Kernel: Allow calling sys$waitid on traced, non-child processes

Previously, attempting to call sys$waitid on non-child processes
returned ECHILD.

That prevented debugging non-child processes by attaching to them during
runtime (as opposed to forking and debugging the child, which is what
was previously supported).

We now allow calling sys$waitid on a any process that is being traced
by us, even if it's not our child.

1 files changed, 5 insertions, 2 deletions

diff --git a/Kernel/Syscalls/waitid.cpp b/Kernel/Syscalls/waitid.cpp
index de0948745e..f94025d917 100644
--- a/Kernel/Syscalls/waitid.cpp
+++ b/Kernel/Syscalls/waitid.cpp
@@ -32,9 +32,12 @@ KResultOr<FlatPtr> Process::sys$waitid(Userspace<const Syscall::SC_waitid_params
         break;
     case P_PID: {
         auto waitee_process = Process::from_pid(params.id);
-        if (!waitee_process || waitee_process->ppid() != Process::current().pid()) {
+        if (!waitee_process)
+            return ECHILD;
+        bool waitee_is_child = waitee_process->ppid() == Process::current().pid();
+        bool waitee_is_our_tracee = waitee_process->has_tracee_thread(Process::current().pid());
+        if (!waitee_is_child && !waitee_is_our_tracee)
             return ECHILD;
-        }
         waitee = waitee_process.release_nonnull();
         break;
     }