pkg/sys: add GetUsernsForNamespace interface

Signed-off-by: Wei Fu <fuweid89@gmail.com>

pkg/sys/namespace_linux.go

@@ -0,0 +1,38 @@
+/*
+   Copyright The containerd 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 sys
+
+import (
+	"fmt"
+	"os"
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+// GetUsernsForNamespace returns a file descriptor that refers to the owning
+// user namespace for the namespace referred to by fd.
+//
+// REF: https://man7.org/linux/man-pages/man2/ioctl_ns.2.html
+func GetUsernsForNamespace(fd uintptr) (*os.File, error) {
+	fd, _, errno := unix.Syscall(syscall.SYS_IOCTL, fd, uintptr(unix.NS_GET_USERNS), 0)
+	if errno != 0 {
+		return nil, fmt.Errorf("failed to get user namespace fd: %w", errno)
+	}
+
+	return os.NewFile(fd, fmt.Sprintf("/proc/%d/fd/%d", os.Getpid(), fd)), nil
+}