linux-kernel - Re: WARNING in get

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Date:   Mon, 5 Oct 2020 15:49:24 +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 10/02, Eric W. Biederman wrote:
>
> syzbot <syzbot+3485e3773f7da290eecc@...kaller.appspotmail.com> writes:
>
> > Hello,
> >
> > syzbot found the following issue on:
>
> So this is:
>
> static void do_jobctl_trap(void)
> {
> 	struct signal_struct *signal = current->signal;
> 	int signr = current->jobctl & JOBCTL_STOP_SIGMASK;
>
> 	if (current->ptrace & PT_SEIZED) {
> 		if (!signal->group_stop_count &&
> 		    !(signal->flags & SIGNAL_STOP_STOPPED))
> 			signr = SIGTRAP;
> 		WARN_ON_ONCE(!signr);
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> 		ptrace_do_notify(signr, signr | (PTRACE_EVENT_STOP << 8),
> 				 CLD_STOPPED);
> 	} else {
> 		WARN_ON_ONCE(!signr);
> 		ptrace_stop(signr, CLD_STOPPED, 0, NULL);
> 		current->exit_code = 0;
> 	}
> }
>
> I have the state of this paged out of my head at the moment.
>
> Oleg or Tejun do you remember what is supposed to keep signr from being
> NULL?

This nearly killed me, but I seem to understand whats going on.
ptrace_init_task() does task_set_jobctl_pending(JOBCTL_TRAP_STOP) while
SIGNAL_STOP_STOPPED, child->jobctl & JOBCTL_STOP_SIGMASK == 0.

> It looks like this code was introduced in commit 73ddff2bee15 ("job
> control: introduce JOBCTL_TRAP_STOP and use it for group stop trap").

Yes, but I bet this was broken later, _may be_ by 924de3b8c9410c4.

I need to take a rest and read this code again. I too forgot how this
all supposed to work.

Oleg.