syscall: skip non-root user namespace test if kernel forbids

The unprivileged_userns_clone sysctl prevents unpriviledged users from
creating namespaces, which the AmbientCaps test does. It's set to 0 by
default in a few Linux distributions (Debian and Arch, possibly
others), so we need to check it before running the test.

I've verified that setting

  echo 1 > /proc/sys/kernel/unprivileged_userns_clone

and then running the test *without this patch* makes it pass, which
proves that checking unprivileged_userns_clone is indeed sufficient.

Fixes #30698

Change-Id: Ib2079b5e714d7f2440ddf979c3e7cfda9a9c5005
Reviewed-on: https://go-review.googlesource.com/c/go/+/166460Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/syscall/exec_linux_test.go

@@ -539,6 +539,13 @@ func testAmbientCaps(t *testing.T, userns bool) {
 		t.Skip("skipping test on Kubernetes-based builders; see Issue 12815")
 	}
 
+	// Skip the test if the sysctl that prevents unprivileged user
+	// from creating user namespaces is enabled.
+	data, errRead := ioutil.ReadFile("/proc/sys/kernel/unprivileged_userns_clone")
+	if errRead == nil && data[0] == '0' {
+		t.Skip("kernel prohibits user namespace in unprivileged process")
+	}
+
 	// skip on android, due to lack of lookup support
 	if runtime.GOOS == "android" {
 		t.Skip("skipping test on android; see Issue 27327")