linux-kernel - Re: piping core dump to a program escapes container

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Message-ID: <56679399.6080306@cn.fujitsu.com>
Date:	Wed, 9 Dec 2015 10:36:09 +0800
From:	Dongsheng Yang <yangds.fnst@...fujitsu.com>
To:	Shayan Pooya <shayan@...eve.org>, <cgroups@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>,
	Richard Weinberger <richard@....at>,
	<containers@...ts.linux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: piping core dump to a program escapes container

On 12/09/2015 10:26 AM, Dongsheng Yang wrote:
> On 10/25/2015 05:54 AM, Shayan Pooya wrote:
>> I noticed the following core_pattern behavior in my linux box while
>> running docker containers. I am not sure if it is bug, but it is
>> inconsistent and not documented.
>>
>> If the core_pattern is set on the host, the containers will observe
>> and use the pattern for dumping cores (there is no per cgroup
>> core_pattern). According to core(5) for setting core_pattern one can:
>>
>> 1. echo "/tmp/cores/core.%e.%p" > /proc/sys/kernel/core_pattern
>> 2. echo "|/bin/custom_core /tmp/cores/ %e %p " >
>> /proc/sys/kernel/core_pattern
>>
>> The former pattern evaluates the /tmp/cores path in the container's
>> filesystem namespace. Which means, the host does not see a core file
>> in /tmp/cores.
>>
>> However, the latter evaluates the /bin/custom_core path in the global
>> filesystem namespace. Moreover, if /bin/core decides to write the core
>> to a path (/tmp/cores in this case as shown by the arg to
>> custom_core), the path will be evaluated in the global filesystem
>> namespace as well.
>>
>> The latter behaviour is counter-intuitive and error-prone as the
>> container can fill up the core-file directory which it does not have
>> direct access to (which means the core is also not accessible for
>> debugging if someone only has access to the container).
>
> Hi Shayan,
>      We found the same problem with what you described here.
> Is there any document for this behaviour? I want to know is
> that intentional or as you said a 'bug'. Maybe that's intentional
> to provide a way for admin to collect core dumps from all containers as
> Richard said. I am interested in it too.
>
> Anyone can help here?

In addition, is that a good idea to make core_pattern to be seperated
in different namespace?

Yang
>
> Yang
>>
>> Currently, I work around this issue by detecting that the process is
>> crashing from a container (by comparing the namespace pid to the
>> global pid) and refuse to dump the core if it is from a container.
>>
>> Tested on Ubuntu (kernel 3.16) and Fedora (kernel 4.1).
>> --
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>
>
>
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