syscall: use clone3 syscall with CLONE_NEWTIME

CLONE_NEWTIME can only be used with the clone3 and unshare system calls,
see torvalds/linux@769071a:

> All available clone flags have been used, so CLONE_NEWTIME uses the highest
> bit of CSIGNAL. It means that it can be used only with the unshare() and
> the clone3() system calls.

The clone3 syscall was added in Linux kernel version 5.3 and
CLONE_NEWTIME was added in version 5.6. However, it was non-functional
until version 6.3 (and stable versions with the corresponding fix [1]).

[1] https://lore.kernel.org/lkml/20230308105126.10107-1-tklauser@distanz.ch/

In case CLONE_NEWTIME is set in SysProcAttr.Cloneflags on an unsupported
kernel version, the fork/exec call will fail.

Fixes golang#49779

Change-Id: Ic3ecfc2b601bafaab12b1805d7f9512955a8c7e2
Reviewed-on: https://go-review.googlesource.com/c/go/+/474356
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/syscall/exec_linux.go

@@ -293,6 +293,11 @@ func forkAndExecInChild1(argv0 *byte, argv, envv []*byte, chroot, dir *byte, att
 			exitSignal: uint64(SIGCHLD),
 			cgroup:     uint64(sys.CgroupFD),
 		}
+	} else if flags&CLONE_NEWTIME != 0 {
+		clone3 = &cloneArgs{
+			flags:      uint64(flags),
+			exitSignal: uint64(SIGCHLD),
+		}
 	}
 
 	// About to call fork.

src/syscall/exec_linux_test.go

@@ -487,6 +487,52 @@ func TestUseCgroupFDHelper(*testing.T) {
 	fmt.Print(string(selfCg))
 }
 
+func TestCloneTimeNamespace(t *testing.T) {
+	testenv.MustHaveExec(t)
+
+	if os.Getenv("GO_WANT_HELPER_PROCESS") == "1" {
+		timens, err := os.Readlink("/proc/self/ns/time")
+		if err != nil {
+			fmt.Fprintln(os.Stderr, err)
+			os.Exit(2)
+		}
+		fmt.Print(string(timens))
+		os.Exit(0)
+	}
+
+	exe, err := os.Executable()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	cmd := testenv.Command(t, exe, "-test.run=TestCloneTimeNamespace")
+	cmd.Env = append(cmd.Environ(), "GO_WANT_HELPER_PROCESS=1")
+	cmd.SysProcAttr = &syscall.SysProcAttr{
+		Cloneflags: syscall.CLONE_NEWTIME,
+	}
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		if isNotSupported(err) {
+			// CLONE_NEWTIME does not appear to be supported.
+			t.Skipf("skipping, CLONE_NEWTIME not supported: %v", err)
+		}
+		t.Fatalf("Cmd failed with err %v, output: %s", err, out)
+	}
+
+	// Inode numer of the time namespaces should be different.
+	// Based on https://man7.org/linux/man-pages/man7/time_namespaces.7.html#EXAMPLES
+	timens, err := os.Readlink("/proc/self/ns/time")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	parentTimeNS := string(timens)
+	childTimeNS := string(out)
+	if childTimeNS == parentTimeNS {
+		t.Fatalf("expected child time namespace to be different from parent time namespace: %s", parentTimeNS)
+	}
+}
+
 type capHeader struct {
 	version uint32
 	pid     int32