[PATCH V2] exit: trigger panic when global init has exited

[Qianli Zhao <zhaoqianligood@gmail.com>]

From: Qianli Zhao <zhaoqianli@xiaomi.com>

When init sub-threads running on different CPUs exit at the same time,
zap_pid_ns_processe()->BUG() may be happened.
And every thread status is abnormal after exit(PF_EXITING set,task->mm=NULL etc),
which makes it difficult to parse coredump from fulldump normally.
In order to fix the above problem, when any one init has been set to SIGNAL_GROUP_EXIT,
trigger panic immediately, and prevent other init threads from continuing to exit.

[   24.705376] Kernel panic - not syncing: Attempted to kill init! exitcode=0x00007f00
[   24.705382] CPU: 4 PID: 552 Comm: init Tainted: G S         O    4.14.180-perf-g4483caa8ae80-dirty #1
[   24.705390] kernel BUG at include/linux/pid_namespace.h:98!

PID: 552   CPU: 4   COMMAND: "init"
PID: 1     CPU: 7   COMMAND: "init"
core4                           core7
...                             sys_exit_group()
                                do_group_exit()
                                   - sig->flags = SIGNAL_GROUP_EXIT
                                   - zap_other_threads()
                                do_exit() //PF_EXITING is set
ret_to_user()
do_notify_resume()
get_signal()
    - signal_group_exit
    - goto fatal;
do_group_exit()
do_exit() //PF_EXITING is set
    - panic("Attempted to kill init! exitcode=0x%08x\n")
                                exit_notify()
                                find_alive_thread() //no alive sub-threads
                                zap_pid_ns_processes()//CONFIG_PID_NS is not set
                                BUG()

Signed-off-by: Qianli Zhao <zhaoqianli@xiaomi.com>
---
V2:
- Changelog update
- Remove wrong useage of SIGNAL_UNKILLABLE. 
- Add thread_group_empty() test to handle single init thread exit

---
 kernel/exit.c | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 04029e3..9d2716b 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -767,6 +767,17 @@ void __noreturn do_exit(long code)
 	validate_creds_for_do_exit(tsk);
 
 	/*
+	 * If global init has exited,
+	 * panic immediately to get a useable coredump.
+	 */
+	if (unlikely(is_global_init(tsk) &&
+	    (thread_group_empty(tsk) ||
+	    (tsk->signal->flags & SIGNAL_GROUP_EXIT)))) {
+			panic("Attempted to kill init! exitcode=0x%08x\n",
+				tsk->signal->group_exit_code ?: (int)code);
+	}
+
+	/*
 	 * We're taking recursive faults here in do_exit. Safest is to just
 	 * leave this task alone and wait for reboot.
 	 */
@@ -786,14 +797,6 @@ 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);
-- 
1.9.1