lxc-start: Add command to retrieve the clone flags used to start the container.

Add the LXC_COMMAND_CLONE_FLAGS that retrieves the flags passed to clone(2)
when the container was started. This allows external programs to determine
which namespaces the container was unshared from.

Signed-off-by: Christian Seiler <christian@iwakd.de>
Cc: Daniel Lezcano <daniel.lezcano@free.fr>
Cc: Serge Hallyn <serge.hallyn@canonical.com>
lxc-attach: Remodel cgroup attach logic and attach to namespaces again in parent process

With the introduction of lxc-attach's functionality to attach to cgroups,
the setns() calls were put in the child process after the fork() and not the
parent process before the fork() so the parent process remained outside the
namespaces and could add the child to the correct cgroup.

Unfortunately, the pid namespace really affects only children of the current
process and not the process itself, which has several drawbacks: The
attached program does not have a pid inside the container and the context
that is used when remounting /proc from that process is wrong. Thus, the
previous logic of first setting the namespaces and then forking so the child
process (which then exec()s to the desired program) is a real member of the
container.

However, inside the container, there is no guarantee that the cgroup
filesystem is still be mounted and that we are allowed to write to it (which
is why the setns() was moved in the first place).

To work around both problems, we separate the cgroup attach functionality
into two parts: Preparing the attach process, which just opens the tasks
files of all cgroups and keeps the file descriptors open and the writing to
those fds part. This allows us to open all the tasks files in lxc_attach,
then call setns(), then fork, in the child process close them completely and
in the parent process just write the pid of the child process to all those
fds.

Signed-off-by: Christian Seiler <christian@iwakd.de>
Cc: Daniel Lezcano <daniel.lezcano@free.fr>
Cc: Serge Hallyn <serge.hallyn@canonical.com>
lxc-attach: Detect which namespaces to attach to dynamically

Use the command interface to contact lxc-start to receive the set of
flags passed to clone() when starting the container. This allows lxc-attach
to determine which namespaces were used for the container and select only
those to attach to.

Signed-off-by: Christian Seiler <christian@iwakd.de>
Cc: Daniel Lezcano <daniel.lezcano@free.fr>
Cc: Serge Hallyn <serge.hallyn@canonical.com>
lxc-unshare: Move functions to determine clone flags from command line options to namespace.c

In order to be able to reuse code in lxc-attach, the functions
lxc_namespace_2_cloneflag and lxc_fill_namespace_flags are moved from
lxc_unshare.c to namespace.c.

Signed-off-by: Christian Seiler <christian@iwakd.de>
Cc: Daniel Lezcano <daniel.lezcano@free.fr>
Cc: Serge Hallyn <serge.hallyn@canonical.com>
lxc-attach: Add -s option to select namespaces to attach to

This patch allows the user to select any list of namespaces (network, pid,
mount, uts, ipc, user) that lxc-attach should use when attaching to the
container; all other namespaces will not be attached to.

This allows the user to for example attach to just the network namespace and
use the host's (and not the container's) network tools to reconfigure the
network of the container.

Signed-off-by: Christian Seiler <christian@iwakd.de>
Cc: Daniel Lezcano <daniel.lezcano@free.fr>
Cc: Serge Hallyn <serge.hallyn@canonical.com>
lxc-attach: Add -R option to remount /sys and /proc when only partially attaching

When attaching to only some namespaces of the container but not the mount
namespace, the contents of /sys and /proc of the host system do not properly
reflect the context of the container's pid and/or network namespaces, and
possibly others.

The introduced -R option adds the possibility to additionally unshare the
mount namespace (when it is not being attached) and remount /sys and /proc
in order for those filesystems to properly reflect the container's context
even when only attaching to some of the namespaces.

Signed-off-by: Christian Seiler <christian@iwakd.de>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Cc: Daniel Lezcano <daniel.lezcano@free.fr>