syscall: don't call Setgroups if Credential.Groups is empty

Setgroups with zero-length groups is no-op for changing groups and
supposed to be used only for determining curent groups length. Also
because we deny setgroups by default if use GidMappings we have
unnecessary error from that no-op syscall.

Change-Id: I8f74fbca9190a3dcbbef1d886c518e01fa05eb62
Reviewed-on: https://go-review.googlesource.com/13938
Reviewed-by: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: LK4D4

src/syscall/exec_linux.go

@@ -191,13 +191,12 @@ 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))
-		var groups unsafe.Pointer
 		if ngroups > 0 {
-			groups = unsafe.Pointer(&cred.Groups[0])
-		}
-		_, _, err1 = RawSyscall(SYS_SETGROUPS, ngroups, uintptr(groups), 0)
-		if err1 != 0 {
-			goto childerror
+			groups := unsafe.Pointer(&cred.Groups[0])
+			_, _, err1 = RawSyscall(SYS_SETGROUPS, ngroups, uintptr(groups), 0)
+			if err1 != 0 {
+				goto childerror
+			}
 		}
 		_, _, err1 = RawSyscall(SYS_SETGID, uintptr(cred.Gid), 0, 0)
 		if err1 != 0 {

src/syscall/exec_linux_test.go

@@ -109,3 +109,11 @@ func TestCloneNEWUSERAndRemapNoRootSetgroupsEnableSetgroups(t *testing.T) {
 		t.Fatalf("Unprivileged gid_map rewriting with GidMappingsEnableSetgroups must fail")
 	}
 }
+
+func TestEmptyCredGroupsDisableSetgroups(t *testing.T) {
+	cmd := whoamiCmd(t, os.Getuid(), os.Getgid(), false)
+	cmd.SysProcAttr.Credential = &syscall.Credential{}
+	if err := cmd.Run(); err != nil {
+		t.Fatal(err)
+	}
+}