Kernel: Hoist allocation of main program FD in sys$execve()

When executing a dynamically linked program, we need to pass the main
program executable via a file descriptor to the dynamic loader.

Before this patch, we were allocating an FD for this purpose long after
it was safe to do anything fallible. If we were unable to allocate an
FD we would simply panic the kernel(!)

We now hoist the allocation so it can fail before we've committed to
a new executable.

Kernel/Syscalls/execve.cpp

@@ -40,7 +40,7 @@ struct LoadResult {
  WeakPtr<Memory::Region> stack_region;
 };
 
-static Vector<ELF::AuxiliaryValue> generate_auxiliary_vector(FlatPtr load_base, FlatPtr entry_eip, UserID uid, UserID euid, GroupID gid, GroupID egid, String executable_path, int main_program_fd);
+static Vector<ELF::AuxiliaryValue> generate_auxiliary_vector(FlatPtr load_base, FlatPtr entry_eip, UserID uid, UserID euid, GroupID gid, GroupID egid, String executable_path, Optional<Process::ScopedDescriptionAllocation> const& main_program_fd_allocation);
 
 static bool validate_stack_size(const Vector<String>& arguments, const Vector<String>& environment)
 {
@@ -461,6 +461,11 @@ KResult Process::do_exec(NonnullRefPtr<FileDescription> main_program_description
  auto signal_trampoline_region = TRY(load_result.space->allocate_region_with_vmobject(signal_trampoline_range, g_signal_trampoline_region->vmobject(), 0, "Signal trampoline", PROT_READ | PROT_EXEC, true));
  signal_trampoline_region->set_syscall_region(true);
 
+ // (For dynamically linked executable) Allocate an FD for passing the main executable to the dynamic loader.
+ Optional<ScopedDescriptionAllocation> main_program_fd_allocation;
+ if (interpreter_description)
+ main_program_fd_allocation = TRY(m_fds.allocate());
+
  // We commit to the new executable at this point. There is no turning back!
 
  // Prevent other processes from attaching to us with ptrace while we're doing this.
@@ -521,15 +526,11 @@ KResult Process::do_exec(NonnullRefPtr<FileDescription> main_program_description
  file_description_metadata = {};
  });
 
- int main_program_fd = -1;
- if (interpreter_description) {
- auto main_program_fd_wrapper = m_fds.allocate().release_value();
- VERIFY(main_program_fd_wrapper.fd >= 0);
+ if (main_program_fd_allocation.has_value()) {
  auto seek_result = main_program_description->seek(0, SEEK_SET);
  VERIFY(!seek_result.is_error());
  main_program_description->set_readable(true);
- m_fds[main_program_fd_wrapper.fd].set(move(main_program_description), FD_CLOEXEC);
- main_program_fd = main_program_fd_wrapper.fd;
+ m_fds[main_program_fd_allocation->fd].set(move(main_program_description), FD_CLOEXEC);
  }
 
  new_main_thread = nullptr;
@@ -543,7 +544,7 @@ KResult Process::do_exec(NonnullRefPtr<FileDescription> main_program_description
  }
  VERIFY(new_main_thread);
 
- auto auxv = generate_auxiliary_vector(load_result.load_base, load_result.entry_eip, uid(), euid(), gid(), egid(), path, main_program_fd);
+ auto auxv = generate_auxiliary_vector(load_result.load_base, load_result.entry_eip, uid(), euid(), gid(), egid(), path, main_program_fd_allocation);
 
  // NOTE: We create the new stack before disabling interrupts since it will zero-fault
  // and we don't want to deal with faults after this point.
@@ -627,7 +628,7 @@ KResult Process::do_exec(NonnullRefPtr<FileDescription> main_program_description
  return KSuccess;
 }
 
-static Vector<ELF::AuxiliaryValue> generate_auxiliary_vector(FlatPtr load_base, FlatPtr entry_eip, UserID uid, UserID euid, GroupID gid, GroupID egid, String executable_path, int main_program_fd)
+static Vector<ELF::AuxiliaryValue> generate_auxiliary_vector(FlatPtr load_base, FlatPtr entry_eip, UserID uid, UserID euid, GroupID gid, GroupID egid, String executable_path, Optional<Process::ScopedDescriptionAllocation> const& main_program_fd_allocation)
 {
  Vector<ELF::AuxiliaryValue> auxv;
  // PHDR/EXECFD
@@ -658,7 +659,8 @@ static Vector<ELF::AuxiliaryValue> generate_auxiliary_vector(FlatPtr load_base,
 
  auxv.append({ ELF::AuxiliaryValue::ExecFilename, executable_path });
 
- auxv.append({ ELF::AuxiliaryValue::ExecFileDescriptor, main_program_fd });
+ if (main_program_fd_allocation.has_value())
+ auxv.append({ ELF::AuxiliaryValue::ExecFileDescriptor, main_program_fd_allocation->fd });
 
  auxv.append({ ELF::AuxiliaryValue::Null, 0L });
  return auxv;