exit: panic before exit_mm() on global init exit

Currently, when global init and all threads in its thread-group have exited
we panic via:
do_exit()
-> exit_notify()
   -> forget_original_parent()
      -> find_child_reaper()
This makes it hard to extract a useable coredump for global init from a
kernel crashdump because by the time we panic exit_mm() will have already
released global init's mm.
This patch moves the panic futher up before exit_mm() is called. As was the
case previously, we only panic when global init and all its threads in the
thread-group have exited.

Signed-off-by: chenqiwu <chenqiwu@xiaomi.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
[christian.brauner@ubuntu.com: fix typo, rewrite commit message]
Link: https://lore.kernel.org/r/1576736993-10121-1-git-send-email-qiwuchen55@gmail.com
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

diff --git a/kernel/exit.c b/kernel/exit.c
index a46a50d67002d9877b00c8052ba922e02e9740af..fc364272759de1b75259d2a9936e47dc931438fc 100644 (file)
--- 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);
@@ -786,6 +782,14 @@ void __noreturn do_exit(long code)
        acct_update_integrals(tsk);
        group_dead = atomic_dec_and_test(&tsk->signal->live);
        if (group_dead) {
+               /*
+                * If the last thread of global init has exited, panic
+                * immediately to get a useable coredump.
+                */
+               if (unlikely(is_global_init(tsk)))
+                       panic("Attempted to kill init! exitcode=0x%08x\n",
+                               tsk->signal->group_exit_code ?: (int)code);
+
 #ifdef CONFIG_POSIX_TIMERS
                hrtimer_cancel(&tsk->signal->real_timer);
                exit_itimers(tsk->signal);