diff --git a/userns/userns_linux.go b/userns/userns_linux.go
index 87c1c38e..bfd0f768 100644
--- a/userns/userns_linux.go
+++ b/userns/userns_linux.go
@@ -5,8 +5,12 @@ import (
 	"fmt"
 	"os"
 	"sync"
+	"syscall"
 )
 
+// See PROC_USER_INIT_INO in https://github.com/torvalds/linux/blob/v7.1/include/uapi/linux/nsfs.h#L50.
+const procUserInitIno = 0xEFFFFFFD
+
 var inUserNS = sync.OnceValue(runningInUserNS)
 
 // runningInUserNS detects whether we are currently running in a user namespace.
@@ -17,6 +21,11 @@ var inUserNS = sync.OnceValue(runningInUserNS)
 // [libcontainer/runc]: https://github.com/opencontainers/runc/blob/3778ae603c706494fd1e2c2faf83b406e38d687d/libcontainer/userns/userns_linux.go#L12-L49
 // [lcx/incus]: https://github.com/lxc/incus/blob/e45085dd42f826b3c8c3228e9733c0b6f998eafe/shared/util.go#L678-L700
 func runningInUserNS() bool {
+	var st syscall.Stat_t
+	if err := syscall.Stat("/proc/self/ns/user", &st); err == nil {
+		return st.Ino != procUserInitIno
+	}
+
 	file, err := os.Open("/proc/self/uid_map")
 	if err != nil {
 		// This kernel-provided file only exists if user namespaces are supported.
@@ -30,12 +39,9 @@ func runningInUserNS() bool {
 		return false
 	}
 
-	return uidMapInUserNS(string(l))
-}
-
-func uidMapInUserNS(uidMap string) bool {
+	uidMap := string(l)
 	if uidMap == "" {
-		// File exist but empty (the initial state when userns is created,
+		// File exists but is empty (the initial state when userns is created,
 		// see user_namespaces(7)).
 		return true
 	}
diff --git a/userns/userns_linux_test.go b/userns/userns_linux_test.go
deleted file mode 100644
index 25c4ac30..00000000
--- a/userns/userns_linux_test.go
+++ /dev/null
@@ -1,34 +0,0 @@
-package userns
-
-import "testing"
-
-func TestUIDMapInUserNS(t *testing.T) {
-	cases := []struct {
-		s        string
-		expected bool
-	}{
-		{
-			s:        "         0          0 4294967295\n",
-			expected: false,
-		},
-		{
-			s:        "         0          0          1\n",
-			expected: true,
-		},
-		{
-			s:        "         0       1001          1\n         1     231072      65536\n",
-			expected: true,
-		},
-		{
-			// file exist but empty (the initial state when userns is created. see man 7 user_namespaces)
-			s:        "",
-			expected: true,
-		},
-	}
-	for _, c := range cases {
-		actual := uidMapInUserNS(c.s)
-		if c.expected != actual {
-			t.Fatalf("expected %v, got %v for %q", c.expected, actual, c.s)
-		}
-	}
-}