[FFmpeg-devel] [PATCH] avcodec/mpegaudiodec_template: Check CRCs for layer1 and layer2

[Michael Niedermayer]

This differs from the MPEG specification as the actual real world
files do compute their CRC over variable areas and not the fixed
ones listed in the specification. This is also the reason for
the complexity of this code and the need to perform the CRC
check for layer2 in the middle of layer2 decoding.

Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
---
 libavcodec/mpegaudiodec_template.c | 61 +++++++++++++++++++++---------
 1 file changed, 44 insertions(+), 17 deletions(-)

diff --git a/libavcodec/mpegaudiodec_template.c b/libavcodec/mpegaudiodec_template.c
index 3d7e3ba4f2..d84e4f1cb6 100644
--- a/libavcodec/mpegaudiodec_template.c
+++ b/libavcodec/mpegaudiodec_template.c
@@ -89,6 +89,7 @@ typedef struct MPADecodeContext {
     MPADSPContext mpadsp;
     AVFloatDSPContext *fdsp;
     AVFrame *frame;
+    int crc;
 } MPADecodeContext;
 
 #define HEADER_SIZE 4
@@ -500,12 +501,43 @@ static void imdct12(INTFLOAT *out, SUINTFLOAT *in)
     out[11] = in0 + in5;
 }
 
+static int handle_crc(MPADecodeContext *s, int sec_len)
+{
+    if (s->error_protection && (s->err_recognition & AV_EF_CRCCHECK)) {
+        const uint8_t *buf = s->gb.buffer - HEADER_SIZE;
+        int sec_byte_len  = sec_len >> 3;
+        int sec_rem_bits  = sec_len & 7;
+        const AVCRC *crc_tab = av_crc_get_table(AV_CRC_16_ANSI);
+        uint8_t tmp_buf[4];
+        uint32_t crc_val = av_crc(crc_tab, UINT16_MAX, &buf[2], 2);
+        crc_val = av_crc(crc_tab, crc_val, &buf[6], sec_byte_len);
+
+        AV_WB32(tmp_buf,
+            ((buf[6 + sec_byte_len] & (0xFF00>>sec_rem_bits))<<24) +
+            ((unsigned)(s->crc<<16) >> sec_rem_bits));
+
+        crc_val = av_crc(crc_tab, crc_val, tmp_buf, 3);
+
+        if (crc_val) {
+            av_log(s->avctx, AV_LOG_ERROR, "CRC mismatch %X!\n", crc_val);
+            if (s->err_recognition & AV_EF_EXPLODE)
+                return AVERROR_INVALIDDATA;
+        }
+    }
+    return 0;
+}
+
 /* return the number of decoded frames */
 static int mp_decode_layer1(MPADecodeContext *s)
 {
     int bound, i, v, n, ch, j, mant;
     uint8_t allocation[MPA_MAX_CHANNELS][SBLIMIT];
     uint8_t scale_factors[MPA_MAX_CHANNELS][SBLIMIT];
+    int ret;
+
+    ret = handle_crc(s, (s->nb_channels == 1) ? 8*16  : 8*32);
+    if (ret < 0)
+        return ret;
 
     if (s->mode == MPA_JSTEREO)
         bound = (s->mode_ext + 1) * 4;
@@ -575,6 +607,7 @@ static int mp_decode_layer2(MPADecodeContext *s)
     unsigned char scale_code[MPA_MAX_CHANNELS][SBLIMIT];
     unsigned char scale_factors[MPA_MAX_CHANNELS][SBLIMIT][3], *sf;
     int scale, qindex, bits, steps, k, l, m, b;
+    int ret;
 
     /* select decoding table */
     table = ff_mpa_l2_select_table(s->bit_rate / 1000, s->nb_channels,
@@ -617,6 +650,10 @@ static int mp_decode_layer2(MPADecodeContext *s)
         }
     }
 
+    ret = handle_crc(s, get_bits_count(&s->gb) - 16);
+    if (ret < 0)
+        return ret;
+
     /* scale factors */
     for (i = 0; i < sblimit; i++) {
         for (ch = 0; ch < s->nb_channels; ch++) {
@@ -1310,13 +1347,16 @@ static int mp_decode_layer3(MPADecodeContext *s)
     int gr, ch, blocksplit_flag, i, j, k, n, bits_pos;
     GranuleDef *g;
     int16_t exponents[576]; //FIXME try INTFLOAT
+    int ret;
 
     /* read side info */
     if (s->lsf) {
+        ret = handle_crc(s, ((s->nb_channels == 1) ? 8*9  : 8*17));
         main_data_begin = get_bits(&s->gb, 8);
         skip_bits(&s->gb, s->nb_channels);
         nb_granules = 1;
     } else {
+        ret = handle_crc(s, ((s->nb_channels == 1) ? 8*17 : 8*32));
         main_data_begin = get_bits(&s->gb, 9);
         if (s->nb_channels == 2)
             skip_bits(&s->gb, 3);
@@ -1328,6 +1368,8 @@ static int mp_decode_layer3(MPADecodeContext *s)
             s->granules[ch][1].scfsi = get_bits(&s->gb, 4);
         }
     }
+    if (ret < 0)
+        return ret;
 
     for (gr = 0; gr < nb_granules; gr++) {
         for (ch = 0; ch < s->nb_channels; ch++) {
@@ -1565,23 +1607,8 @@ static int mp_decode_frame(MPADecodeContext *s, OUT_INT **samples,
     OUT_INT *samples_ptr;
 
     init_get_bits(&s->gb, buf + HEADER_SIZE, (buf_size - HEADER_SIZE) * 8);
-
-    if (s->error_protection) {
-        uint16_t crc = get_bits(&s->gb, 16);
-        if (s->err_recognition & AV_EF_CRCCHECK) {
-            const int sec_len = s->lsf ? ((s->nb_channels == 1) ? 9  : 17) :
-                                         ((s->nb_channels == 1) ? 17 : 32);
-            const AVCRC *crc_tab = av_crc_get_table(AV_CRC_16_ANSI);
-            uint32_t crc_cal = av_crc(crc_tab, UINT16_MAX, &buf[2], 2);
-            crc_cal = av_crc(crc_tab, crc_cal, &buf[6], sec_len);
-
-            if (av_bswap16(crc) ^ crc_cal) {
-                av_log(s->avctx, AV_LOG_ERROR, "CRC mismatch!\n");
-                if (s->err_recognition & AV_EF_EXPLODE)
-                    return AVERROR_INVALIDDATA;
-            }
-        }
-    }
+    if (s->error_protection)
+        s->crc = get_bits(&s->gb, 16);
 
     switch(s->layer) {
     case 1:
-- 
2.17.1