linux-kernel - Re: pppd service crash in linux-3.13.6

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Message-ID: <53244C61.5090508@hurleysoftware.com>
Date:	Sat, 15 Mar 2014 08:49:37 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Sree Harsha Totakura <sreeharsha@...akura.in>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: pppd service crash in linux-3.13.6

On 03/14/2014 05:04 PM, Oleg Nesterov wrote:
> On 03/14, Peter Hurley wrote:
>> On 03/14/2014 03:23 PM, Oleg Nesterov wrote:
>>> On 03/14, Peter Hurley wrote:
>>>>
>> Yes, cgroup_release_agent() is the work function that is scheduled.
>>
>>>> which requires both namespace and tty facilities.
>>>
>>> Hmm... why?
>>>
>>> The exiting task obviously can't exec. The only way to spawn a userspace
>>> process is call_usermodehelper(), it should work just fine, no?
>>
>> You're correct, in the immediate sense that the user command exec'd will
>> not inherit open file descriptors.
>>
>> But what if it expects to be able to find the intact children of
>> the foreground process group, and can't because the controlling tty
>> has already been torn down and all the children already sent SIGHUP.
>
> Which group/tty ? call_usermodehelper() asks the workqueue thread
> to kthread_create/exec. See also below...
>
>> Or what if the user command expects to find and join the user namespace
>> of the dying process but now it's already been freed?
>
> But it can't even know who called call_usermodehelper(). Besides,
> cgroup_release_agent() uses UMH_WAIT_EXEC, so the caller can continue
> and disappear completely before the usermode process has any chance
> to do something.

I'm just hypothesizing potential breakage, since the order of teardown
is sensitive to changes, and I didn't do a complete audit of all the
possibilities.

If you feel strongly about moving disassociate_tty(), I won't object.

Regards,
Peter Hurley


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