From 87d7fe28d1e2096f6f1705f0e2a9deacafb722b8 Mon Sep 17 00:00:00 2001
From: Nick Mathewson <nickm@torproject.org>
Date: Mon, 2 Jun 2025 11:54:10 -0400
Subject: [PATCH] Move responsibility for ntorv3 extension lookup to a
 function.

---
 src/core/crypto/onion_crypto.c          | 27 +++++++++++++
 src/core/crypto/onion_crypto.h          |  6 +++
 src/core/or/congestion_control_common.c | 51 +++++--------------------
 3 files changed, 43 insertions(+), 41 deletions(-)

diff --git a/src/core/crypto/onion_crypto.c b/src/core/crypto/onion_crypto.c
index ed3340fa2d..b5cf50a0a9 100644
--- a/src/core/crypto/onion_crypto.c
+++ b/src/core/crypto/onion_crypto.c
@@ -563,3 +563,30 @@ onion_skin_client_handshake(int type,
     return -1;
   }
 }
+
+/**
+ * If there is an extension field of type `ext_type` in `ext`,
+ * return that field.  Otherwise return NULL.
+ */
+const trn_extension_field_t *
+trn_extension_find(const trn_extension_t *ext, uint8_t ext_type)
+{
+  IF_BUG_ONCE(!ext) {
+    return NULL;
+  }
+  size_t n_fields = trn_extension_get_num(ext);
+  if (n_fields == 0)
+    return NULL;
+
+  for (unsigned i = 0; i < n_fields; ++i) {
+    const trn_extension_field_t *field = trn_extension_getconst_fields(ext, i);
+    IF_BUG_ONCE(field == NULL) {
+      return NULL;
+    }
+    if (trn_extension_field_get_field_type(field) == ext_type) {
+      return field;
+    }
+  }
+
+  return NULL;
+}
diff --git a/src/core/crypto/onion_crypto.h b/src/core/crypto/onion_crypto.h
index 5e64726b08..90cab5ace4 100644
--- a/src/core/crypto/onion_crypto.h
+++ b/src/core/crypto/onion_crypto.h
@@ -69,4 +69,10 @@ void server_onion_keys_free_(server_onion_keys_t *keys);
 #define server_onion_keys_free(keys) \
   FREE_AND_NULL(server_onion_keys_t, server_onion_keys_free_, (keys))
 
+struct trn_extension_st;
+struct trn_extension_field_st;
+const struct trn_extension_field_st *trn_extension_find(
+                                        const struct trn_extension_st *ext,
+                                        uint8_t ext_type);
+
 #endif /* !defined(TOR_ONION_CRYPTO_H) */
diff --git a/src/core/or/congestion_control_common.c b/src/core/or/congestion_control_common.c
index 7dc9dbafdd..faba193abb 100644
--- a/src/core/or/congestion_control_common.c
+++ b/src/core/or/congestion_control_common.c
@@ -1073,7 +1073,6 @@ congestion_control_parse_ext_request(const uint8_t *msg, const size_t msg_len)
 {
   ssize_t ret = 0;
   trn_extension_t *ext = NULL;
-  size_t num_fields = 0;
 
   /* Parse extension from payload. */
   ret = trn_extension_parse(&ext, msg, msg_len);
@@ -1081,28 +1080,13 @@ congestion_control_parse_ext_request(const uint8_t *msg, const size_t msg_len)
     goto end;
   }
 
-  /* No extension implies no support for congestion control. In this case, we
-   * simply return 0 to indicate CC is disabled. */
-  if ((num_fields = trn_extension_get_num(ext)) == 0) {
+  if (trn_extension_find(ext, TRUNNEL_EXT_TYPE_CC_FIELD_REQUEST) == NULL) {
+    /* No extension implies no support for congestion control. In this case, we
+     * simply return 0 to indicate CC is disabled. */
     ret = 0;
-    goto end;
-  }
-
-  /* Go over all fields. If any field is TRUNNEL_EXT_TYPE_CC_FIELD_REQUEST,
-   * then congestion control is enabled. Ignore unknown fields. */
-  for (size_t f = 0; f < num_fields; f++) {
-    const trn_extension_field_t *field = trn_extension_get_fields(ext, f);
-    if (field == NULL) {
-      ret = -1;
-      goto end;
-    }
-
+  } else {
     /* For congestion control to be enabled, we only need the field type. */
-    if (trn_extension_field_get_field_type(field) ==
-        TRUNNEL_EXT_TYPE_CC_FIELD_REQUEST) {
-      ret = 1;
-      break;
-    }
+    ret = 1;
   }
 
  end:
@@ -1233,8 +1217,8 @@ congestion_control_parse_ext_response(const uint8_t *msg,
                                       circuit_params_t *params_out)
 {
   ssize_t ret = 0;
-  size_t num_fields = 0;
   trn_extension_t *ext = NULL;
+  const trn_extension_field_t *field = NULL;
   trn_extension_field_cc_t *cc_field = NULL;
 
   /* We will only accept this response (and this circuit) if sendme_inc
@@ -1250,24 +1234,11 @@ congestion_control_parse_ext_response(const uint8_t *msg,
     goto end;
   }
 
-  if ((num_fields = trn_extension_get_num(ext)) == 0) {
-    ret = 0;
-    goto end;
-  }
-
-  /* Go over all fields. If any field is TRUNNEL_EXT_TYPE_CC_FIELD_RESPONSE,
-   * then congestion control is enabled. Ignore unknown fields. */
-  for (size_t f = 0; f < num_fields; f++) {
-    const trn_extension_field_t *field = trn_extension_get_fields(ext, f);
-    if (field == NULL) {
-      ret = -1;
-      goto end;
-    }
-
-    /* Only examine TRUNNEL_EXT_TYPE_CC_FIELD_RESPONSE; ignore other fields */
-    if (trn_extension_field_get_field_type(field) ==
-        TRUNNEL_EXT_TYPE_CC_FIELD_RESPONSE) {
+  field = trn_extension_find(ext, TRUNNEL_EXT_TYPE_CC_FIELD_RESPONSE);
 
+  if (field == 0) {
+    ret = 0;
+  } else {
       /* Parse the field into the congestion control field. */
       ret = trn_extension_field_cc_parse(&cc_field,
                 trn_extension_field_getconstarray_field(field),
@@ -1286,8 +1257,6 @@ congestion_control_parse_ext_response(const uint8_t *msg,
       /* All good. Get value and break */
       params_out->sendme_inc_cells = sendme_inc_cells;
       ret = 1;
-      break;
-    }
   }
 
  end:
-- 
2.47.2