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

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Date:	Sun, 1 Nov 2015 21:25:40 +0100
From:	Richard Weinberger <richard.weinberger@...il.com>
To:	Shayan Pooya <shayan@...eve.org>
Cc:	cgroups mailinglist <cgroups@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux Containers <containers@...ts.linux-foundation.org>
Subject: Re: piping core dump to a program escapes container

On Sat, Oct 24, 2015 at 11:54 PM, Shayan Pooya <shayan@...eve.org> 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).
>
> 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.

IMHO this is another example of the question whether a container should
be a full blown Linux system or not.
In your opinion containers should own core_pattern and be able to install
their own dump helpers.
Currently this is not the case as this is setting is only writable by
the global root.
Others might argue that containers are not a full virtual Linux with
all features and
an administrator may want to collect cores from all containers.

-- 
Thanks,
//richard
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