[PATCH] fs: hfs: Fix comparison bug in hfs_cat_keycmp

[PATCH] fs: hfs: Fix comparison bug in hfs_cat_keycmp

[Rasmus Villemoes]

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@rasmusvillemoes.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

Re: [PATCH] fs: hfs: Fix comparison bug in hfs_cat_keycmp

[Vyacheslav Dubeyko]

On Wed, 2014-12-10 at 17:32 +0100, Rasmus Villemoes wrote:
> 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.
> 

Looks good for me. Thank you for the fix.

Reviewed-by: Vyacheslav Dubeyko <slava@dubeyko.com>

Thanks,
Vyacheslav Dubeyko.

> Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.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 */