linux-kernel - [PATCH 3.2 025/164] jhash: Update jhash_[321]words functions to use correct initval

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Message-ID: <lsq.1438473757.147114363@decadent.org.uk>
Date:	Sun, 02 Aug 2015 01:02:37 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org, "David S. Miller" <davem@...emloft.net>,
	"Alexander Duyck" <alexander.h.duyck@...hat.com>
Subject: [PATCH 3.2 025/164] jhash: Update jhash_[321]words functions to
 use correct initval

3.2.70-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Alexander Duyck <alexander.h.duyck@...hat.com>

commit 2e7056c433216f406b90a003aa0ba42e19d3bdcf upstream.

Looking over the implementation for jhash2 and comparing it to jhash_3words
I realized that the two hashes were in fact very different.  Doing a bit of
digging led me to "The new jhash implementation" in which lookup2 was
supposed to have been replaced with lookup3.

In reviewing the patch I noticed that jhash2 had originally initialized a
and b to JHASH_GOLDENRATIO and c to initval, but after the patch a, b, and
c were initialized to initval + (length << 2) + JHASH_INITVAL.  However the
changes in jhash_3words simply replaced the initialization of a and b with
JHASH_INITVAL.

This change corrects what I believe was an oversight so that a, b, and c in
jhash_3words all have the same value added consisting of initval + (length
<< 2) + JHASH_INITVAL so that jhash2 and jhash_3words will now produce the
same hash result given the same inputs.

Fixes: 60d509c823cca ("The new jhash implementation")
Signed-off-by: Alexander Duyck <alexander.h.duyck@...hat.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 include/linux/jhash.h | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

--- a/include/linux/jhash.h
+++ b/include/linux/jhash.h
@@ -145,11 +145,11 @@ static inline u32 jhash2(const u32 *k, u
 }
 
 
-/* jhash_3words - hash exactly 3, 2 or 1 word(s) */
-static inline u32 jhash_3words(u32 a, u32 b, u32 c, u32 initval)
+/* __jhash_nwords - hash exactly 3, 2 or 1 word(s) */
+static inline u32 __jhash_nwords(u32 a, u32 b, u32 c, u32 initval)
 {
-	a += JHASH_INITVAL;
-	b += JHASH_INITVAL;
+	a += initval;
+	b += initval;
 	c += initval;
 
 	__jhash_final(a, b, c);
@@ -157,14 +157,19 @@ static inline u32 jhash_3words(u32 a, u3
 	return c;
 }
 
+static inline u32 jhash_3words(u32 a, u32 b, u32 c, u32 initval)
+{
+	return __jhash_nwords(a, b, c, initval + JHASH_INITVAL + (3 << 2));
+}
+
 static inline u32 jhash_2words(u32 a, u32 b, u32 initval)
 {
-	return jhash_3words(a, b, 0, initval);
+	return __jhash_nwords(a, b, 0, initval + JHASH_INITVAL + (2 << 2));
 }
 
 static inline u32 jhash_1word(u32 a, u32 initval)
 {
-	return jhash_3words(a, 0, 0, initval);
+	return __jhash_nwords(a, 0, 0, initval + JHASH_INITVAL + (1 << 2));
 }
 
 #endif /* _LINUX_JHASH_H */

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