[Libguestfs] [hivex PATCH] lib: write: improve key collation compatibility with Windows

[Laszlo Ersek]

There are multiple problems with using strcasecmp() for ordering registry
keys:

(1) strcasecmp() is influenced by LC_CTYPE.

(2) strcasecmp() cannot implement case conversion for multibyte UTF-8
    sequences.

(3) Even with LC_CTYPE=POSIX and key names consisting solely of ASCII
    characters, strcasecmp() converts characters to lowercase, for
    comparison. But on Windows, the CompareStringOrdinal() function
    converts characters to uppercase. This makes a difference when
    comparing a letter to one of the characters that fall between 'Z'
    (0x5A) and 'a' (0x61), namely {'[', '\\', ']', '^', '_', '`'}. For
    example,

      'c' (0x63) > '_' (0x5F)
      'C' (0x43) < '_' (0x5F)

Compare key names byte for byte, eliminating problems (1) and (3).

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1648520
Signed-off-by: Laszlo Ersek <lersek at redhat.com>
---
 lib/write.c | 32 +++++++++++++++++++++++++++++++-
 1 file changed, 31 insertions(+), 1 deletion(-)

diff --git a/lib/write.c b/lib/write.c
index 70105c9d9907..d9a13a3c18b6 100644
--- a/lib/write.c
+++ b/lib/write.c
@@ -462,7 +462,37 @@ compare_name_with_nk_name (hive_h *h, const char *name, hive_node_h nk_offs)
     return 0;
   }
 
-  int r = strcasecmp (name, nname);
+  /* Perform a limited case-insensitive comparison. ASCII letters will be
+   * *upper-cased*. Multibyte sequences will produce nonsensical orderings.
+   */
+  int r = 0;
+  const char *s1 = name;
+  const char *s2 = nname;
+
+  for (;;) {
+    unsigned char c1 = *(s1++);
+    unsigned char c2 = *(s2++);
+
+    if (c1 >= 'a' && c1 <= 'z')
+      c1 = 'A' + (c1 - 'a');
+    if (c2 >= 'a' && c2 <= 'z')
+      c2 = 'A' + (c2 - 'a');
+    if (c1 < c2) {
+      /* Also covers the case when "name" is a prefix of "nname". */
+      r = -1;
+      break;
+    }
+    if (c1 > c2) {
+      /* Also covers the case when "nname" is a prefix of "name". */
+      r = 1;
+      break;
+    }
+    if (c1 == '\0') {
+      /* Both strings end. */
+      break;
+    }
+  }
+
   free (nname);
 
   return r;
-- 
2.19.1.3.g30247aa5d201