kernel: nil check, exited procs handling in postnote()

make sure not to dereference Proc* nil pointer. this can potentially
happen from devip which has code like:

	if(er->read4p)
		postnote(er->read4p, 1, "unbind", 0);

the process it is about to kill can zero er->read4p at any time,
so there is the possibility of the condition to be true and then
er->read4p becoming nil.

check if the process has already exited (p->pid == 0) in postnote()
under p->debug qlock.

1 files changed, 9 insertions, 0 deletions

diff --git a/sys/src/9/port/proc.c b/sys/src/9/port/proc.c
index 41cfb9aff..911bfd608 100644
--- a/sys/src/9/port/proc.c
+++ b/sys/src/9/port/proc.c
@@ -903,9 +903,18 @@ postnote(Proc *p, int dolock, char *n, int flag)
 	int s, ret;
 	QLock *q;
 
+	if(p == nil)
+		return 0;
+
 	if(dolock)
 		qlock(&p->debug);
 
+	if(p->pid == 0){
+		if(dolock)
+			qunlock(&p->debug);
+		return 0;
+	}
+
 	if(n != nil && flag != NUser && (p->notify == 0 || p->notified))
 		p->nnote = 0;