sys: simplify RunningInUserNS to original implementation

Given that we're only interested in detecting if userns is
enabled, and no further details about the mapping, we can
revert this function to go back to its original implementation
in github.com/lxc/lxd/shared/util.go

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

sys/userns_linux.go

@@ -17,22 +17,36 @@
 package sys
 
 import (
-	"github.com/opencontainers/runc/libcontainer/user"
+	"bufio"
+	"fmt"
+	"os"
 )
 
 // RunningInUserNS detects whether we are currently running in a user namespace.
 // Originally copied from github.com/lxc/lxd/shared/util.go
 func RunningInUserNS() bool {
-	uidmap, err := user.CurrentProcessUIDMap()
+	file, err := os.Open("/proc/self/uid_map")
 	if err != nil {
 		// This kernel-provided file only exists if user namespaces are supported
 		return false
 	}
+	defer file.Close()
+
+	buf := bufio.NewReader(file)
+	l, _, err := buf.ReadLine()
+	if err != nil {
+		return false
+	}
+
+	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 len(uidmap) == 1 && uidmap[0].ID == 0 && uidmap[0].ParentID == 0 && uidmap[0].Count == 4294967295 {
+	if a == 0 && b == 0 && c == 4294967295 {
 		return false
 	}
 	return true