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Date: Tue, 20 Oct 2015 12:55:39 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Dmitry Vyukov <dvyukov@google.com>
Cc: LKML <linux-kernel@vger.kernel.org>, Roland McGrath <roland@hack.frob.com>,
        syzkaller@googlegroups.com, Kostya Serebryany <kcc@google.com>,
        Alexander Potapenko <glider@google.com>,
        Robert Swiecki <swiecki@google.com>, Kees Cook <keescook@google.com>,
        Julien Tinnes <jln@google.com>, Eric Dumazet <edumazet@google.com>
Subject: Re: Unkillable processes due to PTRACE_TRACEME
Message-ID: <20151020105539.GA27706@redhat.com>
References: <CACT4Y+Zu9uvt5z_LRAQaPTo+nwfodwBbjkeTRHoxGKRCPqpx5Q@mail.gmail.com> <20151019194911.GA20063@redhat.com> <CACT4Y+aDeSED9pnERC+gyHAEV=+n+awJ-s-8QGBQWMzuww894A@mail.gmail.com> <CACT4Y+bHNEKdZpgOYc+LaKxDyt1GeD+Vf4jLnmAVWQ3Z8=LKaw@mail.gmail.com> <CACT4Y+YmBeTxXVpoWLybcKCy_EwX2uAA5u2b7KbACJX4k3B4FQ@mail.gmail.com>
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On 10/20, Dmitry Vyukov wrote:
>
> On Tue, Oct 20, 2015 at 10:34 AM, Dmitry Vyukov <dvyukov@google.com> wrote:
> > On Mon, Oct 19, 2015 at 10:17 PM, Dmitry Vyukov <dvyukov@google.com> wrote:
> >> On Mon, Oct 19, 2015 at 9:49 PM, Oleg Nesterov <oleg@redhat.com> wrote:
> >>>
> >>> 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.

Thanks. And I just checked Fedora 22, it doesn't use __WALL too.

So I think we should change the kernel even if this is not a bug...
I'll send the patch.

> > But __WALL can't be used with waitid and Ubuntu init uses waitid...

Yes, and I never understood why. Perhaps we should change this too.

> #include <pthread.h>
> #include <unistd.h>
> #include <sys/ptrace.h>
> #include <stdio.h>
> #include <errno.h>
> #include <signal.h>
> #include <sys/types.h>
> #include <sys/wait.h>
>
> void *thr(void *arg) {
>         ptrace(PTRACE_TRACEME, 0, 0, 0);
>         return 0;
> }
>
> int main() {
> int pid = fork();
>         if (pid == 0) {
>                 pthread_t th;
>                 pthread_create(&th, 0, thr, 0);
>                 sleep(1);
>                 return 0;
>         }
>         siginfo_t info = {};
>         int status = 0;
>         int res = waitpid(-1, &status, __WALL);
>         printf("pid=%d res=%d errno=%d\n", pid, res, errno);
>         res = waitpid(-1, &status, __WALL);
>         printf("pid=%d res=%d errno=%d\n", pid, res, errno);
>         return 0;
> }
>
>
> However, I need to wait for a particular child and if I change the
> first waitpid to:
>
>         int res = waitpid(pid, &status, __WALL);
>
> then it does not terminate.
> So how can I wait for such child process?

You can't. This is one of historical oddities. You need to reap the
traced sub-thread first. And PTRACE_DETACH doesn't work.

Oleg.