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MIT 6.S081 Lab traps

Compulsory exercises

Preparation

  • reading
  • To start the lab, switch to the trap branch
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git fetch
git checkout traps
make clean

RISC-V assembly (easy)

  • Which registers contain arguments to functions? For example, which register holds 13 in main’s call to printf?
    • a0-a1 a2-a7; a2
  • Where is the call to function f in the assembly code for main? Where is the call to g? (Hint: the compiler may inline functions.)
    • f in printf 0x34 , g in f 0x14
  • At what address is the function printf located?
    • 0x630
  • What value is in the register ra just after the jalr to printf in main?
    • 0x38
  • Run the following code.
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unsigned int i = 0x00646c72;
printf("H%x Wo%s", 57616, &i);
  • What is the output?
    • 注意字节序是小端序
    • He110 World
  • The output depends on that fact that the RISC-V is little-endian. If the RISC-V were instead big-endian what would you set i to in order to yield the same output? Would you need to change 57616 to a different value?
    • i = 0x72646c00 -> “ord\0”
    • no
  • In the following code, what is going to be printed after ‘y=’? (note: the answer is not a specific value.) Why does this happen?
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printf("x=%d y=%d", 3);
  • x=3 y=寄存器a2的十进制值，因为在printf没有传入第三个参数，a2没有被更新，因此直接把原来a2的值传入

Backtrace (moderate)

  • 该任务是打印出错堆栈前的所有返回地址ra

  • Implement a backtrace() function in kernel/printf.c

  • 添加定义和内联函数(内核中)

defs.h //printf.c
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void            backtrace(void);
riscv.h
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// store frame pointer
static inline uint64
r_fp()
{
  uint64 x;
  asm volatile("mv %0, s0" : "=r" (x) );
  return x;
}

  • 注意ra在fp-8处，上一个fp在fp-16处

  • 使用riscv中的宏定义PGROUNDDOWN(fp)和PGROUNDUP(fp)确定循环终止条件

  • 实现backtrace()

kernel/printf.c
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void backtrace(void)
{
    printf("backtrace:\n");
    uint64 fp = r_fp();
    uint64 start = PGROUNDDOWN(fp);
    uint64 end = PGROUNDUP(fp);

    while (start <= fp && fp <= end) {
        uint64 ra = *(uint64*)(fp - 8);
        fp = *(uint64*)(fp - 16);       
        printf("%p\n", ra);
    }
}

  • 添加至sys_sleep()和panic()

sysproc.c
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backtrace();
kernel/printf.c panic()
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backtrace();

Alarm (hard)

  • 添加系统调用在计时器中断时打印alarm

  • test0: invoke handler

  • 修改proc.h

proc.h struct proc
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int ticks;
uint64 handler;
int gonetick;

  • 需要完成sigalarm系统调用,并且添加sigreturn系统调用（空）

sysproc.c
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uint64
sys_sigalarm(void)
{
    int ticks;
    uint64 handler;
    if (argint(0, &ticks) < 0)
        return -1;
    if (argaddr(1, &handler) < 0)
        return -1;

    struct proc* p = myproc();
    p->ticks = ticks;
    p->handler = handler;

    return 0;
}

uint64
sys_sigreturn(void)
{
    //test0
    return 0;
}

  • 修改usertrap()

trap.c usertrap()
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//test0
if(which_dev == 2) {
  p->gonetick += 1;
  if (p->ticks_pass % p->ticks == 0) {
    p->trapframe->epc = p->handler;
  }
  yield();
}

  • test1/test2(): resume interrupted code

  • test0尚未完成返回，因此程序在alarm后崩溃，需要补全sigreturn

  • 修改proc.h用于保存alarm时的寄存器信息，inhandler防止函数被多次调用

  • 需要初始化的参数可以通过allocproc()初始化

proc.h struct proc >folded
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int ticks;
uint64 handler;
int gonetick;
int inhandler;

/*  40 */ uint64 ra;
/*  48 */ uint64 sp;
/*  56 */ uint64 gp;
/*  64 */ uint64 tp;
/*  72 */ uint64 t0;
/*  80 */ uint64 t1;
/*  88 */ uint64 t2;
/*  96 */ uint64 s0;
/* 104 */ uint64 s1;
/* 112 */ uint64 a0;
/* 120 */ uint64 a1;
/* 128 */ uint64 a2;
/* 136 */ uint64 a3;
/* 144 */ uint64 a4;
/* 152 */ uint64 a5;
/* 160 */ uint64 a6;
/* 168 */ uint64 a7;
/* 176 */ uint64 s2;
/* 184 */ uint64 s3;
/* 192 */ uint64 s4;
/* 200 */ uint64 s5;
/* 208 */ uint64 s6;
/* 216 */ uint64 s7;
/* 224 */ uint64 s8;
/* 232 */ uint64 s9;
/* 240 */ uint64 s10;
/* 248 */ uint64 s11;
/* 256 */ uint64 t3;
/* 264 */ uint64 t4;
/* 272 */ uint64 t5;
/* 280 */ uint64 t6;

uint64 epc;

  • 完成sigreturn系统调用

sysproc.c >folded
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uint64
sys_sigalarm(void)
{
    int ticks;
    uint64 handler;
    if (argint(0, &ticks) < 0)
        return -1;
    if (argaddr(1, &handler) < 0)
        return -1;

    struct proc* p = myproc();
    p->ticks = ticks;
    p->handler = handler;

    return 0;
}

uint64
sys_sigreturn(void)
{
  //test12
  struct proc* p = myproc();
  p->trapframe->ra = p->ra;
  p->trapframe->sp = p->sp;
  p->trapframe->gp = p->gp;
  p->trapframe->tp = p->tp;
  p->trapframe->t0 = p->t0;
  p->trapframe->t1 = p->t1;
  p->trapframe->t2 = p->t2;
  p->trapframe->s0 = p->s0;
  p->trapframe->s1 = p->s1;
  p->trapframe->a0 = p->a0;
  p->trapframe->a1 = p->a1;
  p->trapframe->a2 = p->a2;
  p->trapframe->a3 = p->a3;
  p->trapframe->a4 = p->a4;
  p->trapframe->a5 = p->a5;
  p->trapframe->a6 = p->a6;
  p->trapframe->a7 = p->a7;
  p->trapframe->s2 = p->s2;
  p->trapframe->s3 = p->s3;
  p->trapframe->s4 = p->s4;
  p->trapframe->s5 = p->s5;
  p->trapframe->s6 = p->s6;
  p->trapframe->s7 = p->s7;
  p->trapframe->s8 = p->s8;
  p->trapframe->s9 = p->s9;
  p->trapframe->s10 = p->s10;
  p->trapframe->s11 = p->s11;
  p->trapframe->t3 = p->t3;
  p->trapframe->t4 = p->t4;
  p->trapframe->t5 = p->t5;
  p->trapframe->t6 = p->t6;

  p->trapframe->epc = p->epc;
  p->inhandler = 0;
  return 0;
}

  • 修改usertrap()

trap.c usertrap() >folded
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//test12
if(which_dev == 2 && p->inhandler == 0) {
  p->gonetick += 1;
  if (p->ticks && p->gonetick == p->ticks) {
    p->gonetick = 0;
    p->inhandler = 1;

    p->ra = p->trapframe->ra;
    p->sp = p->trapframe->sp;
    p->gp = p->trapframe->gp;
    p->tp = p->trapframe->tp;
    p->t0 = p->trapframe->t0;
    p->t1 = p->trapframe->t1;
    p->t2 = p->trapframe->t2;
    p->s0 = p->trapframe->s0;
    p->s1 = p->trapframe->s1;
    p->a0 = p->trapframe->a0;
    p->a1 = p->trapframe->a1;
    p->a2 = p->trapframe->a2;
    p->a3 = p->trapframe->a3;
    p->a4 = p->trapframe->a4;
    p->a5 = p->trapframe->a5;
    p->a6 = p->trapframe->a6;
    p->a7 = p->trapframe->a7;
    p->s2 = p->trapframe->s2;
    p->s3 = p->trapframe->s3;
    p->s4 = p->trapframe->s4;
    p->s5 = p->trapframe->s5;
    p->s6 = p->trapframe->s6;
    p->s7 = p->trapframe->s7;
    p->s8 = p->trapframe->s8;
    p->s9 = p->trapframe->s9;
    p->s10 = p->trapframe->s10;
    p->s11 = p->trapframe->s11;
    p->t3 = p->trapframe->t3;
    p->t4 = p->trapframe->t4;
    p->t5 = p->trapframe->t5;
    p->t6 = p->trapframe->t6;   

    p->epc = p->trapframe->epc;     

    p->trapframe->epc = p->handler;
  }
  yield();
}

Optional challenge exercises

  • Print the names of the functions and line numbers in backtrace() instead of numerical addresses (hard).

MIT 6.S081 Lab traps

http://huaeryi.com/2023/04/13/6-S081-Lab-traps/

作者

huayi

发布于

2023-04-13

更新于

2023-04-23

许可协议

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