From 9e179d46bf954c93ba0b23922d40457e57d01325 Mon Sep 17 00:00:00 2001
From: Oliver Jowett <oliver@mutability.co.uk>
Date: Wed, 14 Sep 2016 16:11:31 +0100
Subject: [PATCH] Don't accept all-zeros messages (e.g. off the network - the
 demodulator doesn't accept them already)

---
 mode_s.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mode_s.c b/mode_s.c
index 94ec7af..c28fce4 100644
--- a/mode_s.c
+++ b/mode_s.c
@@ -307,12 +307,12 @@ static inline  __attribute__((always_inline)) unsigned getbits(unsigned char *da
 //   -1: message might be valid, but we couldn't validate the CRC against a known ICAO
 //   -2: bad message or unrepairable CRC error
 
+static unsigned char all_zeros[14] = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
 int scoreModesMessage(unsigned char *msg, int validbits)
 {
     int msgtype, msgbits, crc, iid;
     uint32_t addr;
     struct errorinfo *ei;
-    static unsigned char all_zeros[14] = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
 
     if (validbits < 56)
         return -2;
@@ -434,6 +434,10 @@ int decodeModesMessage(struct modesMessage *mm, unsigned char *msg)
     }
     msg = mm->msg;
 
+    // don't accept all-zeros messages
+    if (!memcmp(all_zeros, msg, 7))
+        return -2;
+
     // Get the message type ASAP as other operations depend on this
     mm->msgtype         = getbits(msg, 1, 5); // Downlink Format
     mm->msgbits         = modesMessageLenByType(mm->msgtype);