syscall: move ptrace setup to right before exec

This will prevent any ptrace calls from getting trace output from the runtime
itself setting up after fork.

Fixes #21428.

Change-Id: I9d835bd5a8f404394eb6237679f2111a72e5bc17
Reviewed-on: https://go-review.googlesource.com/55811
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Avelino <t@avelino.xxx>

src/syscall/exec_linux.go

@@ -199,14 +199,6 @@ func forkAndExecInChild1(argv0 *byte, argv, envv []*byte, chroot, dir *byte, att
 		}
 	}
 
-	// Enable tracing if requested.
-	if sys.Ptrace {
-		_, _, err1 = RawSyscall(SYS_PTRACE, uintptr(PTRACE_TRACEME), 0, 0)
-		if err1 != 0 {
-			goto childerror
-		}
-	}
-
 	// Session ID
 	if sys.Setsid {
 		_, _, err1 = RawSyscall(SYS_SETSID, 0, 0, 0)
@@ -402,6 +394,16 @@ func forkAndExecInChild1(argv0 *byte, argv, envv []*byte, chroot, dir *byte, att
 		}
 	}
 
+	// Enable tracing if requested.
+	// Do this right before exec so that we don't unnecessarily trace the runtime
+	// setting up after the fork. See issue #21428.
+	if sys.Ptrace {
+		_, _, err1 = RawSyscall(SYS_PTRACE, uintptr(PTRACE_TRACEME), 0, 0)
+		if err1 != 0 {
+			goto childerror
+		}
+	}
+
 	// Time to exec.
 	_, _, err1 = RawSyscall(SYS_EXECVE,
 		uintptr(unsafe.Pointer(argv0)),