Currently, there's no way to trap a running ptracee short of sending a
signal which has various side effects.  This patch implements
PTRACE_INTERRUPT which traps ptracee without any signal or job control
related side effect.

The implementation is almost trivial.  It uses the group stop trap -
SIGTRAP | PTRACE_EVENT_STOP << 8.  A new trap flag
JOBCTL_TRAP_INTERRUPT is added, which is set on PTRACE_INTERRUPT and
cleared when any trap happens.  As INTERRUPT should be useable
regardless of the current state of tracee, task_is_traced() test in
ptrace_check_attach() is skipped for INTERRUPT.

PTRACE_INTERRUPT is available iff tracee is attached with
PTRACE_SEIZE.

Test program follows.

  #define PTRACE_SEIZE		0x4206
  #define PTRACE_INTERRUPT	0x4207

  #define PTRACE_SEIZE_DEVEL	0x80000000

  static const struct timespec ts100ms = { .tv_nsec = 100000000 };
  static const struct timespec ts1s = { .tv_sec = 1 };
  static const struct timespec ts3s = { .tv_sec = 3 };

  int main(int argc, char **argv)
  {
	  pid_t tracee;

	  tracee = fork();
	  if (tracee == 0) {
		  nanosleep(&ts100ms, NULL);
		  while (1) {
			  printf("tracee: alive pid=%d\n", getpid());
			  nanosleep(&ts1s, NULL);
		  }
	  }

	  if (argc > 1)
		  kill(tracee, SIGSTOP);

	  nanosleep(&ts100ms, NULL);

	  ptrace(PTRACE_SEIZE, tracee, NULL,
		 (void *)(unsigned long)PTRACE_SEIZE_DEVEL);
	  if (argc > 1) {
		  waitid(P_PID, tracee, NULL, WSTOPPED);
		  ptrace(PTRACE_CONT, tracee, NULL, NULL);
	  }
	  nanosleep(&ts3s, NULL);

	  printf("tracer: INTERRUPT and DETACH\n");
	  ptrace(PTRACE_INTERRUPT, tracee, NULL, NULL);
	  waitid(P_PID, tracee, NULL, WSTOPPED);
	  ptrace(PTRACE_DETACH, tracee, NULL, NULL);
	  nanosleep(&ts3s, NULL);

	  printf("tracer: exiting\n");
	  kill(tracee, SIGKILL);
	  return 0;
  }

When called without argument, tracee is seized from running state,
interrupted and then detached back to running state.

  # ./test-interrupt
  tracee: alive pid=4546
  tracee: alive pid=4546
  tracee: alive pid=4546
  tracer: INTERRUPT and DETACH
  tracee: alive pid=4546
  tracee: alive pid=4546
  tracee: alive pid=4546
  tracer: exiting

When called with argument, tracee is seized from stopped state,
continued, interrupted and then detached back to stopped state.

  # ./test-interrupt  1
  tracee: alive pid=4548
  tracee: alive pid=4548
  tracee: alive pid=4548
  tracer: INTERRUPT and DETACH
  tracer: exiting

Before PTRACE_INTERRUPT, once the tracee was running, there was no way
to trap tracee and do PTRACE_DETACH without causing side effect.

-v2: Updated to use task_set_jobctl_pending() so that it doesn't end
     up scheduling TRAP_STOP if child is dying which may make the
     child unkillable.  Spotted by Oleg.
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>