

Task_struct abstraction of the overall data structure:

tty：控制台fs：文件系统files：文件描述符mm：内存管理signal：信号描述

process state: different from operating system (ready, running, blocked), such as the ready state and running state in Linux are task_running

task_running Specific is ready or implementation, depends on the system's current resource allocation;

Task_zombie also known as the zombie process

two. Creation of processes

--Review of process origins:

道生一（start_kernel....cpu_idle）一生二（kernel_init和kthreadd）二生三（即前面0、1和2三个进程）三生万物（1号进程是所有用户态进程的祖先，2号进程是所有内核线程的祖先）

Process No. 0, is the code to write dead, 1th process replication No. 0 Process PCB, then modify, and then load the executable program.

1.fork Code

1.#include<stdio.h>2.#include<stdlib.h>3.#include<unistd.h>4.int main (int argc,char * argv[])5.{6.int pid;7./* Fork another process */8.pid = fork ();9.if (PID <0)10.{11./* ERROR occurred */12.fprintf (stderr,"Fork failed!");13.exit (-1);14.}15.Elseif (PID = =0)PID = = 0 and the following else will be executed (one is the PID ==0 in the parent process, one in the child process, that is, the PID is not equal to 0)16.{17. /* Child process */18.printf ( "This is Child process!\n"); 19.} 20.else 21.{ 22./* parent process */23.printf ( Span class= "hljs-string" > "This is the Parent process!\n"); 24./* parent would wait for the child to Complete*/25.wait (null);  "child complete!\n");  27.}28.} 
 
 
                /span>     

2. Create a new process in the kernel execution process

Fork, Vfork, and clone three system calls can create a new process, and all are created by calling Do_fork to implement the process;

Linux creates a new process by copying the parent process:

do_fork主要是复制了父进程的task_struct，然后修改必要的信息，从而得到子进程的task_struct。

An imaginary framework for understanding this process:

Copy a pcb--task_struct

err = arch_dup_task_struct(tsk, orig);

To assign a new kernel stack to the new process

 ti = Alloc_thread_info_node (tsk, node) ;tsk->stack = Ti;setup_thread_stack ( tsk, orig) //This is just a copy of Thread_info, not a copy of the kernel stack       
 
 
 
 3. What line of code does a newly created child process (after it obtains the CPU) be executed from? 

• In comparison with previously written my_kernel, kernel can specify where the new process will start (that is, specify the line of code via the EIP register). There are similar mechanisms in fork.

• This involves the kernel stack data state of the child process and the consistency of the SP and IP in the thread record in Task_struct, where is this set? Copy_thread in Copy_process

  1.*childregs = *current_pt_regs(); //复制内核堆栈，并不是全部，只是regs结构体（内核堆栈栈底的程序）2.childregs->ax = 0; //为什么子进程的fork返回0，这里就是原因！3. 4.p->thread.sp = (unsigned long) childregs; //调度到子进程时的内核栈顶5.p->thread.ip = (unsigned long) ret_from_fork; //调度到子进程时的第一条指令地址，也就是说返回的就是子进程的空间了

three. Experiment: analyzing the process of creating a new process for the Linux kernel

1. start Menuos

cd LinuxKernel   rm menu -rfgit clone https://github.com/mengning/menu.gitcd menumv test_fork.c test.cmake rootfs

2.gdb Debug Fork Command

qemu -kernel linux-3.18.6/arch/x86/boot/bzImage -initrd rootfs.img -s -Sgdbfile linux-3.18.6/vmlinuxtarget remote:1234

3. Set Breakpoints:

b sys_cloneb do_forkb dup_task_structb copy_processb copy_threadb ret_from_fork



4.继续执行之后，停在了do_fork的位置。




四.总结
Understanding of the "Linux system creates a new process":

P->THREAD.SP = (
P->thread.ip = (//The first instruction address when dispatched to a child process

The IP of the child process is set to the first address of the ret_ form fork, so the child process is executed from ret_ From_ fork.
The regs parameter of the parent process is assigned to the child process's kernel stack, and the *childregs type is Pt_regs, which holds the arguments to the stack in save all.

20135201 Li Chenxi Description and creation of the sixth week process of Linux kernel analysis