[PATCH v2 3/3] ntfs: only alias volume $UpCase to default on exact match

[DaeMyung Kang]

load_and_init_upcase() currently aliases vol->upcase to the global
default upcase whenever the shared prefix matches, and then truncates
vol->upcase_len to that shorter prefix.  The result is correct only by
accident: upcase[] accesses in name collation are gated by upcase_len,
so the prefix-equality alias produces the same fold output as keeping
the volume's own shorter table.

Still, prefix equality is not equality: the volume table is logically
distinct from the default and should not be replaced by it unless they
are byte-for-byte identical.  Use memcmp() to compare the complete table
in one expression and drop the now-redundant upcase_len rewrite.

No user-visible change is expected for compliant volumes whose $UpCase
has exactly default_upcase_len entries; shorter volume tables are no
longer aliased to the default.

Signed-off-by: DaeMyung Kang <charsyam@xxxxxxxxx>
---
 fs/ntfs/super.c | 12 +++---------
 1 file changed, 3 insertions(+), 9 deletions(-)

diff --git a/fs/ntfs/super.c b/fs/ntfs/super.c
index 14fd244f29fe..971c2822f5e3 100644
--- a/fs/ntfs/super.c
+++ b/fs/ntfs/super.c
@@ -1323,7 +1323,6 @@ static bool load_and_init_upcase(struct ntfs_volume *vol)
 	u8 *addr;
 	pgoff_t index, max_index;
 	unsigned int size;
-	int i, max;
 
 	ntfs_debug("Entering.");
 	/* Read upcase table and setup vol->upcase and vol->upcase_len. */
@@ -1374,16 +1373,11 @@ static bool load_and_init_upcase(struct ntfs_volume *vol)
 		mutex_unlock(&ntfs_lock);
 		return true;
 	}
-	max = default_upcase_len;
-	if (max > vol->upcase_len)
-		max = vol->upcase_len;
-	for (i = 0; i < max; i++)
-		if (vol->upcase[i] != default_upcase[i])
-			break;
-	if (i == max) {
+	if (default_upcase_len == vol->upcase_len &&
+	    !memcmp(vol->upcase, default_upcase,
+		    default_upcase_len * sizeof(*default_upcase))) {
 		kvfree(vol->upcase);
 		vol->upcase = default_upcase;
-		vol->upcase_len = max;
 		ntfs_nr_upcase_users++;
 		mutex_unlock(&ntfs_lock);
 		ntfs_debug("Volume specified $UpCase matches default. Using default.");
-- 
2.43.0