This changes the default ipc mode of daemon/engine to be private,
meaning the containers will not have their /dev/shm bind-mounted
from the host by default. The benefits of doing this are:
1. No leaked mounts. Eliminate a possibility to leak mounts into
other namespaces (and therefore unfortunate errors like "Unable to
remove filesystem for <ID>: remove /var/lib/docker/containers/<ID>/shm:
device or resource busy").
2. Working checkpoint/restore. Make `docker checkpoint`
not lose the contents of `/dev/shm`, but save it to
the dump, and be restored back upon `docker start --checkpoint`
(currently it is lost -- while CRIU handles tmpfs mounts,
the "shareable" mount is seen as external to container,
and thus rightfully ignored).
3. Better security. Currently any container is opened to share
its /dev/shm with any other container.
Obviously, this change will break the following usage scenario:
$ docker run -d --name donor busybox top
$ docker run --rm -it --ipc container:donor busybox sh
Error response from daemon: linux spec namespaces: can't join IPC
of container <ID>: non-shareable IPC (hint: use IpcMode:shareable
for the donor container)
The soution, as hinted by the (amended) error message, is to
explicitly enable donor sharing by using --ipc shareable:
$ docker run -d --name donor --ipc shareable busybox top
Compatibility notes:
1. This only applies to containers created _after_ this change.
Existing containers are not affected and will work fine
as their ipc mode is stored in HostConfig.
2. Old backward compatible behavior ("shareable" containers
by default) can be enabled by either using
`--default-ipc-mode shareable` daemon command line option,
or by adding a `"default-ipc-mode": "shareable"`
line in `/etc/docker/daemon.json` configuration file.
3. If an older client (API < 1.40) is used, a "shareable" container
is created. A test to check that is added.
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
