job control: Fix ptracer wait(2) hang and explain notask_error clearing

wait(2) and friends allow access to stopped/continued states through
zombies, which is required as the states are process-wide and should
be accessible whether the leader task is alive or undead.
wait_consider_task() implements this by always clearing notask_error
and going through wait_task_stopped/continued() for unreaped zombies.

However, while ptraced, the stopped state is per-task and as such if
the ptracee became a zombie, there's no further stopped event to
listen to and wait(2) and friends should return -ECHILD on the tracee.

Fix it by clearing notask_error only if WCONTINUED | WEXITED is set
for ptraced zombies.  While at it, document why clearing notask_error
is safe for each case.

Test case follows.

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

  static void *nooper(void *arg)
  {
	  pause();
	  return NULL;
  }

  int main(void)
  {
	  const struct timespec ts1s = { .tv_sec = 1 };
	  pid_t tracee, tracer;
	  siginfo_t si;

	  tracee = fork();
	  if (tracee == 0) {
		  pthread_t thr;

		  pthread_create(&thr, NULL, nooper, NULL);
		  nanosleep(&ts1s, NULL);
		  printf("tracee exiting\n");
		  pthread_exit(NULL);	/* let subthread run */
	  }

	  tracer = fork();
	  if (tracer == 0) {
		  ptrace(PTRACE_ATTACH, tracee, NULL, NULL);
		  while (1) {
			  if (waitid(P_PID, tracee, &si, WSTOPPED) < 0) {
				  perror("waitid");
				  break;
			  }
			  ptrace(PTRACE_CONT, tracee, NULL,
				 (void *)(long)si.si_status);
		  }
		  return 0;
	  }

	  waitid(P_PID, tracer, &si, WEXITED);
	  kill(tracee, SIGKILL);
	  return 0;
  }

Before the patch, after the tracee becomes a zombie, the tracer's
waitid(WSTOPPED) never returns and the program doesn't terminate.

  tracee exiting
  ^C

After the patch, tracee exiting triggers waitid() to fail.

  tracee exiting
  waitid: No child processes

-v2: Oleg pointed out that exited in addition to continued can happen
     for ptraced dead group leader.  Clear notask_error for ptraced
     child on WEXITED too.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Oleg Nesterov <oleg@redhat.com>

diff --git a/kernel/exit.c b/kernel/exit.c
index b4a935c..84d13d6 100644 (file)
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1550,17 +1550,41 @@ static int wait_consider_task(struct wait_opts *wo, int ptrace,
                return 0;
        }
 
-       /*
-        * We don't reap group leaders with subthreads.
-        */
-       if (p->exit_state == EXIT_ZOMBIE && !delay_group_leader(p))
-               return wait_task_zombie(wo, p);
+       /* slay zombie? */
+       if (p->exit_state == EXIT_ZOMBIE) {
+               /* we don't reap group leaders with subthreads */
+               if (!delay_group_leader(p))
+                       return wait_task_zombie(wo, p);
 
-       /*
-        * It's stopped or running now, so it might
-        * later continue, exit, or stop again.
-        */
-       wo->notask_error = 0;
+               /*
+                * Allow access to stopped/continued state via zombie by
+                * falling through.  Clearing of notask_error is complex.
+                *
+                * When !@ptrace:
+                *
+                * If WEXITED is set, notask_error should naturally be
+                * cleared.  If not, subset of WSTOPPED|WCONTINUED is set,
+                * so, if there are live subthreads, there are events to
+                * wait for.  If all subthreads are dead, it's still safe
+                * to clear - this function will be called again in finite
+                * amount time once all the subthreads are released and
+                * will then return without clearing.
+                *
+                * When @ptrace:
+                *
+                * Stopped state is per-task and thus can't change once the
+                * target task dies.  Only continued and exited can happen.
+                * Clear notask_error if WCONTINUED | WEXITED.
+                */
+               if (likely(!ptrace) || (wo->wo_flags & (WCONTINUED | WEXITED)))
+                       wo->notask_error = 0;
+       } else {
+               /*
+                * @p is alive and it's gonna stop, continue or exit, so
+                * there always is something to wait for.
+                */
+               wo->notask_error = 0;
+       }
 
        if (task_stopped_code(p, ptrace))
                return wait_task_stopped(wo, ptrace, p);