diff --git a/libcontainer/userns/userns.go b/libcontainer/userns/userns.go
index b225f18f2..a07afe07b 100644
--- a/libcontainer/userns/userns.go
+++ b/libcontainer/userns/userns.go
@@ -1,4 +1,8 @@
 package userns
 
-// RunningInUserNS detects whether we are currently running in a user namespace.
-var RunningInUserNS = runningInUserNS
+// RunningInUserNS detects whether we are currently running in a Linux
+// user namespace and memoizes the result. It returns false on non-Linux
+// platforms.
+func RunningInUserNS() bool {
+	return inUserNS()
+}
diff --git a/libcontainer/userns/userns_linux.go b/libcontainer/userns/userns_linux.go
index a6710b321..a1462e13b 100644
--- a/libcontainer/userns/userns_linux.go
+++ b/libcontainer/userns/userns_linux.go
@@ -7,32 +7,26 @@ import (
 	"sync"
 )
 
-var (
-	inUserNS bool
-	nsOnce   sync.Once
-)
+var inUserNS = sync.OnceValue(runningInUserNS)
 
 // runningInUserNS detects whether we are currently running in a user namespace.
 //
 // Originally copied from https://github.com/lxc/incus/blob/e45085dd42f826b3c8c3228e9733c0b6f998eafe/shared/util.go#L678-L700.
 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()
+	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
-		}
+	buf := bufio.NewReader(file)
+	l, _, err := buf.ReadLine()
+	if err != nil {
+		return false
+	}
 
-		inUserNS = uidMapInUserNS(string(l))
-	})
-	return inUserNS
+	return uidMapInUserNS(string(l))
 }
 
 func uidMapInUserNS(uidMap string) bool {
diff --git a/libcontainer/userns/userns_fuzzer.go b/libcontainer/userns/userns_linux_fuzzer.go
similarity index 79%
rename from libcontainer/userns/userns_fuzzer.go
rename to libcontainer/userns/userns_linux_fuzzer.go
index d90cffcf4..26ba2e16e 100644
--- a/libcontainer/userns/userns_fuzzer.go
+++ b/libcontainer/userns/userns_linux_fuzzer.go
@@ -1,4 +1,4 @@
-//go:build gofuzz
+//go:build linux && gofuzz
 
 package userns
 
diff --git a/libcontainer/userns/userns_unsupported.go b/libcontainer/userns/userns_unsupported.go
index 71d4c6353..8ed83072c 100644
--- a/libcontainer/userns/userns_unsupported.go
+++ b/libcontainer/userns/userns_unsupported.go
@@ -2,14 +2,5 @@
 
 package userns
 
-// runningInUserNS is a stub for non-Linux systems
-// Always returns false
-func runningInUserNS() bool {
-	return false
-}
-
-// uidMapInUserNS is a stub for non-Linux systems
-// Always returns false
-func uidMapInUserNS(uidMap string) bool {
-	return false
-}
+// inUserNS is a stub for non-Linux systems. Always returns false.
+func inUserNS() bool { return false }