Merge pull request tock#4053 from tock/ipc-no-double-entry

IPC: Avoid double grant entry

kernel/src/ipc.rs

@@ -73,6 +73,13 @@ impl<const NUM_PROCS: u8> IPC<NUM_PROCS> {
     ) -> Result<(), process::Error> {
         let schedule_on_id = schedule_on.index().ok_or(process::Error::NoSuchApp)?;
         let called_from_id = called_from.index().ok_or(process::Error::NoSuchApp)?;
+
+        // Verify that IPC is not trying to share with the same app. If so, this
+        // will cause a double grant enter if we don't return now.
+        if schedule_on_id == called_from_id {
+            return Err(process::Error::AlreadyInUse);
+        }
+
         self.data.enter(schedule_on, |_, schedule_on_data| {
             self.data.enter(called_from, |_, called_from_data| {
                 // If the other app shared a buffer with us, make