Kernel: Allow moving a process to a new pgrp via setpgid()

Kernel/Syscalls/setpgid.cpp

@@ -124,10 +124,15 @@ int Process::sys$setpgid(pid_t specified_pid, pid_t specified_pgid)
  ProcessGroupID new_pgid = specified_pgid ? ProcessGroupID(specified_pgid) : process->m_pid.value();
  SessionID current_sid = sid();
  SessionID new_sid = get_sid_from_pgid(new_pgid);
- if (current_sid != new_sid) {
+ if (new_sid != -1 && current_sid != new_sid) {
  // Can't move a process between sessions.
  return -EPERM;
  }
+ if (new_sid == -1 && new_pgid != process->m_pid.value()) {
+ // The value of the pgid argument is valid, but is not
+ // the calling pid, and is not an existing process group.
+ return -EPERM;
+ }
  // FIXME: There are more EPERM conditions to check for here..
  process->m_pgid = new_pgid;
  return 0;