Fixes inability to use /dev/null when inside a container

This is a forward port of opencontainers#3620

The original code depended on the origin filesystem to have
/dev/{block,char} populated. This is done by udev normally and while is
very common non-containerized systemd installs, it's very easy to start
systemd in a container created by runc itself and not have
/dev/{block,char} populated. When this occurs, the following error
output is observed:

$ docker run hello-world
docker: Error response from daemon: failed to create shim task: OCI runtime create failed: runc create failed: unable to start container process: error during container init: error reopening /dev/null inside container: open /dev/null: operation not permitted: unknown.

/dev/null can't be opened because it was not added to the
deviceAllowList, as there was no /dev/char directory. The change here
utilizes the fact that when sysfs in in use, there is a
/sys/dev/{block,char} that is kernel maintained that we can check.

Signed-off-by: Evan Phoenix <evan@phx.io>

libcontainer/cgroups/devices/systemd.go

@@ -133,8 +133,18 @@ func systemdProperties(r *configs.Resources) ([]systemdDbus.Property, error) {
 			// rules separately to systemd) we can safely skip entries that don't
 			// have a corresponding path.
 			if _, err := os.Stat(entry.Path); err != nil {
-				logrus.Debugf("skipping device %s for systemd: %s", entry.Path, err)
-				continue
+				// Also check /sys/dev so that we don't depend on /dev/{block,char}
+				// being populated. (/dev/{block,char} is populated by udev, which
+				// isn't strictly required for systemd). Ironically, this happens most
+				// easily when starting containerd within a runc created container
+				// itself.
+
+				// We don't bother with securejoin here because we create entry.Path
+				// right above here, so we know it's safe.
+				if _, err := os.Stat("/sys" + entry.Path); err != nil {
+					logrus.Warnf("skipping device %s for systemd: %s", entry.Path, err)
+					continue
+				}
 			}
 		}
 		deviceAllowList = append(deviceAllowList, entry)