netdev - [PATCH] net: compare_ether_addr[

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Date:	Mon, 07 May 2012 15:39:06 +0200
From:	Johannes Berg <johannes@...solutions.net>
To:	netdev <netdev@...r.kernel.org>
Subject: [PATCH] net: compare_ether_addr[_64bits]() has no ordering

From: Johannes Berg <johannes.berg@...el.com>

Neither compare_ether_addr() nor compare_ether_addr_64bits()
(as it can fall back to the former) have comparison semantics
like memcmp() where the sign of the return value indicates sort
order. We had a bug in the wireless code due to a blind memcmp
replacement because of this.

A cursory look suggests that the wireless bug was the only one
due to this semantic difference.

Signed-off-by: Johannes Berg <johannes.berg@...el.com>
---
 include/linux/etherdevice.h |   11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

--- a/include/linux/etherdevice.h	2012-04-12 05:40:35.000000000 +0200
+++ b/include/linux/etherdevice.h	2012-05-07 15:34:28.000000000 +0200
@@ -159,7 +159,8 @@ static inline void eth_hw_addr_random(st
  * @addr1: Pointer to a six-byte array containing the Ethernet address
  * @addr2: Pointer other six-byte array containing the Ethernet address
  *
- * Compare two ethernet addresses, returns 0 if equal
+ * Compare two ethernet addresses, returns 0 if equal, non-zero otherwise.
+ * Unlike memcmp(), it doesn't return a value suitable for sorting.
  */
 static inline unsigned compare_ether_addr(const u8 *addr1, const u8 *addr2)
 {
@@ -184,10 +185,10 @@ static inline unsigned long zap_last_2by
  * @addr1: Pointer to an array of 8 bytes
  * @addr2: Pointer to an other array of 8 bytes
  *
- * Compare two ethernet addresses, returns 0 if equal.
- * Same result than "memcmp(addr1, addr2, ETH_ALEN)" but without conditional
- * branches, and possibly long word memory accesses on CPU allowing cheap
- * unaligned memory reads.
+ * Compare two ethernet addresses, returns 0 if equal, non-zero otherwise.
+ * Unlike memcmp(), it doesn't return a value suitable for sorting.
+ * The function doesn't need any conditional branches and possibly uses
+ * word memory accesses on CPU allowing cheap unaligned memory reads.
  * arrays = { byte1, byte2, byte3, byte4, byte6, byte7, pad1, pad2}
  *
  * Please note that alignment of addr1 & addr2 is only guaranted to be 16 bits.


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