can anyone explain about this XV6 inline asm validateint() test function that uses a pointer as ESP for a system call?

Hi~ I'm working on xv6 and I'm stuck on the validate test in usertests.c. There is asm code and I'm quite confused about what exactly is going on here. Would you like to explain that for me?

// try to crash the kernel by passing in a badly placed integer
void
validateint(int *p)
{
  int res;
  printf(stdout,"in validateint\n");
  asm("mov %%esp, %%ebx\n\t"
      "mov %3, %%esp\n\t"
      "int %2\n\t"
      "mov %%ebx, %%esp" :
      "=a" (res) :
      "a" (SYS_sleep), "n" (T_SYSCALL), "c" (p) :
      "ebx");
  printf(stdout,"%d \n",res);
}

I found that res is increasing with p but at some point res becomes 0 and then it becomes -1. The whole function stucks as long as res turn to -1. I have no idea what happened here.