syscall: call setgroups for no groups on GNU/Linux

Skip setgroups only for one particular case: GidMappings != nil and
GidMappingsEnableSetgroup == false and list of supplementary groups is
empty.
This patch returns pre-1.5 behavior for simple exec and still allows to
use GidMappings with non-empty Credential.

Change-Id: Ia91c77e76ec5efab7a7f78134ffb529910108fc1
Reviewed-on: https://go-review.googlesource.com/23524Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/syscall/exec_linux.go

@@ -206,9 +206,15 @@ func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr
 	// User and groups
 	if cred := sys.Credential; cred != nil {
 		ngroups := uintptr(len(cred.Groups))
+		groups := uintptr(0)
 		if ngroups > 0 {
-			groups := unsafe.Pointer(&cred.Groups[0])
-			_, _, err1 = RawSyscall(SYS_SETGROUPS, ngroups, uintptr(groups), 0)
+			groups = uintptr(unsafe.Pointer(&cred.Groups[0]))
+		}
+		// Don't call setgroups in case of user namespace, gid mappings
+		// and disabled setgroups, because otherwise unprivileged user namespace
+		// will fail with any non-empty SysProcAttr.Credential.
+		if !(sys.GidMappings != nil && !sys.GidMappingsEnableSetgroups && ngroups == 0) {
+			_, _, err1 = RawSyscall(SYS_SETGROUPS, ngroups, groups, 0)
 			if err1 != 0 {
 				goto childerror
 			}

src/syscall/exec_linux_test.go

@@ -26,7 +26,7 @@ func isChrooted(t *testing.T) bool {
 	return root.Sys().(*syscall.Stat_t).Ino != 2
 }
 
-func whoamiCmd(t *testing.T, uid, gid int, setgroups bool) *exec.Cmd {
+func checkUserNS(t *testing.T) {
 	if _, err := os.Stat("/proc/self/ns/user"); err != nil {
 		if os.IsNotExist(err) {
 			t.Skip("kernel doesn't support user namespaces")
@@ -56,6 +56,10 @@ func whoamiCmd(t *testing.T, uid, gid int, setgroups bool) *exec.Cmd {
 	if os.Getenv("GO_BUILDER_NAME") != "" && os.Getenv("IN_KUBERNETES") == "1" {
 		t.Skip("skipping test on Kubernetes-based builders; see Issue 12815")
 	}
+}
+
+func whoamiCmd(t *testing.T, uid, gid int, setgroups bool) *exec.Cmd {
+	checkUserNS(t)
 	cmd := exec.Command("whoami")
 	cmd.SysProcAttr = &syscall.SysProcAttr{
 		Cloneflags: syscall.CLONE_NEWUSER,
@@ -161,3 +165,58 @@ func TestUnshare(t *testing.T) {
 		t.Fatalf("Expected 3 lines of output, got %d", len(lines))
 	}
 }
+
+func TestGroupCleanup(t *testing.T) {
+	if os.Getuid() != 0 {
+		t.Skip("we need root for credential")
+	}
+	cmd := exec.Command("id")
+	cmd.SysProcAttr = &syscall.SysProcAttr{
+		Credential: &syscall.Credential{
+			Uid: 0,
+			Gid: 0,
+		},
+	}
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("Cmd failed with err %v, output: %s", err, out)
+	}
+	strOut := strings.TrimSpace(string(out))
+	expected := "uid=0(root) gid=0(root) groups=0(root)"
+	if strOut != expected {
+		t.Fatalf("id command output: %s, expected: %s", strOut, expected)
+	}
+}
+
+func TestGroupCleanupUserNamespace(t *testing.T) {
+	if os.Getuid() != 0 {
+		t.Skip("we need root for credential")
+	}
+	checkUserNS(t)
+	cmd := exec.Command("id")
+	uid, gid := os.Getuid(), os.Getgid()
+	cmd.SysProcAttr = &syscall.SysProcAttr{
+		Cloneflags: syscall.CLONE_NEWUSER,
+		Credential: &syscall.Credential{
+			Uid: uint32(uid),
+			Gid: uint32(gid),
+		},
+		UidMappings: []syscall.SysProcIDMap{
+			{ContainerID: 0, HostID: uid, Size: 1},
+		},
+		GidMappings: []syscall.SysProcIDMap{
+			{ContainerID: 0, HostID: gid, Size: 1},
+		},
+	}
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("Cmd failed with err %v, output: %s", err, out)
+	}
+	strOut := strings.TrimSpace(string(out))
+	// there are two possible outs
+	expected1 := "uid=0(root) gid=0(root) groups=0(root)"
+	expected2 := "uid=0(root) gid=0(root) groups=0(root),65534(nobody)"
+	if strOut != expected1 && strOut != expected2 {
+		t.Fatalf("id command output: %s, expected: %s or %s", strOut, expected1, expected2)
+	}
+}