Re: piping core dump to a program escapes container

Dongsheng Yang
2015-12-10T03:06:16Z

On 12/09/2015 11:29 AM, Eric W. Biederman wrote:
> Dongsheng Yang <yangds.fnst@cn.fujitsu.com> writes:
>
>> 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).
>
>>From a container perspective it is perhaps counter intuitive from
> the perspective of the operator of the machine nothing works specially
> about core_pattern and it works as designed with no unusual danages.
>
>>> 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?
>
> The behavior was the best we could do at the time last time this issue
> was examined. There is enough information available to be able to
> write a core dumping program that can reliably place your core dumps
> in your container.
>
> There has not yet been an obvious namespace in which to stick
> core_pattern, and even worse exactly how to appropriate launch a process
> in a container has not been figured out.

Hi Eric,
Could you provide an reference to these discussion?? In
addition, is there a already infrastructure to do this kind of thing?

Thanx
Yang
>
> If those tricky problems can be solved we can have a core_pattern in a
> container. What we have now is the best we have been able to figure out
> so far.
>
> Eric
>
>
>>
>> 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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