cri: Expose runtimeHandler support for userns

Since kubernetes 1.30, the kubelet will query the runtime handlers
features and only start pods with userns if the runtime handler used for
that pod supports it.

Let's expose the user namespace support to the kubelet.

Signed-off-by: Rodrigo Campos <rodrigoca@microsoft.com>

Author: rata

internal/cri/server/service.go

@@ -381,6 +381,9 @@ func (c *criService) introspectRuntimeHandlers(ctx context.Context) ([]*runtime.
 					log.G(ctx).Debugf("runtime %q supports recursive read-only mounts, but the kernel does not", name)
 				}
 			}
+			userns := supportsCRIUserns(rawFeatures)
+			h.Features.UserNamespaces = userns
+			log.G(ctx).Debugf("runtime %q supports CRI userns: %v", name, userns)
 		}
 		res = append(res, &h)
 		if name == c.config.DefaultRuntimeName {
@@ -438,3 +441,20 @@ func introspectRuntimeFeatures(ctx context.Context, intro introspection.Service,
 	}
 	return features, nil
 }
+
+func supportsCRIUserns(f *features.Features) bool {
+	if f == nil {
+		return false
+	}
+	userns := slices.Contains(f.Linux.Namespaces, "user")
+
+	var idmap bool
+	if m := f.Linux.MountExtensions; m != nil && m.IDMap != nil && m.IDMap.Enabled != nil {
+		if *m.IDMap.Enabled {
+			idmap = true
+		}
+	}
+
+	// user namespace support in CRI requires userns and idmap support.
+	return userns && idmap
+}