fscrypt: improve a few comments

Improve a few comments.  These were extracted from the patch
"fscrypt: add support for hardware-wrapped keys"
(https://lore.kernel.org/r/20211021181608.54127-4-ebiggers@kernel.org).

Link: https://lore.kernel.org/r/20211026021042.6581-1-ebiggers@kernel.orgSigned-off-by: Eric Biggers <ebiggers@google.com>

fs/crypto/fscrypt_private.h

@@ -20,6 +20,11 @@
 
 #define FSCRYPT_FILE_NONCE_SIZE	16
 
+/*
+ * Minimum size of an fscrypt master key.  Note: a longer key will be required
+ * if ciphers with a 256-bit security strength are used.  This is just the
+ * absolute minimum, which applies when only 128-bit encryption is used.
+ */
 #define FSCRYPT_MIN_KEY_SIZE	16
 
 #define FSCRYPT_CONTEXT_V1	1
@@ -413,7 +418,11 @@ struct fscrypt_master_key_secret {
 	 */
 	struct fscrypt_hkdf	hkdf;
 
-	/* Size of the raw key in bytes.  Set even if ->raw isn't set. */
+	/*
+	 * Size of the raw key in bytes.  This remains set even if ->raw was
+	 * zeroized due to no longer being needed.  I.e. we still remember the
+	 * size of the key even if we don't need to remember the key itself.
+	 */
 	u32			size;
 
 	/* For v1 policy keys: the raw key.  Wiped for v2 policy keys. */

fs/crypto/keysetup.c

@@ -122,8 +122,9 @@ fscrypt_allocate_skcipher(struct fscrypt_mode *mode, const u8 *raw_key,
 
 /*
  * Prepare the crypto transform object or blk-crypto key in @prep_key, given the
- * raw key, encryption mode, and flag indicating which encryption implementation
- * (fs-layer or blk-crypto) will be used.
+ * raw key, encryption mode (@ci->ci_mode), flag indicating which encryption
+ * implementation (fs-layer or blk-crypto) will be used (@ci->ci_inlinecrypt),
+ * and IV generation method (@ci->ci_policy.flags).
  */
 int fscrypt_prepare_key(struct fscrypt_prepared_key *prep_key,
 			const u8 *raw_key, const struct fscrypt_info *ci)