linux-kernel - Re: Unkillable processes due to PTRACE

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Date:	Tue, 20 Oct 2015 12:55:39 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Dmitry Vyukov <dvyukov@...gle.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 10/20, Dmitry Vyukov wrote:
>
> On Tue, Oct 20, 2015 at 10:34 AM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
> > 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:
> >>>
> >>> 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.

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