job control: Don't set group_stop exit_code if re-entering job control stop

While ptraced, a task may be resumed while the containing process is
still job control stopped.  If the task receives another stop signal
in this state, it will still initiate group stop, which generates
group_exit_code, which the real parent would be able to see once the
ptracer detaches.

In this scenario, the real parent may see two consecutive CLD_STOPPED
events from two stop signals without intervening SIGCONT, which
normally is impossible.

Test case follows.

  #include <stdio.h>
  #include <unistd.h>
  #include <sys/ptrace.h>
  #include <sys/wait.h>

  int main(void)
  {
	  pid_t tracee;
	  siginfo_t si;

	  tracee = fork();
	  if (!tracee)
		  while (1)
			  pause();

	  kill(tracee, SIGSTOP);
	  waitid(P_PID, tracee, &si, WSTOPPED);

	  if (!fork()) {
		  ptrace(PTRACE_ATTACH, tracee, NULL, NULL);
		  waitid(P_PID, tracee, &si, WSTOPPED);
		  ptrace(PTRACE_CONT, tracee, NULL, (void *)(long)si.si_status);
		  waitid(P_PID, tracee, &si, WSTOPPED);
		  ptrace(PTRACE_CONT, tracee, NULL, (void *)(long)si.si_status);
		  waitid(P_PID, tracee, &si, WSTOPPED);
		  ptrace(PTRACE_DETACH, tracee, NULL, NULL);
		  return 0;
	  }

	  while (1) {
		  si.si_pid = 0;
		  waitid(P_PID, tracee, &si, WSTOPPED | WNOHANG);
		  if (si.si_pid)
			  printf("st=%02d c=%02d\n", si.si_status, si.si_code);
	  }
	  return 0;
  }

Before the patch, the latter waitid() in polling mode reports the
second stopped event generated by the implied SIGSTOP of
PTRACE_ATTACH.

  st=19 c=05
  ^C

After the patch, the second event is not reported.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Oleg Nesterov <oleg@redhat.com>

kernel/signal.c

@@ -1827,10 +1827,27 @@ static int do_signal_stop(int signr)
 		    unlikely(signal_group_exit(sig)))
 			return 0;
 		/*
-		 * There is no group stop already in progress.
-		 * We must initiate one now.
+		 * There is no group stop already in progress.  We must
+		 * initiate one now.
+		 *
+		 * While ptraced, a task may be resumed while group stop is
+		 * still in effect and then receive a stop signal and
+		 * initiate another group stop.  This deviates from the
+		 * usual behavior as two consecutive stop signals can't
+		 * cause two group stops when !ptraced.
+		 *
+		 * The condition can be distinguished by testing whether
+		 * SIGNAL_STOP_STOPPED is already set.  Don't generate
+		 * group_exit_code in such case.
+		 *
+		 * This is not necessary for SIGNAL_STOP_CONTINUED because
+		 * an intervening stop signal is required to cause two
+		 * continued events regardless of ptrace.
 		 */
-		sig->group_exit_code = signr;
+		if (!(sig->flags & SIGNAL_STOP_STOPPED))
+			sig->group_exit_code = signr;
+		else
+			WARN_ON_ONCE(!task_ptrace(current));
 
 		current->group_stop &= ~GROUP_STOP_SIGMASK;
 		current->group_stop |= signr | gstop;