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btrfs-progs: image: add a function to calculate CRC32C collisions

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The function uses the reverse CRC32C table to quickly calculate a
4-byte suffix, that when added to the original data will make it
match desired checksum.

Author: Piotr Pawlow <pp@siedziba.pl>
[ minor adjustments ]
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Piotr Pawlow 2017-08-11 15:06:17 +02:00 committed by David Sterba
parent a172244163
commit c859336f47
1 changed files with 23 additions and 0 deletions
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23
image/main.c
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@ -408,6 +408,29 @@ static const u32 crc32c_rev_table[256] = {
0x588982AFL,0x5D65F45EL,0x53516F4DL,0x56BD19BCL
};
/*
* Calculate a 4-byte suffix to match desired CRC32C
*
* @current_crc: CRC32C checksum of all bytes before the suffix
* @desired_crc: the checksum that we want to get after adding the suffix
*
* Outputs: @suffix: pointer to where the suffix will be written (4-bytes)
*/
static void find_collision_calc_suffix(unsigned long current_crc,
unsigned long desired_crc,
char *suffix)
{
int i;
for(i = 3; i >= 0; i--) {
desired_crc = (desired_crc << 8)
^ crc32c_rev_table[desired_crc >> 24 & 0xFF]
^ ((current_crc >> i * 8) & 0xFF);
}
for (i = 0; i < 4; i++)
suffix[i] = (desired_crc >> i * 8) & 0xFF;
}
static int find_collision_brute_force(struct name *val, u32 name_len)
{
unsigned long checksum;