Re: piping core dump to a program escapes container

[Dongsheng Yang]

On 12/09/2015 11:29 AM, Eric W. Biederman wrote:
> Dongsheng Yang <yangds.fnst@xxxxxxxxxxxxxx> 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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>
>
> .



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