fsverity: simplify fsverity_get_digest()

Instead of looking up the algorithm by name in hash_algo_name[] to get
its hash_algo ID, just store the hash_algo ID in the fsverity_hash_alg
struct.  Verify at boot time that every fsverity_hash_alg has a valid
hash_algo ID with matching digest size.

Remove an unnecessary memset() of the whole digest array to 0 before the
digest is copied into it.

Finally, remove the pr_debug statement.  There is already a pr_debug for
the fsverity digest when the file is opened.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
Link: https://lore.kernel.org/r/20221129045139.69803-1-ebiggers@kernel.org

fs/verity/fsverity_private.h

@@ -32,6 +32,11 @@ struct fsverity_hash_alg {
 	unsigned int digest_size; /* digest size in bytes, e.g. 32 for SHA-256 */
 	unsigned int block_size;  /* block size in bytes, e.g. 64 for SHA-256 */
 	mempool_t req_pool;	  /* mempool with a preallocated hash request */
+	/*
+	 * The HASH_ALGO_* constant for this algorithm.  This is different from
+	 * FS_VERITY_HASH_ALG_*, which uses a different numbering scheme.
+	 */
+	enum hash_algo algo_id;
 };
 
 /* Merkle tree parameters: hash algorithm, initial hash state, and topology */

fs/verity/hash_algs.c

@@ -16,11 +16,13 @@ struct fsverity_hash_alg fsverity_hash_algs[] = {
 		.name = "sha256",
 		.digest_size = SHA256_DIGEST_SIZE,
 		.block_size = SHA256_BLOCK_SIZE,
+		.algo_id = HASH_ALGO_SHA256,
 	},
 	[FS_VERITY_HASH_ALG_SHA512] = {
 		.name = "sha512",
 		.digest_size = SHA512_DIGEST_SIZE,
 		.block_size = SHA512_BLOCK_SIZE,
+		.algo_id = HASH_ALGO_SHA512,
 	},
 };
 
@@ -324,5 +326,9 @@ void __init fsverity_check_hash_algs(void)
 		 */
 		BUG_ON(!is_power_of_2(alg->digest_size));
 		BUG_ON(!is_power_of_2(alg->block_size));
+
+		/* Verify that there is a valid mapping to HASH_ALGO_*. */
+		BUG_ON(alg->algo_id == 0);
+		BUG_ON(alg->digest_size != hash_digest_size[alg->algo_id]);
 	}
 }

fs/verity/measure.c

@@ -65,8 +65,7 @@ EXPORT_SYMBOL_GPL(fsverity_ioctl_measure);
  * @alg: (out) pointer to the hash algorithm enumeration
  *
  * Return the file hash algorithm and digest of an fsverity protected file.
- * Assumption: before calling fsverity_get_digest(), the file must have been
- * opened.
+ * Assumption: before calling this, the file must have been opened.
  *
  * Return: 0 on success, -errno on failure
  */
@@ -76,27 +75,13 @@ int fsverity_get_digest(struct inode *inode,
 {
 	const struct fsverity_info *vi;
 	const struct fsverity_hash_alg *hash_alg;
-	int i;
 
 	vi = fsverity_get_info(inode);
 	if (!vi)
 		return -ENODATA; /* not a verity file */
 
 	hash_alg = vi->tree_params.hash_alg;
-	memset(digest, 0, FS_VERITY_MAX_DIGEST_SIZE);
-
-	/* convert the verity hash algorithm name to a hash_algo_name enum */
-	i = match_string(hash_algo_name, HASH_ALGO__LAST, hash_alg->name);
-	if (i < 0)
-		return -EINVAL;
-	*alg = i;
-
-	if (WARN_ON_ONCE(hash_alg->digest_size != hash_digest_size[*alg]))
-		return -EINVAL;
 	memcpy(digest, vi->file_digest, hash_alg->digest_size);
-
-	pr_debug("file digest %s:%*phN\n", hash_algo_name[*alg],
-		 hash_digest_size[*alg], digest);
-
+	*alg = hash_alg->algo_id;
 	return 0;
 }