syscall: add Unshare flags to SysProcAttr on Linux

This patch adds Unshare flags to SysProcAttr for Linux systems.

Fixes #1954

Change-Id: Id819c3f92b1474e5a06dd8d55f89d74a43eb770c
Reviewed-on: https://go-review.googlesource.com/23233
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/syscall/exec_linux.go

@@ -32,6 +32,7 @@ type SysProcAttr struct {
 	Pgid        int            // Child's process group ID if Setpgid.
 	Pdeathsig   Signal         // Signal that the process will get when its parent dies (Linux only)
 	Cloneflags  uintptr        // Flags for clone calls (Linux only)
+	Unshare     uintptr        // Flags for unshare calls (Linux only)
 	UidMappings []SysProcIDMap // User ID mappings for user namespaces.
 	GidMappings []SysProcIDMap // Group ID mappings for user namespaces.
 	// GidMappingsEnableSetgroups enabling setgroups syscall.
@@ -194,6 +195,14 @@ func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr
 		}
 	}
 
+	// Unshare
+	if sys.Unshare != 0 {
+		_, _, err1 = RawSyscall(SYS_UNSHARE, sys.Unshare, 0, 0)
+		if err1 != 0 {
+			goto childerror
+		}
+	}
+
 	// User and groups
 	if cred := sys.Credential; cred != nil {
 		ngroups := uintptr(len(cred.Groups))

src/syscall/exec_linux_test.go

@@ -125,3 +125,39 @@ func TestEmptyCredGroupsDisableSetgroups(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func TestUnshare(t *testing.T) {
+	// Make sure we are running as root so we have permissions to use unshare
+	// and create a network namespace.
+	if os.Getuid() != 0 {
+		t.Skip("kernel prohibits unshare in unprivileged process, unless using user namespace")
+	}
+
+	// When running under the Go continuous build, skip tests for
+	// now when under Kubernetes. (where things are root but not quite)
+	// Both of these are our own environment variables.
+	// See Issue 12815.
+	if os.Getenv("GO_BUILDER_NAME") != "" && os.Getenv("IN_KUBERNETES") == "1" {
+		t.Skip("skipping test on Kubernetes-based builders; see Issue 12815")
+	}
+
+	cmd := exec.Command("ip", "a")
+	cmd.SysProcAttr = &syscall.SysProcAttr{
+		Unshare: syscall.CLONE_NEWNET,
+	}
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("Cmd failed with err %v, output: %s", err, out)
+	}
+
+	// Check there is only the local network interface
+	sout := strings.TrimSpace(string(out))
+	if !strings.Contains(sout, "lo") {
+		t.Fatalf("Expected lo network interface to exist, got %s", sout)
+	}
+
+	lines := strings.Split(sout, "\n")
+	if len(lines) != 2 {
+		t.Fatalf("Expected 2 lines of output, got %d", len(lines))
+	}
+}