Kernel: Do a bit more of do_exec() before disabling interrupts.

We definitely need to replace m_executable before clearing interrupts, since
otherwise we might call ~Custody() which would make it assert in locking.
Also avoid calling FileDescriptor::metadata() repeatedly and just cache the
result from the first call.

I also added a comment at the point where we've decided to commit to the new
executable and follow through with the swap.

Kernel/Process.cpp

@@ -315,13 +315,13 @@ int Process::do_exec(String path, Vector<String> arguments, Vector<String> envir
  if (result.is_error())
  return result.error();
  auto descriptor = result.value();
+ auto metadata = descriptor->metadata();
 
- if (!descriptor->metadata().may_execute(m_euid, m_gids))
+ if (!metadata.may_execute(m_euid, m_gids))
  return -EACCES;
 
- if (!descriptor->metadata().size) {
+ if (!metadata.size)
  return -ENOTIMPL;
- }
 
  dword entry_eip = 0;
  // FIXME: Is there a race here?
@@ -339,7 +339,7 @@ int Process::do_exec(String path, Vector<String> arguments, Vector<String> envir
 #else
  vmo->set_name("ELF image");
 #endif
- RetainPtr<Region> region = allocate_region_with_vmo(LinearAddress(), descriptor->metadata().size, vmo.copy_ref(), 0, "executable", PROT_READ);
+ RetainPtr<Region> region = allocate_region_with_vmo(LinearAddress(), metadata.size, vmo.copy_ref(), 0, "executable", PROT_READ);
  ASSERT(region);
 
  if (this != &current->process()) {
@@ -388,10 +388,17 @@ int Process::do_exec(String path, Vector<String> arguments, Vector<String> envir
  return -ENOEXEC;
  }
 
+ // NOTE: At this point, we've committed to the new executable.
  entry_eip = loader->entry().get();
  }
 
  m_elf_loader = move(loader);
+ m_executable = descriptor->custody();
+
+ if (metadata.is_setuid())
+ m_euid = metadata.uid;
+ if (metadata.is_setgid())
+ m_egid = metadata.gid;
 
  current->m_kernel_stack_for_signal_handler_region = nullptr;
  current->m_signal_stack_user_region = nullptr;
@@ -435,13 +442,6 @@ int Process::do_exec(String path, Vector<String> arguments, Vector<String> envir
  main_thread().m_tss.esp0 = old_esp0;
  main_thread().m_tss.ss2 = m_pid;
 
- m_executable = descriptor->custody();
-
- if (descriptor->metadata().is_setuid())
- m_euid = descriptor->metadata().uid;
- if (descriptor->metadata().is_setgid())
- m_egid = descriptor->metadata().gid;
-
 #ifdef TASK_DEBUG
  kprintf("Process %u (%s) exec'd %s @ %p\n", pid(), name().characters(), path.characters(), main_thread().tss().eip);
 #endif