dm-verity: make real_digest and want_digest fixed-length

Change the digest fields in struct dm_verity_io from variable-length to
fixed-length, since their maximum length is fixed at
HASH_MAX_DIGESTSIZE, i.e. 64 bytes, which is not too big.  This is
simpler and makes the fields a bit faster to access.

(HASH_MAX_DIGESTSIZE did not exist when this code was written, which may
explain why it wasn't used.)

This makes the verity_io_real_digest() and verity_io_want_digest()
functions trivial, but this patch leaves them in place temporarily since
most of their callers will go away in a later patch anyway.
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

drivers/md/dm-verity-target.c

@@ -1529,8 +1529,7 @@ static int verity_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 		goto bad;
 	}
 
-	ti->per_io_data_size = sizeof(struct dm_verity_io) +
-				v->ahash_reqsize + v->digest_size * 2;
+	ti->per_io_data_size = sizeof(struct dm_verity_io) + v->ahash_reqsize;
 
 	r = verity_fec_ctr(v);
 	if (r)

drivers/md/dm-verity.h

@@ -91,15 +91,12 @@ struct dm_verity_io {
 
 	char *recheck_buffer;
 
+	u8 real_digest[HASH_MAX_DIGESTSIZE];
+	u8 want_digest[HASH_MAX_DIGESTSIZE];
+
 	/*
-	 * Three variably-size fields follow this struct:
-	 *
-	 * u8 hash_req[v->ahash_reqsize];
-	 * u8 real_digest[v->digest_size];
-	 * u8 want_digest[v->digest_size];
-	 *
-	 * To access them use: verity_io_hash_req(), verity_io_real_digest()
-	 * and verity_io_want_digest().
+	 * This struct is followed by a variable-sized struct ahash_request of
+	 * size v->ahash_reqsize.  To access it, use verity_io_hash_req().
 	 */
 };
 
@@ -112,13 +109,13 @@ static inline struct ahash_request *verity_io_hash_req(struct dm_verity *v,
 static inline u8 *verity_io_real_digest(struct dm_verity *v,
 					struct dm_verity_io *io)
 {
-	return (u8 *)(io + 1) + v->ahash_reqsize;
+	return io->real_digest;
 }
 
 static inline u8 *verity_io_want_digest(struct dm_verity *v,
 					struct dm_verity_io *io)
 {
-	return (u8 *)(io + 1) + v->ahash_reqsize + v->digest_size;
+	return io->want_digest;
 }
 
 extern int verity_for_bv_block(struct dm_verity *v, struct dm_verity_io *io,