Kernel: Fix behaviour of PT_TRACEME in ptrace

The behaviour of the PT_TRACEME feature has been broken for some time,
this change fixes it.

When this ptrace flag is used, the traced process should be paused
before exiting execve.
We previously were sending the SIGSTOP signal at a stage where
interrupts are disabled, and the traced process continued executing
normally, without pausing and waiting for the tracer.
This change fixes it.

Kernel/Arch/i386/CPU.cpp

@@ -1190,10 +1190,6 @@ extern "C" void context_first_init(Thread* from_thread, Thread* to_thread, TrapF
 
     Scheduler::enter_current(*from_thread);
 
-    if (to_thread->process().wait_for_tracer_at_next_execve()) {
-        to_thread->send_urgent_signal_to_self(SIGSTOP);
-    }
-
     // Since we got here and don't have Scheduler::context_switch in the
     // call stack (because this is the first time we switched into this
     // context), we need to notify the scheduler so that it can release

Kernel/Syscalls/execve.cpp

@@ -269,6 +269,9 @@ int Process::do_exec(NonnullRefPtr<FileDescription> main_program_description, Ve
     //       and we don't want to deal with faults after this point.
     u32 new_userspace_esp = new_main_thread->make_userspace_stack_for_main_thread(move(arguments), move(environment), move(auxv));
 
+    if (wait_for_tracer_at_next_execve())
+        Thread::current()->send_urgent_signal_to_self(SIGSTOP);
+
     // We enter a critical section here because we don't want to get interrupted between do_exec()
     // and Processor::assume_context() or the next context switch.
     // If we used an InterruptDisabler that sti()'d on exit, we might timer tick'd too soon in exec().
@@ -557,7 +560,7 @@ int Process::sys$execve(Userspace<const Syscall::SC_execve_params*> user_params)
     if (params.arguments.length > ARG_MAX || params.environment.length > ARG_MAX)
         return -E2BIG;
 
-    if (m_wait_for_tracer_at_next_execve)
+    if (wait_for_tracer_at_next_execve())
         Thread::current()->send_urgent_signal_to_self(SIGSTOP);
 
     String path;
@@ -597,5 +600,4 @@ int Process::sys$execve(Userspace<const Syscall::SC_execve_params*> user_params)
     ASSERT(rc < 0); // We should never continue after a successful exec!
     return rc;
 }
-
 }