[systemd-devel] Docker vs PrivateTmp

Daniel J Walsh dwalsh at redhat.com
Fri Jan 23 08:31:40 PST 2015

On 01/22/2015 10:02 PM, Lennart Poettering wrote:
> On Sat, 17.01.15 23:02, Lars Kellogg-Stedman (lars at redhat.com) wrote:
>> See the `devicemapper` mountpoint created by Docker for the container:
>>     # grep devicemapper/mnt /proc/mounts
>>     /dev/mapper/docker-253:6-98310-e68df3f45d6151259ce84a0e467a3117840084e99ef3bbc654b33f08d2d6dd62
>>     /var/lib/docker/devicemapper/mnt/e68df3f45d6151259ce84a0e467a3117840084e99ef3bbc654b33f08d2d6dd62
>>     ext4
>>     rw,context="system_u:object_r:svirt_sandbox_file_t:s0:c261,c1018",relatime,discard,stripe=16,data=ordered
>>     0 0
> I am not sure why docker makes these mounts visible in the host
> namespace at all. This smells like a bug.
>> Watch Docker fail to destroy the container because it is unable to remove the mountpoint directory:
>>     Jan 17 22:43:03 pk115wp-lkellogg docker-1.4.1-dev[18239]:
>>     time="2015-01-17T22:43:03-05:00" level="error" msg="Handler for DELETE
>>     /containers/{name:.*} returned error: Cannot destroy container e68df3f45d61:
>>     Driver devicemapper failed to remove root filesystem
>>     e68df3f45d6151259ce84a0e467a3117840084e99ef3bbc654b33f08d2d6dd62: Device is
>>     Busy"
> This smells as if Docker incorrectly sets the mount propagation bits
> on its own mounts.
> It would be good checking /proc/self/mountinfo inside and outside of
> docker's own namespace, and checking how the propagation bits are set
> for the individual mounts. It's a bit hard to read, but the
> interesting bits are in the 7th column of that file.
> In general: docker should do the equivalent of "mount --make-rslave /"
> as first thing after opening its mount namespace, so that from that
> point on mounts and especiall *un*mounts propagate from the host into
> the container, but not vice versa.
> If they do not invoke that, then the propagation will stay at
> "shared", which means the mounts will appear in the host and vice
> versa, which is certainly undesired.
> Also, they should not use "mount --make-rprivate /", as that means
> anything the host mounted will stay mounted in the container forever,
> which is a problem.
> Also, they really need to make this recursive, so that all mount
> points they have access too are detached from the host!
> Lennart

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