hdlc: compare CRC against known good value rather than found CRC

Instead of bothering to pull the found CRC out of the decapsulated
frame, we can just compare the calculated CRC to a known good value
due to the way CRC-CCITT works.

1 files changed, 12 insertions, 25 deletions

diff --git a/src/mm-hdlc.c b/src/mm-hdlc.c
index 731721c00..4fad9e96f 100644
--- a/src/mm-hdlc.c
+++ b/src/mm-hdlc.c
@@ -67,16 +67,11 @@ crc_update (guint16 crc, guint8 val)
     return crc_table[(crc ^ val) & 0xff] ^ (crc >> 8);
 }
 
-static guint16
-crc_finish (guint16 crc)
-{
-    return ~crc;
-}
-
 const guint8  HDLC_CONTROL  = 0x7E;  /* Control character */
 const guint8  HDLC_ESC_CHAR = 0x7D;  /* Escape sequence 1st character value */
 const guint8  HDLC_ESC_MASK = 0x20;  /* Escape sequence complement value */
 const guint16 HDLC_CRC_INIT = 0xFFFF;
+const guint16 HDLC_CRC_GOOD = 0xF0B8;
 
 static guint
 framed_len (GByteArray *bytes)
@@ -136,8 +131,8 @@ mm_hdlc_encapsulate (GByteArray *bytes, GError **error)
         escape_and_add (framed, bytes->data[i]);
     }
 
-    /* Append and escape the CRC */
-    crc = crc_finish (crc);
+    /* Append and escape the complement of the CRC */
+    crc = ~crc;
     escape_and_add (framed, crc & 0xFF);
     escape_and_add (framed, (crc >> 8) & 0xFF);
 
@@ -178,7 +173,6 @@ mm_hdlc_decapsulate (GByteArray *bytes,
     guint i, frame_len = 0;
     gboolean unescaping = FALSE;
     guint16 crc = HDLC_CRC_INIT;
-    guint16 found_crc = 0;
 
     if (out_bytes_used)
         *out_bytes_used = 0;
@@ -214,33 +208,26 @@ mm_hdlc_decapsulate (GByteArray *bytes,
     unframed = g_byte_array_sized_new (frame_len);
 
     for (i = 0; i < frame_len; i++) {
-        guint8 val;
+        guint8 val = bytes->data[i];
 
-        if (bytes->data[i] == HDLC_ESC_CHAR) {
+        if (val == HDLC_ESC_CHAR) {
             unescaping = TRUE;
             continue;
-        }
-
-        if (unescaping) {
-            val = bytes->data[i] ^ HDLC_ESC_MASK;
+        } else if (unescaping) {
+            val ^= HDLC_ESC_MASK;
             unescaping = FALSE;
-        } else
-            val = bytes->data[i];
+        }
+        crc = crc_update (crc, val);
 
-        /* Don't CRC the CRC */
-        if (i < frame_len - 2) {
-            crc = crc_update (crc, val);
+        /* Ignore the CRC when returning the frame */
+        if (i < frame_len - 2)
             g_byte_array_append (unframed, &val, 1);
-        } else if (i == frame_len - 2)
-            found_crc = val;
-        else if (i == frame_len - 1)
-            found_crc |= val << 8;
     }
 
     if (out_bytes_used)
         *out_bytes_used = frame_len + 1;
 
-    if (crc_finish (crc) != found_crc) {
+    if (crc != HDLC_CRC_GOOD) {
         g_set_error (error, G_IO_ERROR, G_IO_ERROR_FAILED, "HDLC CRC mismatch");
         g_byte_array_free (unframed, TRUE);
         return NULL;