XXX for next year

<h2>System call tracing</h2>

<p>
<div class="required">
  In this exercise you will add a system call tracing feature that
  may help you when debugging. You'll create a new <tt>trace</tt>
  system call that will control tracing. It
  should take one argument, an integer "mask", whose bits specify
  which system calls to trace.  For example, to trace the fork system
  call, you call <tt>trace(1 << SYS_fork)</tt>. You have to
  modify the xv6 kernel to print out a line when each system call
  is about to return, if the system call's number is set in the mask.
  It is enough
  to print process id, the name of the system call and the return
  value; you don't need to print the system call arguments.
  The <tt>trace</tt> system call should enable tracing just
  for the process that calls it and that process's descendants.
  Finally,
  you will write a
  <tt>trace</tt> user-level program that runs another program
  with tracing enabled.
</div>

<p>
When you're done, you should see output like this when tracing
grep:

<pre>
$ trace 32 grep hello README
3: syscall read -> 1023
3: syscall read -> 959
3: syscall read -> 0
$ 
$ trace 2147483647 grep hello README
4: syscall trace -> 0
4: syscall exec -> 3
4: syscall open -> 3
4: syscall read -> 1023
4: syscall read -> 959
4: syscall read -> 0
4: syscall close -> 0
$
</pre>

<p> In the first example, trace invokes grep tracing just the read system call.
  In the second, trace runs grep while tracing all system calls.

<p>Hint: add a sys_trace() function that implements the new system
   call by remembering its argument in a new variable in the proc structure.

<p>Hint: modify the syscall() function in kernel/syscall.c to
   print the trace output.

<p>Optional: print the system call arguments.