菜单OS更新：调试与内核上下文切换剖析-优快云博客

本文链接：https://blog.youkuaiyun.com/LOOP66/article/details/127839059

本文介绍了如何在终端操作中更新menuOS，涉及代码替换、rootfs制作及关键的内核调试过程，特别是switch_to函数的深入解析。通过GDB和QEMU，读者将学习如何设置断点并观察代码执行情况。

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更新menuOS

在终端中输入以下内容，将test.c替换为test_exec.c

cd LinuxKernel
rm menu -rf
git clone https://github.com/mengning/menu.git cd menu
cd menu
mv test_exec.c test.c
make rootfs

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冻结menuOS并设置断点

//code in shell 1
cd ..
qemu -kernel linux-3.18.6/arch/x86/boot/bzImage -initrd rootfs.img -S -s
//code in shell 2
gdb
file linux-3.18.6/vmlinux
target remote:1234
b schedule
b context_switch
b switch_to
b pick_next_task

进行调试并查看对应代码

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switch_to代码如下

#define switch_to(prev, next, last)                    
do {                                 
  /*                              
   * Context-switching clobbers all registers, so we clobber  
   * them explicitly, via unused output variables.     
   * (EAX and EBP is not listed because EBP is saved/restored  
   * explicitly for wchan access and EAX is the return value of   
   * __switch_to())                     
   */                                
  unsigned long ebx, ecx, edx, esi, edi;                
                                  
  asm volatile("pushfl\n\t"      /* 保存当前进程flags */   
           "pushl %%ebp\n\t"        /* 当前进程堆栈基址压栈*/ 
           "movl %%esp,%[prev_sp]\n\t"  /*保存ESP，将当前堆栈栈顶保存起来*/ 
           "movl %[next_sp],%%esp\n\t"  /*更新ESP，将下一栈顶保存到ESP中*/ 
        
            //完成内核堆栈的切换
           "movl $1f,%[prev_ip]\n\t"    /*保存当前进程EIP*/ 
           "pushl %[next_ip]\n\t"   /*将next进程起点压入堆栈，即next进程的栈顶为起点*/    
         
            //完成EIP的切换
             __switch_canary    
            //next_ip一般是$1f，对于新创建的子进程时ret_from_fork               
           "jmp __switch_to\n"  /*prev进程中，设置next进程堆栈*/ 
            //jmp不同于call是通过寄存器传递参数
           "1:\t"                //next进程开始执行
           "popl %%ebp\n\t"       
           "popfl\n"         
                                  
           /*输出变量定义*/                
           : [prev_sp] "=m" (prev->thread.sp),     //[prev_sp]定义内核堆栈栈顶
             [prev_ip] "=m" (prev->thread.ip),     //[prev_ip]当前进程EIP  
             "=a" (last),                 
                                  
             /* 要破坏的寄存器: */     
             "=b" (ebx), "=c" (ecx), "=d" (edx),      
             "=S" (esi), "=D" (edi)             
                                       
             __switch_canary_oparam                
                                  
            /* 输入变量: */                
           : [next_sp]  "m" (next->thread.sp),     //[next_sp]下一个内核堆栈栈顶    
             [next_ip]  "m" (next->thread.ip),     
           //[next_ip]下一个进程执行起点，，一般是$1f，对于新创建的子进程是ret_from_fork 
   
             /* regparm parameters for __switch_to(): */  
             [prev]     "a" (prev),              
             [next]     "d" (next)               
                                  
             __switch_canary_iparam                
                                 
           : /* 重新加载段寄存器 */           
          "memory");                  
} while (0)