fix: allow group and other access to Unix socket

internal/proxy/proxy.go

@@ -548,6 +548,13 @@ func newSocketMount(ctx context.Context, conf *Config, pc *portConfig, inst Inst
 	if err != nil {
 		return nil, err
 	}
+	// Change file permisions to allow access for user, group, and other.
+	if network == "unix" {
+		// Best effort. If this call fails, group and other won't have write
+		// access.
+		_ = os.Chmod(address, 0777)
+	}
+
 	m := &socketMount{inst: inst.Name, listener: ln}
 	return m, nil
 }

internal/proxy/proxy_other_test.go

@@ -0,0 +1,33 @@
+// Copyright 2022 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build !windows
+// +build !windows
+
+package proxy_test
+
+import (
+	"os"
+	"testing"
+)
+
+func verifySocketPermissions(t *testing.T, addr string) {
+	fi, err := os.Stat(addr)
+	if err != nil {
+		t.Fatalf("os.Stat(%v): %v", addr, err)
+	}
+	if fm := fi.Mode(); fm != 0777|os.ModeSocket {
+		t.Fatalf("file mode: want = %v, got = %v", 0777|os.ModeSocket, fm)
+	}
+}

internal/proxy/proxy_test.go

@@ -244,6 +244,8 @@ func TestClientInitialization(t *testing.T) {
 			}
 
 			for _, addr := range tc.wantUnixAddrs {
+				verifySocketPermissions(t, addr)
+
 				conn, err := net.Dial("unix", addr)
 				if err != nil {
 					t.Fatalf("want error = nil, got = %v", err)

internal/proxy/proxy_windows_test.go

@@ -0,0 +1,25 @@
+// Copyright 2022 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package proxy_test
+
+import (
+	"testing"
+)
+
+func verifySocketPermissions(t *testing.T, addr string) {
+	// On Linux and Darwin, we check that the socket named by addr exists with
+	// os.Stat. That operation is not supported on Windows.
+	// See https://github.com/microsoft/Windows-Containers/issues/97#issuecomment-887713195
+}