linux-kernel - Re: WARNING in get

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Date:   Thu, 15 Oct 2020 14:49:26 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     syzbot <syzbot+3485e3773f7da290eecc@...kaller.appspotmail.com>,
        axboe@...nel.dk, christian@...uner.io,
        linux-kernel@...r.kernel.org, liuzhiqiang26@...wei.com,
        syzkaller-bugs@...glegroups.com, Tejun Heo <tj@...nel.org>
Subject: Re: WARNING in get_signal

On 10/06, Oleg Nesterov wrote:
>
> I still do not see a good fix. I am crying ;)

Sorry for delay... Finally I think I have a simple and clean fix.
We can leave ptrace_init_task() alone and fix task_join_group_stop().

I need to test it a bit and write the changelog, do you see any problem
in the patch below?

(TODO: SIGCONT should clear JOBCTL_STOP_SIGMASK, needs another patch)

Oleg.

--- a/kernel/signal.c
+++ b/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)
 {
+	struct signal_struct *sig = current->signal;
+	unsigned long mask = current->jobctl & JOBCTL_STOP_SIGMASK;
+
+	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);
 }
 
 /*