irda: small read past the end of array in debug code

The "reason" can come from skb->data[] and it hasn't been capped so it
can be from 0-255 instead of just 0-6.  For example in irlmp_state_dtr()
the code does:

	reason = skb->data[3];
	...
	irlmp_disconnect_indication(self, reason, skb);

Also LMREASON has a couple other values which don't have entries in the
irlmp_reasons[] array.  And 0xff is a valid reason as well which means
"unknown".

So far as I can see we don't actually care about "reason" except for in
the debug code.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/irda/irlmp.h

@@ -256,7 +256,8 @@ static inline __u32 irlmp_get_daddr(const struct lsap_cb *self)
 	return (self && self->lap) ? self->lap->daddr : 0;
 }
 
-extern const char *irlmp_reasons[];
+const char *irlmp_reason_str(LM_REASON reason);
+
 extern int sysctl_discovery_timeout;
 extern int sysctl_discovery_slots;
 extern int sysctl_discovery;

net/irda/iriap.c

@@ -303,7 +303,8 @@ static void iriap_disconnect_indication(void *instance, void *sap,
 {
 	struct iriap_cb *self;
 
-	IRDA_DEBUG(4, "%s(), reason=%s\n", __func__, irlmp_reasons[reason]);
+	IRDA_DEBUG(4, "%s(), reason=%s [%d]\n", __func__,
+		   irlmp_reason_str(reason), reason);
 
 	self = instance;
 

net/irda/irlmp.c

@@ -66,8 +66,15 @@ const char *irlmp_reasons[] = {
 	"LM_LAP_RESET",
 	"LM_INIT_DISCONNECT",
 	"ERROR, NOT USED",
+	"UNKNOWN",
 };
 
+const char *irlmp_reason_str(LM_REASON reason)
+{
+	reason = min_t(size_t, reason, ARRAY_SIZE(irlmp_reasons) - 1);
+	return irlmp_reasons[reason];
+}
+
 /*
  * Function irlmp_init (void)
  *
@@ -747,7 +754,8 @@ void irlmp_disconnect_indication(struct lsap_cb *self, LM_REASON reason,
 {
 	struct lsap_cb *lsap;
 
-	IRDA_DEBUG(1, "%s(), reason=%s\n", __func__, irlmp_reasons[reason]);
+	IRDA_DEBUG(1, "%s(), reason=%s [%d]\n", __func__,
+		   irlmp_reason_str(reason), reason);
 	IRDA_ASSERT(self != NULL, return;);
 	IRDA_ASSERT(self->magic == LMP_LSAP_MAGIC, return;);