btrfs-progs: image: use CRC32C reversing instead of brute force to find collisions

Author: Piotr Pawlow <pp@siedziba.pl>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/image/main.c b/image/main.c
index c743cb4..4cffbdb 100644 (file)
--- a/image/main.c
+++ b/image/main.c
@@ -447,17 +447,26 @@ static int find_collision_is_suffix_valid(const char *suffix)
        return 1;
 }
 
-static int find_collision_brute_force(struct name *val, u32 name_len)
+static int find_collision_reverse_crc32c(struct name *val, u32 name_len)
 {
        unsigned long checksum;
+       unsigned long current_checksum;
        int found = 0;
        int i;
 
+       /* There are no same length collisions of 4 or less bytes */
+       if (name_len <= 4)
+               return 0;
        checksum = crc32c(~1, val->val, name_len);
+       name_len -= 4;
        memset(val->sub, ' ', name_len);
        i = 0;
        while (1) {
-               if (crc32c(~1, val->sub, name_len) == checksum &&
+               current_checksum = crc32c(~1, val->sub, name_len);
+               find_collision_calc_suffix(current_checksum,
+                                          checksum,
+                                          val->sub + name_len);
+               if (find_collision_is_suffix_valid(val->sub + name_len) &&
                    memcmp(val->sub, val->val, val->len)) {
                        found = 1;
                        break;
@@ -524,7 +533,7 @@ static char *find_collision(struct metadump_struct *md, char *name,
                return NULL;
        }
 
-       found = find_collision_brute_force(val, name_len);
+       found = find_collision_reverse_crc32c(val, name_len);
 
        if (!found) {
                warning(