1 files changed, 84 insertions, 0 deletions
diff --git a/src/or/proto_cell.c b/src/or/proto_cell.c
new file mode 100644
index 0000000000..75eb2a7e7f
--- /dev/null
+++ b/src/or/proto_cell.c
@@ -0,0 +1,84 @@
+/* Copyright (c) 2001 Matej Pfajfar.
+ * Copyright (c) 2001-2004, Roger Dingledine.
+ * Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson.
+ * Copyright (c) 2007-2017, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+#include "or.h"
+#include "buffers.h"
+#include "proto_cell.h"
+
+#include "connection_or.h"
+
+/** True iff the cell command <b>command</b> is one that implies a
+ * variable-length cell in Tor link protocol <b>linkproto</b>. */
+static inline int
+cell_command_is_var_length(uint8_t command, int linkproto)
+{
+  /* If linkproto is v2 (2), CELL_VERSIONS is the only variable-length cells
+   * work as implemented here. If it's 1, there are no variable-length cells.
+   * Tor does not support other versions right now, and so can't negotiate
+   * them.
+   */
+  switch (linkproto) {
+  case 1:
+    /* Link protocol version 1 has no variable-length cells. */
+    return 0;
+  case 2:
+    /* In link protocol version 2, VERSIONS is the only variable-length cell */
+    return command == CELL_VERSIONS;
+  case 0:
+  case 3:
+  default:
+    /* In link protocol version 3 and later, and in version "unknown",
+     * commands 128 and higher indicate variable-length. VERSIONS is
+     * grandfathered in. */
+    return command == CELL_VERSIONS || command >= 128;
+  }
+}
+
+/** Check <b>buf</b> for a variable-length cell according to the rules of link
+ * protocol version <b>linkproto</b>.  If one is found, pull it off the buffer
+ * and assign a newly allocated var_cell_t to *<b>out</b>, and return 1.
+ * Return 0 if whatever is on the start of buf_t is not a variable-length
+ * cell.  Return 1 and set *<b>out</b> to NULL if there seems to be the start
+ * of a variable-length cell on <b>buf</b>, but the whole thing isn't there
+ * yet. */
+int
+fetch_var_cell_from_buf(buf_t *buf, var_cell_t **out, int linkproto)
+{
+  char hdr[VAR_CELL_MAX_HEADER_SIZE];
+  var_cell_t *result;
+  uint8_t command;
+  uint16_t length;
+  const int wide_circ_ids = linkproto >= MIN_LINK_PROTO_FOR_WIDE_CIRC_IDS;
+  const int circ_id_len = get_circ_id_size(wide_circ_ids);
+  const unsigned header_len = get_var_cell_header_size(wide_circ_ids);
+  *out = NULL;
+  if (buf_datalen(buf) < header_len)
+    return 0;
+  buf_peek(buf, hdr, header_len);
+
+  command = get_uint8(hdr + circ_id_len);
+  if (!(cell_command_is_var_length(command, linkproto)))
+    return 0;
+
+  length = ntohs(get_uint16(hdr + circ_id_len + 1));
+  if (buf_datalen(buf) < (size_t)(header_len+length))
+    return 1;
+
+  result = var_cell_new(length);
+  result->command = command;
+  if (wide_circ_ids)
+    result->circ_id = ntohl(get_uint32(hdr));
+  else
+    result->circ_id = ntohs(get_uint16(hdr));
+
+  buf_drain(buf, header_len);
+  buf_peek(buf, (char*) result->payload, length);
+  buf_drain(buf, length);
+
+  *out = result;
+  return 1;
+}
+