Move RunningInUserNS() to its own package

This allows using the utility without bringing whole of "sys" with it.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

pkg/userns/userns_linux.go

@@ -0,0 +1,62 @@
+/*
+   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 userns
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"sync"
+)
+
+var (
+	inUserNS bool
+	nsOnce   sync.Once
+)
+
+// RunningInUserNS detects whether we are currently running in a user namespace.
+// Originally copied from github.com/lxc/lxd/shared/util.go
+func RunningInUserNS() bool {
+	nsOnce.Do(func() {
+		file, err := os.Open("/proc/self/uid_map")
+		if err != nil {
+			// This kernel-provided file only exists if user namespaces are supported
+			return
+		}
+		defer file.Close()
+
+		buf := bufio.NewReader(file)
+		l, _, err := buf.ReadLine()
+		if err != nil {
+			return
+		}
+
+		line := string(l)
+		var a, b, c int64
+		fmt.Sscanf(line, "%d %d %d", &a, &b, &c)
+
+		/*
+		 * We assume we are in the initial user namespace if we have a full
+		 * range - 4294967295 uids starting at uid 0.
+		 */
+		if a == 0 && b == 0 && c == 4294967295 {
+			return
+		}
+		inUserNS = true
+	})
+	return inUserNS
+}