fscrypt: add comments that describe the HKDF info strings

Each HKDF context byte is associated with a specific format of the
remaining part of the application-specific info string.  Add comments so
that it's easier to keep track of what these all are.

Link: https://lore.kernel.org/r/20200708215529.146890-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

diff --git a/fs/crypto/fscrypt_private.h b/fs/crypto/fscrypt_private.h
index 5bb40d0..0f154bd 100644 (file)
--- a/fs/crypto/fscrypt_private.h
+++ b/fs/crypto/fscrypt_private.h
@@ -312,13 +312,13 @@ int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
  * outputs are unique and cryptographically isolated, i.e. knowledge of one
  * output doesn't reveal another.
  */
-#define HKDF_CONTEXT_KEY_IDENTIFIER    1
-#define HKDF_CONTEXT_PER_FILE_ENC_KEY  2
-#define HKDF_CONTEXT_DIRECT_KEY                3
-#define HKDF_CONTEXT_IV_INO_LBLK_64_KEY        4
-#define HKDF_CONTEXT_DIRHASH_KEY       5
-#define HKDF_CONTEXT_IV_INO_LBLK_32_KEY        6
-#define HKDF_CONTEXT_INODE_HASH_KEY    7
+#define HKDF_CONTEXT_KEY_IDENTIFIER    1 /* info=<empty>               */
+#define HKDF_CONTEXT_PER_FILE_ENC_KEY  2 /* info=file_nonce            */
+#define HKDF_CONTEXT_DIRECT_KEY                3 /* info=mode_num              */
+#define HKDF_CONTEXT_IV_INO_LBLK_64_KEY        4 /* info=mode_num||fs_uuid     */
+#define HKDF_CONTEXT_DIRHASH_KEY       5 /* info=file_nonce            */
+#define HKDF_CONTEXT_IV_INO_LBLK_32_KEY        6 /* info=mode_num||fs_uuid     */
+#define HKDF_CONTEXT_INODE_HASH_KEY    7 /* info=<empty>               */
 
 int fscrypt_hkdf_expand(const struct fscrypt_hkdf *hkdf, u8 context,
                        const u8 *info, unsigned int infolen,