[release-branch.go1.22] syscall: skip TestAmbientCapsUserns when restricted, document

For #67088
Fixes #69366

Change-Id: I42e7a8d02b161187772f147e3e136ab6e0f71d7f
Reviewed-on: https://go-review.googlesource.com/c/go/+/585059
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
(cherry picked from commit d05af62695)
Reviewed-on: https://go-review.googlesource.com/c/go/+/612475

src/syscall/exec_linux.go

@@ -53,6 +53,10 @@ const (
 
 // SysProcIDMap holds Container ID to Host ID mappings used for User Namespaces in Linux.
 // See user_namespaces(7).
+//
+// Note that User Namespaces are not available on a number of popular Linux
+// versions (due to security issues), or are available but subject to AppArmor
+// restrictions like in Ubuntu 24.04.
 type SysProcIDMap struct {
 	ContainerID int // Container ID.
 	HostID      int // Host ID.

src/syscall/exec_linux_test.go

@@ -642,6 +642,10 @@ func TestAmbientCaps(t *testing.T) {
 }
 
 func TestAmbientCapsUserns(t *testing.T) {
+	b, err := os.ReadFile("/proc/sys/kernel/apparmor_restrict_unprivileged_userns")
+	if err == nil && strings.TrimSpace(string(b)) == "1" {
+		t.Skip("AppArmor restriction for unprivileged user namespaces is enabled")
+	}
 	testAmbientCaps(t, true)
 }