merge branch 'pr-3623'

Evan Phoenix (1):
  Fixes inability to use /dev/null when inside a container

LGTMs: kolyshkin cyphar
Closes #3623

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)