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Date: Mon, 5 Dec 2016 12:00:17 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Dmitry Vyukov <dvyukov@google.com>
Cc: Pavel Machek <pavel@ucw.cz>, Denys Vlasenko <dvlasenk@redhat.com>,
        jan.kratochvil@redhat.com, palves@redhat.com,
        Roland McGrath <roland@hack.frob.com>,
        syzkaller <syzkaller@googlegroups.com>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: Unkillable processes due to PTRACE_TRACEME again
Message-ID: <20161205110016.GA29776@redhat.com>
References: <CACT4Y+ZnT3ngSvbs=U8=41TAMpaS8YqbVb7KdiK9QTM7Jd+JgA@mail.gmail.com> <20161205104652.GA29197@redhat.com>
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On 12/05, Oleg Nesterov wrote:
>
> On 12/02, Dmitry Vyukov wrote:
> >
> > I am not on 2caceb3294a78c389b462e7e236a4e744a53a474 (Dec 1). And see
> > the same unwaitable zombie processes.
>
> This is another thing, and notabug. This is how ptrace works,
>
> > void *thr(void *arg)
> > {
> >   ptrace(PTRACE_TRACEME, 0, 0, 0);
> > }
> >
> > int main()
> > {
> >   int pid = fork();
> >   if (pid == 0) {
> >     pthread_t th;
> >     pthread_create(&th, 0, thr, 0);
> >     usleep(100000);
> >     exit(0);
> >   }
> >   usleep(200000);
> >   kill(pid, SIGKILL);
> >   int status = 0;
> >   waitpid(pid, &status, __WALL);
>
> waitpid(pid) hangs because you need to reap the sub-thread first.

I'm afraid I wasn't clear...

So the child process has 2 threads, the leader thread L and the sub-thread T.
waitpid(pid == L->pid) will block until all the threads go away, but since T is
traced it won't autoreap, the tracer should do waitpid(T->pid) first to reap
this zombie. waitpid(-1) should work too.

Oleg.