linux-kernel - Re: [PATCH v2] kernel/exit: do panic earlier to get coredump if global init task exit

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Date:   Tue, 17 Dec 2019 18:50:42 +0800
From:   chenqiwu <qiwuchen55@...il.com>
To:     Christian Brauner <christian.brauner@...ntu.com>, oleg@...hat.com,
        peterz@...radead.org, mingo@...nel.org
Cc:     kernel-team@...roid.com, linux-kernel@...r.kernel.org,
        chenqiwu@...omi.com
Subject: Re: [PATCH v2] kernel/exit: do panic earlier to get coredump if
 global init task exit

On Mon, Dec 16, 2019 at 06:44:11PM +0100, Christian Brauner wrote:
> On Mon, Dec 16, 2019 at 06:28:41PM +0100, Oleg Nesterov wrote:
> > On 12/16, qiwuchen55@...il.com wrote:
> > >
> > > +	 * If all threads of global init have exited, do panic imeddiately
> > > +	 * to get the coredump to find any clue for init task in userspace.
> > > +	 */
> > > +	group_dead = atomic_dec_and_test(&tsk->signal->live);
> > > +	if (unlikely(is_global_init(tsk) && group_dead))
> > > +		panic("Attempted to kill init! exitcode=0x%08lx\n", code);
> >                                                                     ^^^^
> > 
> > No, we should not throw out the useful info, please use
> > 
> > 	signal->group_exit_code ?: code
> > 
> > as the current code does.
> > 
> > And I am worried atomic_dec_and_test() is called too early...
> > 
> > Say, acct_process() can report the exit while some sub-thread sleeps
> 
> Hm, I'm not following here. I might just be slow. acct_process() doesn't
> seem to report exit status and has been called after group_dead before.
> 
> Christian

I agree using signal->group_exit_code ?: code is better, because there is a
possibilty that the exit of main init thread is called by complete_and_exit()
not by a normal exit route (get_signal()->do_group_exit()). For this case,
signal->group_exit_code maybe NULL, so the arg of code should be the exitcode.

And I agree we should not move up group_dead too much. What Oleg has mentioned
is just one case, what's more, there is a possibilty that some sub-threads sleep
in exit_signals()->threadgroup_change_begin() and not do real exit, the main
thread will do panic if all sub-threads do exit since the group_dead is 0.

For workaroud such case, I think the patch should be:

diff --git a/kernel/exit.c b/kernel/exit.c
index bcbd598..58d90e1 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -517,10 +517,6 @@ static struct task_struct *find_child_reaper(struct task_struct *father,
        }

        write_unlock_irq(&tasklist_lock);
-       if (unlikely(pid_ns == &init_pid_ns)) {
-               panic("Attempted to kill init! exitcode=0x%08x\n",
-                       father->signal->group_exit_code ?: father->exit_code);
-       }

        list_for_each_entry_safe(p, n, dead, ptrace_entry) {
                list_del_init(&p->ptrace_entry);
@@ -728,6 +724,15 @@ void __noreturn do_exit(long code)
                panic("Attempted to kill the idle task!");

        /*
+        * If all threads of global init have exited, do panic imeddiately
+        * to get the coredump to find any clue for init task in userspace.
+        */
+       if (unlikely(is_global_init(tsk) &&
+               (atomic_read(&tsk->signal->live) == 1)))
+               panic("Attempted to kill init! exitcode=0x%08x\n",
+                       tsk->signal->group_exit_code ?: (int)code);
+
+       /*
         * If do_exit is called because this processes oopsed, it's possible
         * that get_fs() was left as KERNEL_DS, so reset it to USER_DS before
         * continuing. Amongst other possible reasons, this is to prevent