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Clarify cross-generation lookup limitation and safety in import_dealloc during rehash

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Andrea Mazzoleni 2026-08-26 10:23:42 +02:00
parent f25fbd0c1b
commit 8396d3aaa1
1 changed files with 10 additions and 4 deletions
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14
cmdline/import.c
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@ -183,10 +183,16 @@ static void import_dealloc(struct snapraid_state* state, const char* dir, struct
/* if we are in a rehash state */
if (state->prevhash != HASH_UNDEFINED) {
/*
* The deallocation record doesn't identify which hash algorithm
* produced the stored digest, so index it under both algorithms.
* This is safe because state_import_fetch() hashes the candidate
* bytes with the requested algorithm before accepting them.
* The deallocation record stores only one digest per block without
* recording which hash algorithm generated it. To avoid disk I/O at
* startup, index the stored digest under both algorithms.
*
* This is safe against false positives because state_import_fetch()
* validates candidate bytes with the requested algorithm upon read.
*
* Note that cross-generation matches (an old-hash deallocation supplying
* a new-hash block, or vice-versa) will miss in hash lookup and fall
* back to parity reconstruction. Only same-generation matches will succeed.
*/
memcpy(block->prevhash, block->hash, BLOCK_HASH_SIZE);
tommy_hashdyn_insert(&state->previmportset, &block->prevnodeset, block, import_block_hash(block->prevhash));