The existing check doesn't work, because when you statically
link a program against libc, any functions not called are not
included.  So cap_init() which we check for is not there in
the built binary.

So instead just check whether a "gcc -lcap -static" works.
If libcap.a is not available it will fail, if it is it will
succeed.
Signed-off-by: Serge Hallyn <shallyn@cisco.com>

lxccontainer: revert set_running_config_item()

Support configurations without root mapping

Closes #2033.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

If no id mapping for the container's root id is defined try to us the id
mappings specified via lxc.init.{g,u}id.

Closes #2033.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

Closes #2033.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

Closes #2033.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

- mapped_hostid_entry()
- idmap_add()

Closes #2033.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

- As discussed we will have a proper API extension that will allow updating
  various parts of a running container. The prior approach wasn't a good idea.

- Revert this is not a problem since we haven't released any version with the
  set_running_config_item() API extension.

- I'm not simply reverting so that master users can still call into new
  liblxc's without crashing the container. This is achieved by keeping the
  commands callback struct member number identical.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

mainloop: capture output of short-lived init procs

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

We don't allow non-pty devices anyway so don't let open() create unneeded
files.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

lxc_config: Add -h and --help flags handler

As the other tools already handle, show usage message when -h or --help
are used.
Signed-off-by: Marcos Paulo de Souza <marcos.souza.org@gmail.com>

The handler for the signal fd will detect when the init process of a container
has exited and cause the mainloop to close. However, this can happen before the
console handlers - or any other events for that matter - are handled. So in the
case of init exiting we still need to allow for all buffered input to the
console to be handled before exiting. This allows us to capture output from
short-lived init processes.

This is conceptually equivalent to my implementation of ExecReaderToChannel()
https://github.com/lxc/lxd/blob/master/shared/util_linux.go#L527

Closes #1694.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

This makes it clearer why handlers return what value.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

start: fix death signal

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

On set{g,u}id() the kernel does:

 	/* dumpability changes */
	if (!uid_eq(old->euid, new->euid) ||
	    !gid_eq(old->egid, new->egid) ||
	    !uid_eq(old->fsuid, new->fsuid) ||
	    !gid_eq(old->fsgid, new->fsgid) ||
	    !cred_cap_issubset(old, new)) {
		if (task->mm)
			set_dumpable(task->mm, suid_dumpable);
		task->pdeath_signal = 0;
		smp_wmb();
	}

which means we need to re-enable the deat signal after the set{g,u}id().
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

start: simplify cgroup namespace preservation

Since we are now dumpable we can open /proc/<child-pid>/ns/cgroup so let's
avoid the overhead of sending around fds.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

When set set{u,g}id() the kernel will make us undumpable. This is unnecessary
since we can guarantee that whatever is running inside the child process at
this point this is fully trusted by the parent. Making us dumpable let's users
use debuggers on the child process before the exec as well and also allows us
to open /proc/<child-pid> files in lieu of the child.
Note, that we only need to perform the prctl(PR_SET_DUMPABLE, ...) if our
effective uid on the host is not 0. If our effective uid on the host is 0 then
we will keep all capabilities in the child user namespace across set{g,u}id().
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

start: tweaks + bugfixes

btrfs: fix unprivileged snapshot creation

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

This way we can rely on the kernel's copy-on-write support similar to fork().
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

This is a copy-on-write (no stack passed) variant of lxc_clone().
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

This is to avoid bad surprises caused by older glibc's pid cache (up to 2.25)
when using clone().
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

Because of older glibc's pid cache (up to 2.25) whenever clone() is called the
child must must retrieve it's own pid via lxc_raw_getpid().
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

- test CLONE_VFORK
- test CLONE_FILES
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

attach: simplify significantly

Receive fd for LSM security module before we set{g,u}id(). The reason is that
on set{g,u}id() the kernel will a) make us undumpable and b) we will change our
effective uid. This means our effective uid will be different from the
effective uid of the process that created us which means that this processs no
longer has capabilities in our namespace including CAP_SYS_PTRACE. This means
we will not be able to read and /proc/<pid> files for the process anymore when
/proc is mounted with hidepid={1,2}. So let's get the lsm label fd before the
set{g,u}id().
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

This let's us simplify the whole file a lot and makes things way clearer. It
also let's us avoid the infamous pid cache.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

cgfsng: Add new macro to print errors

Improve the dhclient hook for OCI compat

At this point, macros such DEBUG or ERROR does not take effect because
this code is called from cgroup_ops_init(cgroup.c), which runs with
__attribute__((constructor)), before any log level is set form any tool
like lxc-start, so these messages are lost.

For now on, use the same LXC_DEBUG_CGFSNG environment variable to
control these messages.
Signed-off-by: Marcos Paulo de Souza <marcos.souza.org@gmail.com>

Signed-off-by: Jonathan Calmels <jcalmels@nvidia.com>