fsverity: store log2(digest_size) precomputed

Add log_digestsize to struct merkle_tree_params so that it can be used
in verify.c.  Also save memory by using u8 for all the log_* fields.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Andrey Albershteyn <aalbersh@redhat.com>
Tested-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Link: https://lore.kernel.org/r/20221223203638.41293-4-ebiggers@kernel.org

fs/verity/fsverity_private.h

@@ -42,8 +42,9 @@ struct merkle_tree_params {
 	unsigned int digest_size;	/* same as hash_alg->digest_size */
 	unsigned int block_size;	/* size of data and tree blocks */
 	unsigned int hashes_per_block;	/* number of hashes per tree block */
-	unsigned int log_blocksize;	/* log2(block_size) */
-	unsigned int log_arity;		/* log2(hashes_per_block) */
+	u8 log_digestsize;		/* log2(digest_size) */
+	u8 log_blocksize;		/* log2(block_size) */
+	u8 log_arity;			/* log2(hashes_per_block) */
 	unsigned int num_levels;	/* number of levels in Merkle tree */
 	u64 tree_size;			/* Merkle tree size in bytes */
 	unsigned long tree_pages;	/* Merkle tree size in pages */

fs/verity/open.c

@@ -76,7 +76,8 @@ int fsverity_init_merkle_tree_params(struct merkle_tree_params *params,
 		err = -EINVAL;
 		goto out_err;
 	}
-	params->log_arity = params->log_blocksize - ilog2(params->digest_size);
+	params->log_digestsize = ilog2(params->digest_size);
+	params->log_arity = log_blocksize - params->log_digestsize;
 	params->hashes_per_block = 1 << params->log_arity;
 
 	/*

fs/verity/verify.c

@@ -35,7 +35,7 @@ static void hash_at_level(const struct merkle_tree_params *params,
 
 	/* Offset of the wanted hash (in bytes) within the hash block */
 	*hoffset = (position & ((1 << params->log_arity) - 1)) <<
-		   (params->log_blocksize - params->log_arity);
+		   params->log_digestsize;
 }
 
 static inline int cmp_hashes(const struct fsverity_info *vi,