6.1810-system calls

接下来我们要开始看系统调用的真面目啦

Using gdb

这次算是我第一次认真gdb了，之前都是直接用vsc封装好的调试的

对于我的环境wsl ubuntu20.04来说这里还有一些小坑

需要启动riscv-gdb,设置架构，手动连接端口并加载符号表

qemu的端口号和kernel符号表的位置可能会有所不同（？，这里是chat老师的样例，不过用vsc remote的话端口号是好找的（

gdb-multiarch

(gdb) set architecture riscv:rv64
(gdb) target remote localhost:1234
(gdb) file path/to/your/kernel/kernel

不过反正是自学所以也没有ta查，就随便往txt里写了点

1. Threads call usertrap()

(gdb) bt
#0  syscall () at kernel/syscall.c:133
#1  0x0000000080001d70 in usertrap () at kernel/trap.c:67

2. reg a7 represent SYSCALL type

(gdb) p/x*p
$1 = {lock = {locked = 0x0, name = 0x800081b8, cpu = 0x0}, 
state = 0x4, chan = 0x0, killed = 0x0, xstate = 0x0, pid = 0x1, 
parent = 0x0, kstack = 0x3fffffd000, sz = 0x4000, 
pagetable = 0x87f6c000, trapframe = 0x87f74000, context = {
ra = 0x800014c2, sp = 0x3fffffdd10, s0 = 0x3fffffdd40, 
s1 = 0x80008d50, s2 = 0x80008920, s3 = 0x0, s4 = 0x80021d38, 
s5 = 0xffffffffffffffff, s6 = 0x3f78, s7 = 0x930, s8 = 0x64, 
s9 = 0x6c, s10 = 0x78, s11 = 0x70}, ofile = {0x80018a50, 
0x80018a50, 0x80018a50, 0x0 <repeats 13 times>}, 
cwd = 0x80016e60, name = {0x69, 0x6e, 0x69, 0x74, 0x0, 0x0, 0x64, 
0x65, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0}}

(gdb) p/x ((struct trapframe*)0x87f74000)->a7
$2 = 0x10

3. sstatus means Supervisor status register , previous privilege is user mode
   SPP (Supervisor previous privilege) = 0
   SIE (Supervisor Interrupt Enable) = 1

(gdb) p /x $sstatus
$3 = 0x22

4. so variable num in reg a3
   sepc=0x0000000080002050
   num = * (int *) 0;
   80002050: 00002683 lw a3,0(zero) # 0 <_entry-0x80000000>

5. In xv6 , 0-0x1000 is Unused.
   It’s confirmed by scause:
   scause 0x000000000000000d means Load Page Fault

(gdb) p p->name
$1 = "initcode\000\000\000\000\000\000\000"
(gdb) p p->pid
$2 = 1

trace

通过掩码来追踪系统调用

这个掩码需要记录在进程信息中，所以在kernel/proc.h中的struct proc中添加

struct proc {
  ...
  int trace_mask; 
  ...
}

在内核的系统调用过程中(kernel/syscall.c)处理掩码

if (p->trace_mask !=0 ){
  for (int i = 0; i < NELEM(syscalls); i++) {
    if (p->trace_mask & (1 << i)) {
      if (num == i) {
        printf("%d: syscall %s -> %d\n", p->pid , syscallNames[num] , p->trapframe->a0);
      }
    }
  }
}

根据已经处理好的用户态的trace实现，去补全sys_trace()函数

uint64
sys_trace(void)
{
  int mask;
  struct proc *p = myproc();

  argint(0, &mask);

  p->trace_mask = mask;

  return 0;
}

注意使用argint向内核态传参

接下来确保子进程会继承追踪掩码，在kernel/proc.c中的fork函数中添加

int
fork(void)
{ 
  ...
  np->trace_mask = p->trace_mask;
  ...
}

sysinfo

在内核态收集信息（进程和内存）之后将结构体复制到用户态即可。

遍历proc数组收集进程数量

int
proc_collect(void)
{ 
  int count = 0;
  struct proc *p;
  for(p = proc; p < &proc[NPROC]; p++){
    if(p->state != UNUSED)
      count++;
  }
  return count;
}

遍历链表收集内存

int
count_free_memory(void)
{
  int free_memory = 0;
  struct run *r;

  acquire(&kmem.lock);
  for(r = kmem.freelist; r ; r = r->next) {
      free_memory += PGSIZE;
  }
  release(&kmem.lock);

  return free_memory;
}

最后通过copyout将数据拷贝到内核态的相应地址

uint64
sys_sysinfo(void){
  struct proc *p = myproc();
  struct sysinfo kinfo;
  uint64 pr;
  kinfo.freemem = count_free_memory();
  kinfo.nproc = proc_collect();
  argaddr(0,&pr);
  if (copyout(p->pagetable,pr,(char *)&kinfo,sizeof(kinfo)) < 0){
    return -1;
  }
  return 0;
}