[PATCH] hfsplus: Prevent overflows in comparisons

[Dave Vasilevsky <dave@vasilevsky.ca>]

When very high Catalog Node IDs are in use, comparing CNIDs via
subtraction may overflow. This causes files and folders to appear to be
missing in the btree. It's safer to just use comparison operations
rather than subtraction.

Also fixes the parent field in grub_hfsplus_catkey_internal, which is
currently using an unqualified int to hold an unsigned value.

=== modified file 'grub-core/fs/hfsplus.c'
--- grub-core/fs/hfsplus.c	2010-01-20 08:12:47 +0000
+++ grub-core/fs/hfsplus.c	2010-12-30 07:10:03 +0000
@@ -178,7 +178,7 @@
 /* Internal representation of a catalog key.  */
 struct grub_hfsplus_catkey_internal
 {
-  int parent;
+  grub_uint32_t parent;
   char *name;
 };
 
@@ -520,9 +520,12 @@
   int i;
   int diff;
 
-  diff = grub_be_to_cpu32 (catkey_a->parent) - catkey_b->parent;
-  if (diff)
-    return diff;
+  /* Safe unsigned comparison */
+  grub_uint32_t aparent = grub_be_to_cpu32 (catkey_a->parent);
+  if (aparent > catkey_b->parent)
+    return 1;
+  if (aparent < catkey_b->parent)
+    return -1;
 
   /* Change the filename in keya so the endianness is correct.  */
   for (i = 0; i < grub_be_to_cpu16 (catkey_a->namelen); i++)
@@ -555,15 +558,21 @@
 {
   struct grub_hfsplus_extkey *extkey_a = &keya->extkey;
   struct grub_hfsplus_extkey_internal *extkey_b = &keyb->extkey;
-  int diff;
-
-  diff = grub_be_to_cpu32 (extkey_a->fileid) - extkey_b->fileid;
-
-  if (diff)
-    return diff;
-
-  diff = grub_be_to_cpu32 (extkey_a->start) - extkey_b->start;
-  return diff;
+  grub_uint32_t akey;
+
+  /* Safe unsigned comparison */
+  akey = grub_be_to_cpu32 (extkey_a->fileid);
+  if (akey > extkey_b->fileid)
+    return 1;
+  if (akey < extkey_b->fileid)
+    return -1;
+  
+  akey = grub_be_to_cpu32 (extkey_a->start);
+  if (akey > extkey_b->start)
+    return 1;
+  if (akey < extkey_b->start)
+    return -1;
+  return 0;
 }
 
 static char *