Re: piping core dump to a program escapes container

Dongsheng Yang
2015-12-09T02:34:37Z

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?

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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