linux-kernel - Re: Threads stuck in zap_pid_ns

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Date:   Thu, 1 Jun 2017 11:45:49 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Vovo Yang <vovoy@...gle.com>, Ingo Molnar <mingo@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: Threads stuck in zap_pid_ns_processes()

On Thu, Jun 01, 2017 at 12:08:58PM -0500, Eric W. Biederman wrote:
> Guenter Roeck <linux@...ck-us.net> writes:
> >
> > I think you nailed it. If I drop CLONE_NEWPID from the reproducer I get
> > a zombie process.
> >
> > I guess the only question left is if zap_pid_ns_processes() should (or could)
> > somehow detect that situation and return instead of waiting forever.
> > What do you think ?
> 
> Any chance you can point me at the chromium code that is performing the
> ptrace?
> 
> I want to conduct a review of the kernel semantics to see if the current
> semantics make it unnecessarily easy to get into hang situations.  If
> the semantics make it really easy to get into a hang situation I want
> to see if there is anything we can do to delicately change the semantics
> to avoid the hangs without breaking existing userspace.
> 
The internal bug should be accessible to you.

https://bugs.chromium.org/p/chromium/issues/detail?id=721298&desc=2

It has some additional information, and points to the following code in Chrome.

https://cs.chromium.org/chromium/src/breakpad/src/client/linux/minidump_writer/linux_ptrace_dumper.cc?rcl=47e51739fd00badbceba5bc26b8abc8bbd530989&l=85

With the information we have, I don't really have a good idea what we could or
should change in Chrome to make the problem disappear, so I just concluded that
we'll have to live with the forever-sleeping task.

Thanks,
Guenter