Re: piping core dump to a program escapes container

Eric W. Biederman
2015-12-09T03:37:42Z

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.

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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