commit 0cc1faf015a253ef64b97a8453b6fc959c0ee512
from: Russ Cox <rsc@swtch.com>
date: Thu Jan 14 14:59:03 2021 UTC

lib9: reject postnote with special pids

commit - fdcf3d70c24886dddb5fd7052dfada67d33d5c75
commit + 0cc1faf015a253ef64b97a8453b6fc959c0ee512
blob - 68e6d2f669fc7ef180e18ccded9257ec368bedad
blob + d750c69dc68ba3543fa1210da3dcb39018df2746
--- src/lib9/postnote.c
+++ src/lib9/postnote.c
@@ -18,6 +18,11 @@ postnote(int who, int pid, char *msg)
 		return -1;
 	}
 
+	if(pid <= 0){
+		werrstr("bad pid in postnote");
+		return -1;
+	}
+
 	switch(who){
 	default:
 		werrstr("bad who in postnote");