fscrypt: factor out fscrypt_policy_to_key_spec()

Factor out a function that builds the fscrypt_key_specifier for an
fscrypt_policy.  Before this was only needed when finding the key for a
file, but now it will also be needed for test_dummy_encryption support.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20220501050857.538984-4-ebiggers@kernel.org

fs/crypto/fscrypt_private.h

@@ -623,6 +623,8 @@ int fscrypt_setup_v1_file_key_via_subscribed_keyrings(struct fscrypt_info *ci);
 
 bool fscrypt_policies_equal(const union fscrypt_policy *policy1,
 			    const union fscrypt_policy *policy2);
+int fscrypt_policy_to_key_spec(const union fscrypt_policy *policy,
+			       struct fscrypt_key_specifier *key_spec);
 bool fscrypt_supported_policy(const union fscrypt_policy *policy_u,
 			      const struct inode *inode);
 int fscrypt_policy_from_context(union fscrypt_policy *policy_u,

fs/crypto/keysetup.c

@@ -425,23 +425,9 @@ static int setup_file_encryption_key(struct fscrypt_info *ci,
 	if (err)
 		return err;
 
-	switch (ci->ci_policy.version) {
-	case FSCRYPT_POLICY_V1:
-		mk_spec.type = FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR;
-		memcpy(mk_spec.u.descriptor,
-		       ci->ci_policy.v1.master_key_descriptor,
-		       FSCRYPT_KEY_DESCRIPTOR_SIZE);
-		break;
-	case FSCRYPT_POLICY_V2:
-		mk_spec.type = FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER;
-		memcpy(mk_spec.u.identifier,
-		       ci->ci_policy.v2.master_key_identifier,
-		       FSCRYPT_KEY_IDENTIFIER_SIZE);
-		break;
-	default:
-		WARN_ON(1);
-		return -EINVAL;
-	}
+	err = fscrypt_policy_to_key_spec(&ci->ci_policy, &mk_spec);
+	if (err)
+		return err;
 
 	key = fscrypt_find_master_key(ci->ci_inode->i_sb, &mk_spec);
 	if (IS_ERR(key)) {

fs/crypto/policy.c

@@ -32,6 +32,26 @@ bool fscrypt_policies_equal(const union fscrypt_policy *policy1,
 	return !memcmp(policy1, policy2, fscrypt_policy_size(policy1));
 }
 
+int fscrypt_policy_to_key_spec(const union fscrypt_policy *policy,
+			       struct fscrypt_key_specifier *key_spec)
+{
+	switch (policy->version) {
+	case FSCRYPT_POLICY_V1:
+		key_spec->type = FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR;
+		memcpy(key_spec->u.descriptor, policy->v1.master_key_descriptor,
+		       FSCRYPT_KEY_DESCRIPTOR_SIZE);
+		return 0;
+	case FSCRYPT_POLICY_V2:
+		key_spec->type = FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER;
+		memcpy(key_spec->u.identifier, policy->v2.master_key_identifier,
+		       FSCRYPT_KEY_IDENTIFIER_SIZE);
+		return 0;
+	default:
+		WARN_ON(1);
+		return -EINVAL;
+	}
+}
+
 static const union fscrypt_policy *
 fscrypt_get_dummy_policy(struct super_block *sb)
 {