From 7b10c2afced6c5e03629dc491dbd9e267f136219 Mon Sep 17 00:00:00 2001
From: =?utf8?q?Sebastian=20Dr=C3=B6ge?= <sebastian.droege@collabora.co.uk>
Date: Fri, 26 Mar 2010 18:42:28 +0100
Subject: [PATCH] flacparse: Check the CRC-8 of the headers before accepting a
 frame as valid

This makes false-positives during seeking much less likely and detection of
them much faster.
---
 gst/audioparsers/gstflacparse.c | 60 +++++++++++++++++++++++++++++++++++++++--
 1 file changed, 58 insertions(+), 2 deletions(-)

diff --git a/gst/audioparsers/gstflacparse.c b/gst/audioparsers/gstflacparse.c
index 2ea30ec..ba9d7c4 100644
--- a/gst/audioparsers/gstflacparse.c
+++ b/gst/audioparsers/gstflacparse.c
@@ -65,6 +65,55 @@
 GST_DEBUG_CATEGORY_STATIC (flacparse_debug);
 #define GST_CAT_DEFAULT flacparse_debug
 
+/* CRC-8, poly = x^8 + x^2 + x^1 + x^0, init = 0 */
+static const guint8 crc8_table[256] = {
+  0x00, 0x07, 0x0E, 0x09, 0x1C, 0x1B, 0x12, 0x15,
+  0x38, 0x3F, 0x36, 0x31, 0x24, 0x23, 0x2A, 0x2D,
+  0x70, 0x77, 0x7E, 0x79, 0x6C, 0x6B, 0x62, 0x65,
+  0x48, 0x4F, 0x46, 0x41, 0x54, 0x53, 0x5A, 0x5D,
+  0xE0, 0xE7, 0xEE, 0xE9, 0xFC, 0xFB, 0xF2, 0xF5,
+  0xD8, 0xDF, 0xD6, 0xD1, 0xC4, 0xC3, 0xCA, 0xCD,
+  0x90, 0x97, 0x9E, 0x99, 0x8C, 0x8B, 0x82, 0x85,
+  0xA8, 0xAF, 0xA6, 0xA1, 0xB4, 0xB3, 0xBA, 0xBD,
+  0xC7, 0xC0, 0xC9, 0xCE, 0xDB, 0xDC, 0xD5, 0xD2,
+  0xFF, 0xF8, 0xF1, 0xF6, 0xE3, 0xE4, 0xED, 0xEA,
+  0xB7, 0xB0, 0xB9, 0xBE, 0xAB, 0xAC, 0xA5, 0xA2,
+  0x8F, 0x88, 0x81, 0x86, 0x93, 0x94, 0x9D, 0x9A,
+  0x27, 0x20, 0x29, 0x2E, 0x3B, 0x3C, 0x35, 0x32,
+  0x1F, 0x18, 0x11, 0x16, 0x03, 0x04, 0x0D, 0x0A,
+  0x57, 0x50, 0x59, 0x5E, 0x4B, 0x4C, 0x45, 0x42,
+  0x6F, 0x68, 0x61, 0x66, 0x73, 0x74, 0x7D, 0x7A,
+  0x89, 0x8E, 0x87, 0x80, 0x95, 0x92, 0x9B, 0x9C,
+  0xB1, 0xB6, 0xBF, 0xB8, 0xAD, 0xAA, 0xA3, 0xA4,
+  0xF9, 0xFE, 0xF7, 0xF0, 0xE5, 0xE2, 0xEB, 0xEC,
+  0xC1, 0xC6, 0xCF, 0xC8, 0xDD, 0xDA, 0xD3, 0xD4,
+  0x69, 0x6E, 0x67, 0x60, 0x75, 0x72, 0x7B, 0x7C,
+  0x51, 0x56, 0x5F, 0x58, 0x4D, 0x4A, 0x43, 0x44,
+  0x19, 0x1E, 0x17, 0x10, 0x05, 0x02, 0x0B, 0x0C,
+  0x21, 0x26, 0x2F, 0x28, 0x3D, 0x3A, 0x33, 0x34,
+  0x4E, 0x49, 0x40, 0x47, 0x52, 0x55, 0x5C, 0x5B,
+  0x76, 0x71, 0x78, 0x7F, 0x6A, 0x6D, 0x64, 0x63,
+  0x3E, 0x39, 0x30, 0x37, 0x22, 0x25, 0x2C, 0x2B,
+  0x06, 0x01, 0x08, 0x0F, 0x1A, 0x1D, 0x14, 0x13,
+  0xAE, 0xA9, 0xA0, 0xA7, 0xB2, 0xB5, 0xBC, 0xBB,
+  0x96, 0x91, 0x98, 0x9F, 0x8A, 0x8D, 0x84, 0x83,
+  0xDE, 0xD9, 0xD0, 0xD7, 0xC2, 0xC5, 0xCC, 0xCB,
+  0xE6, 0xE1, 0xE8, 0xEF, 0xFA, 0xFD, 0xF4, 0xF3
+};
+
+static guint8
+gst_flac_calculate_crc8 (const guint8 * data, guint length)
+{
+  guint8 crc = 0;
+
+  while (length--) {
+    crc = crc8_table[crc ^ *data];
+    ++data;
+  }
+
+  return crc;
+}
+
 static GstStaticPadTemplate src_factory = GST_STATIC_PAD_TEMPLATE ("src",
     GST_PAD_SRC,
     GST_PAD_ALWAYS,
@@ -207,6 +256,7 @@ gst_flac_parse_get_frame_size (GstFlacParse * flacparse, GstBuffer * buffer,
   guint8 tmp;
   gint i;
   guint8 channel_assignment = 0;
+  guint8 actual_crc, expected_crc;
 
   /* Skip 14 bit sync code */
   if (!gst_bit_reader_skip (&reader, 14))
@@ -442,10 +492,16 @@ gst_flac_parse_get_frame_size (GstFlacParse * flacparse, GstBuffer * buffer,
       goto error;
   }
 
-  /* skip crc-8 for the header */
-  if (!gst_bit_reader_skip (&reader, 8))
+  /* check crc-8 for the header */
+  if (!gst_bit_reader_get_bits_uint8 (&reader, &expected_crc, 8))
     goto need_more_data;
 
+  actual_crc =
+      gst_flac_calculate_crc8 (GST_BUFFER_DATA (buffer),
+      (gst_bit_reader_get_pos (&reader) / 8) - 1);
+  if (actual_crc != expected_crc)
+    goto error;
+
   /* parse subframes, one subframe per channel */
   for (i = 0; i < flacparse->channels; i++) {
     guint8 sf_type;
-- 
2.7.4