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From:   qiwuchen55@gmail.com
To:     peterz@infradead.org, mingo@kernel.org, oleg@redhat.com,
        christian.brauner@ubuntu.com
Cc:     kernel-team@android.com, linux-kernel@vger.kernel.org,
        chenqiwu <chenqiwu@xiaomi.com>
Subject: [PATCH] kernel/exit: do panic earlier to get coredump if global init task exit
Date:   Thu, 12 Dec 2019 14:14:15 +0800
Message-Id: <1576131255-3433-1-git-send-email-qiwuchen55@gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

From: chenqiwu <chenqiwu@xiaomi.com>

When global init task get a chance to be killed, panic will happen in
later calling steps by do_exit()->exit_notify()->forget_original_parent()
->find_child_reaper() if all init threads have exited.

However, it's hard to extract the coredump of init task from a kernel
crashdump, since exit_mm() has released its mm before panic. In order
to get the backtrace of init task in userspace, it's better to do panic
earlier at the beginning of exitting route.

It's worth noting that we must take case of a multi-threaded init exitting
issue. We need the test for signal_group_exit() to ensure that it is all
threads exiting and not just the current thread.

Of course testing signal_group_exit() is not sufficient. It is still
possible that this is someone calling exit(2) in an old binary or someone
calling pthread_exit(3) from the last thread of init. To handle that case,
we need the test to be:

(signal_group_exit(tsk->signal) || thread_group_empty(tsk)).

Because exiting the final thread of init should also trigger the panic.

Signed-off-by: chenqiwu <chenqiwu@xiaomi.com>
---
 kernel/exit.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index e10de98..ba7d1aa 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -578,10 +578,6 @@ 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);
@@ -785,6 +781,9 @@ void __noreturn do_exit(long code)
 		panic("Aiee, killing interrupt handler!");
 	if (unlikely(!tsk->pid))
 		panic("Attempted to kill the idle task!");
+	if (unlikely(is_global_init(tsk) &&
+		(signal_group_exit(tsk->signal) || thread_group_empty(tsk))))
+		panic("Attempted to kill init! exitcode=0x%08lx\n", code);
 
 	/*
 	 * If do_exit is called because this processes oopsed, it's possible
-- 
1.9.1