ptrace: fix task_join_group_stop() for the case when current is traced

This testcase

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

	void *tf(void *arg)
	{
		return NULL;
	}

	int main(void)
	{
		int pid = fork();
		if (!pid) {
			kill(getpid(), SIGSTOP);

			pthread_t th;
			pthread_create(&th, NULL, tf, NULL);

			return 0;
		}

		waitpid(pid, NULL, WSTOPPED);

		ptrace(PTRACE_SEIZE, pid, 0, PTRACE_O_TRACECLONE);
		waitpid(pid, NULL, 0);

		ptrace(PTRACE_CONT, pid, 0,0);
		waitpid(pid, NULL, 0);

		int status;
		int thread = waitpid(-1, &status, 0);
		assert(thread > 0 && thread != pid);
		assert(status == 0x80137f);

		return 0;
	}

fails and triggers WARN_ON_ONCE(!signr) in do_jobctl_trap().

This is because task_join_group_stop() has 2 problems when current is traced:

	1. We can't rely on the "JOBCTL_STOP_PENDING" check, a stopped tracee
	   can be woken up by debugger and it can clone another thread which
	   should join the group-stop.

	   We need to check group_stop_count || SIGNAL_STOP_STOPPED.

	2. If SIGNAL_STOP_STOPPED is already set, we should not increment
	   sig->group_stop_count and add JOBCTL_STOP_CONSUME. The new thread
	   should stop without another do_notify_parent_cldstop() report.

To clarify, the problem is very old and we should blame
ptrace_init_task().  But now that we have task_join_group_stop() it makes
more sense to fix this helper to avoid the code duplication.

Reported-by: syzbot+3485e3773f7da290eecc@syzkaller.appspotmail.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Christian Brauner <christian@brauner.io>
Cc: "Eric W . Biederman" <ebiederm@xmission.com>
Cc: Zhiqiang Liu <liuzhiqiang26@huawei.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: <stable@vger.kernel.org>
Link: https://lkml.kernel.org/r/20201019134237.GA18810@redhat.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: oleg-nesterov

kernel/signal.c

@@ -391,16 +391,17 @@ static bool task_participate_group_stop(struct task_struct *task)
 
 void task_join_group_stop(struct task_struct *task)
 {
+	unsigned long mask = current->jobctl & JOBCTL_STOP_SIGMASK;
+	struct signal_struct *sig = current->signal;
+
+	if (sig->group_stop_count) {
+		sig->group_stop_count++;
+		mask |= JOBCTL_STOP_CONSUME;
+	} else if (!(sig->flags & SIGNAL_STOP_STOPPED))
+		return;
+
 	/* Have the new thread join an on-going signal group stop */
-	unsigned long jobctl = current->jobctl;
-	if (jobctl & JOBCTL_STOP_PENDING) {
-		struct signal_struct *sig = current->signal;
-		unsigned long signr = jobctl & JOBCTL_STOP_SIGMASK;
-		unsigned long gstop = JOBCTL_STOP_PENDING | JOBCTL_STOP_CONSUME;
-		if (task_set_jobctl_pending(task, signr | gstop)) {
-			sig->group_stop_count++;
-		}
-	}
+	task_set_jobctl_pending(task, mask | JOBCTL_STOP_PENDING);
 }
 
 /*