linux-kernel - [PATCH] fs: hfs: Fix comparison bug in hfs_cat

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Date:	Wed, 10 Dec 2014 17:32:44 +0100
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Vyacheslav Dubeyko <slava@...eyko.com>
Cc:	Rasmus Villemoes <linux@...musvillemoes.dk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] fs: hfs: Fix comparison bug in hfs_cat_keycmp

Relying on the sign (after casting to int) of the difference of two
quantities for comparison is usually wrong. For example, should a-b
turn out to be 2^31, the return value of cmp(a,b) is -2^31; but that
would also be the return value from cmp(b, a). So a compares less than
b and b compares less than a. One can also easily find three values
a,b,c such that a compares less than b, b compares less than c, but a
does not compare less than c.

Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
---
 fs/hfs/catalog.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/fs/hfs/catalog.c b/fs/hfs/catalog.c
index ff0316b925a5..db458ee3a546 100644
--- a/fs/hfs/catalog.c
+++ b/fs/hfs/catalog.c
@@ -162,14 +162,16 @@ err2:
  */
 int hfs_cat_keycmp(const btree_key *key1, const btree_key *key2)
 {
-	int retval;
+	__be32 k1p, k2p;
 
-	retval = be32_to_cpu(key1->cat.ParID) - be32_to_cpu(key2->cat.ParID);
-	if (!retval)
-		retval = hfs_strcmp(key1->cat.CName.name, key1->cat.CName.len,
-				    key2->cat.CName.name, key2->cat.CName.len);
+	k1p = key1->cat.ParID;
+	k2p = key2->cat.ParID;
 
-	return retval;
+	if (k1p != k2p)
+		return be32_to_cpu(k1p) < be32_to_cpu(k2p) ? -1 : 1;
+
+	return hfs_strcmp(key1->cat.CName.name, key1->cat.CName.len,
+			  key2->cat.CName.name, key2->cat.CName.len);
 }
 
 /* Try to get a catalog entry for given catalog id */
-- 
2.1.3

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