Make binary file and directory name consistent

This change make binary file and directory name consistent.
In addition, change the package name cni to server because cni
is a bit umbiguous for cni plugin's repository.

pkg/server/socket.go

@@ -0,0 +1,69 @@
+// Copyright (c) 2021 Multus Authors
+//
+// 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 server
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"syscall"
+)
+
+const (
+	serverSocketName                   = "multus-cni.sock"
+	fullReadWriteExecutePermissions    = 0777
+	thickPluginSocketRunDirPermissions = 0700
+)
+
+// FilesystemPreRequirements ensures the target `rundir` features the correct
+// permissions.
+func FilesystemPreRequirements(rundir string) error {
+	socketpath := SocketPath(rundir)
+	if err := os.RemoveAll(rundir); err != nil && !os.IsNotExist(err) {
+		info, err := os.Stat(rundir)
+		if err != nil {
+			return fmt.Errorf("failed to stat old pod info socket directory %s: %v", rundir, err)
+		}
+		// Owner must be root
+		tmp := info.Sys()
+		statt, ok := tmp.(*syscall.Stat_t)
+		if !ok {
+			return fmt.Errorf("failed to read pod info socket directory stat info: %T", tmp)
+		}
+		if statt.Uid != 0 {
+			return fmt.Errorf("insecure owner of pod info socket directory %s: %v", rundir, statt.Uid)
+		}
+
+		// Check permissions
+		if info.Mode()&fullReadWriteExecutePermissions != thickPluginSocketRunDirPermissions {
+			return fmt.Errorf("insecure permissions on pod info socket directory %s: %v", rundir, info.Mode())
+		}
+
+		// Finally remove the socket file so we can re-create it
+		if err := os.Remove(socketpath); err != nil && !os.IsNotExist(err) {
+			return fmt.Errorf("failed to remove old pod info socket %s: %v", socketpath, err)
+		}
+	}
+	if err := os.MkdirAll(rundir, thickPluginSocketRunDirPermissions); err != nil {
+		return fmt.Errorf("failed to create pod info socket directory %s: %v", rundir, err)
+	}
+	return nil
+}
+
+// SocketPath returns the path of the multus CNI socket
+func SocketPath(rundir string) string {
+	return filepath.Join(rundir, serverSocketName)
+}