linux-kernel - Re: Unkillable processes due to PTRACE

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Date:	Tue, 20 Oct 2015 10:34:48 +0200
From:	Dmitry Vyukov <dvyukov@...gle.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Roland McGrath <roland@...k.frob.com>,
	syzkaller@...glegroups.com, Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Robert Swiecki <swiecki@...gle.com>,
	Kees Cook <keescook@...gle.com>,
	Julien Tinnes <jln@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: Unkillable processes due to PTRACE_TRACEME

On Mon, Oct 19, 2015 at 10:17 PM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
> On Mon, Oct 19, 2015 at 9:49 PM, Oleg Nesterov <oleg@...hat.com> wrote:
>> On 10/19, Dmitry Vyukov wrote:
>>>
>>> The following program hangs in some interesting state and is not
>>> killable (started by a normal user, not root):
>>
>> Thanks.
>>
>>> #include <pthread.h>
>>> #include <unistd.h>
>>> #include <sys/ptrace.h>
>>> #include <stdio.h>
>>> #include <signal.h>
>>>
>>> void *thr(void *arg) {
>>>         ptrace(PTRACE_TRACEME, 0, 0, 0);
>>>         sleep(3);
>>>         kill(getpid(), SIGCHLD);
>>>         return 0;
>>> }
>>>
>>> int main() {
>>>         if (fork() == 0) {
>>>                 sleep(1);
>>>                 pthread_t th;
>>>                 pthread_create(&th, 0, thr, 0);
>>>                 sleep(1);
>>>         }
>>>         return 0;
>>> }
>>>
>>>
>>> The child process attaches as tracee to init process
>>
>> Yes, although in a racy manner, the parent can exit after
>> PTRACE_TRACEME in this case the kernel will untrace the task
>> before reparenting. Not that this matters.
>>
>>> and then hangs in
>>> a state that I don't understand. When I did a similar thing but
>>> attached it to a normal parent process (shell), I still was able to
>>> get rid of it by killing parent (shell).
>>
>> See above.
>>
>> So I bet the problem is that your /sbin/init doesn't use __WALL,
>> so wait() doesn't reap the traced zombie sub-thread, and thus it
>> can't release the non-empty thread group.
>>
>> Could you please verify? Just do "strace -p1" and send SIGCHLD to
>> init.
>>
>> perhaps eligible_child() should assume WALL if ptrace && ZOMBIE...
>
>
> I am using Ubuntu.
> Here strace output from init:
>
> waitid(P_ALL, 0, {}, WNOHANG|WEXITED|WSTOPPED|WCONTINUED, NULL) = 0
>
> So what should be fixed here? Kernel of distro init?

waitpid(__WALL) indeed joins these processes.
But __WALL can't be used with waitid and Ubuntu init uses waitid...
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