1.Lab-2 system calls

Lab: system calls

1.Using gdb (easy)

打开两个shell，都进入xv6文件夹

第一个shell输入
make qemu-gdb
第二个shell输入
riscv64-unknown-elf-gdb

1. Looking at the backtrace output, which function called syscall？

   (gdb) backtrace
   #0  syscall () at kernel/syscall.c:138
   #1  0x0000000080001d74 in usertrap () at kernel/trap.c:67
   #2  0x0505050505050505 in ?? ()
   

   所在的函数是kernel/trap.c --> usertrap()

2. What is the value of p->trapframe->a7 and what does that value represent? (Hint: look user/initcode.S, the first user program xv6 starts.)

   (gdb) p /x p->trapframe->a7
   $3 = 0x7
   

   xv6使用a7寄存器存储系统调用编号，查看kernel/syscall.h

   #define SYS_exec    7
   

   7号系统调用是SYS_exec

3. What was the previous mode that the CPU was in?

   (gdb) p /x $sstatus
   $4 = 0x200000022
   

   查看riscv-privileged.pdf 文档

   sstatus的SPP位，即Supervisor Previous Privilege，指示在进入监督者模式之前的机器模式

   SPP在第8位，0x200000022从右往左第8位为0，根据文档，0代表用户模式

   所以是用户模式

4. Write down the assembly instruction the kernel is panicing at. Which register corresponds to the variable num?

   运行xv6，出错信息如下

   make qemu
   qemu-system-riscv64 -machine virt -bios none -kernel kernel/kernel -m 128M -smp 3 -nographic -global virtio-mmio.force-legacy=false -drive file=fs.img,if=none,format=raw,id=x0 -device virtio-blk-device,drive=x0,bus=virtio-mmio-bus.0
   
   xv6 kernel is booting
   
   hart 2 starting
   hart 1 starting
   scause 0x000000000000000d
   sepc=0x0000000080002054 stval=0x0000000000000000
   panic: kerneltrap
   QEMU: Terminated
   

   80002054对应的汇编代码是

   num = *(int *)0;
   	80002054:	00002683          	lw	a3,0(zero) # 0 <_entry-0x80000000>
   

   num对应的寄存器是a3

5. Why does the kernel crash? Hint: look at figure 3-3 in the text; is address 0 mapped in the kernel address space? Is that confirmed by the value in scause above? (See description of scause in RISC-V privileged instructions)

   1. 运行gdb，打断点到0x0000000080002054

      b *0x0000000080002054
      

   2. scause的值是：13，访问的目标地址是0

   3. 查看xv6 book的内存分布图

      

      0对应的是Unused的地址，不是内核地址

      查看RISC-V privileged instructions文档，图4.2，scause等于13时，对应的错误是Load page fault

      

6. What is the name of the binary that was running when the kernel paniced? What is its process id (pid)?

   出错时，运行的是kernel/syscall.c文件，编译在kernel二进制文件中

   (gdb) p p->pid
   $2 = 1
   

   pid是1

2.System call tracing (moderate)

代码：github

1. 添加trace到 Makefile的UPROGS

2. 添加trace函数定义到user/user.h

3. 添加trace到user/usys.pl

4. 添加SYS_trace到kernel/syscall.h

   #define SYS_trace 22
   

5. 添加trace_mask到kernel/proc.h的struct proc中，记录当前线程的trace_mask

6. 修改kernel/syscall.c，使的SYS_trace和trace函数能连接到

   添加一个函数调用到系统调用名的映射syscall_names，用于获取系统调用名

7. 修改kernel/proc.c/fork函数，将trace_mask复制给子进程

3.Sysinfo (moderate)

代码：github