Staging: batman-adv: Fixing wrap-around bug in vis

[android-x86/kernel.git] / drivers / staging / batman-adv / vis.c

diff --git a/drivers/staging/batman-adv/vis.c b/drivers/staging/batman-adv/vis.c
index 222db01..28eac7e 100644 (file)
--- a/drivers/staging/batman-adv/vis.c
+++ b/drivers/staging/batman-adv/vis.c
@@ -27,6 +27,22 @@
 #include "hard-interface.h"
 #include "hash.h"
 
+/* Returns the smallest signed integer in two's complement with the sizeof x */
+#define smallest_signed_int(x) (1u << (7u + 8u * (sizeof(x) - 1u)))
+
+/* Checks if a sequence number x is a predecessor/successor of y.
+   they handle overflows/underflows and can correctly check for a
+   predecessor/successor unless the variable sequence number has grown by
+   more then 2**(bitwidth(x)-1)-1.
+   This means that for a uint8_t with the maximum value 255, it would think:
+    * when adding nothing - it is neither a predecessor nor a successor
+    * before adding more than 127 to the starting value - it is a predecessor,
+    * when adding 128 - it is neither a predecessor nor a successor,
+    * after adding more than 127 to the starting value - it is a successor */
+#define seq_before(x, y) ({typeof(x) _dummy = (x - y); \
+                         _dummy > smallest_signed_int(_dummy); })
+#define seq_after(x, y) seq_before(y, x)
+
 struct hashtable_t *vis_hash;
 DEFINE_SPINLOCK(vis_hash_lock);
 static DEFINE_SPINLOCK(recv_list_lock);
@@ -212,7 +228,7 @@ static struct vis_info *add_packet(struct vis_packet *vis_packet,
        old_info = hash_find(vis_hash, &search_elem);
 
        if (old_info != NULL) {
-               if (vis_packet->seqno - old_info->packet.seqno <= 0) {
+               if (!seq_after(vis_packet->seqno, old_info->packet.seqno)) {
                        if (old_info->packet.seqno == vis_packet->seqno) {
                                recv_list_add(&old_info->recv_list,
                                              vis_packet->sender_orig);