Re: [FFmpeg-devel] [PATCH v3] avcodec/dovi_rpu: verify RPU data CRC32

[quietvoid]

On 26/10/2023 17.44, Michael Niedermayer wrote:

On Wed, Aug 09, 2023 at 01:46:57PM -0400, quietvoid wrote:

The Dolby Vision RPU contains a CRC32 to validate the payload against.
The implementation is CRC32/MPEG-2.

The CRC is only verified with the AV_EF_CRCCHECK flag.

Signed-off-by: quietvoid<tcchlis...@gmail.com>
---
  libavcodec/dovi_rpu.c | 46 ++++++++++++++++++++++++++++++++++++++++---
  libavcodec/dovi_rpu.h |  3 ++-
  libavcodec/hevcdec.c  |  3 ++-
  3 files changed, 47 insertions(+), 5 deletions(-)

diff --git a/libavcodec/dovi_rpu.c b/libavcodec/dovi_rpu.c
index dd38936552..1dfeee7564 100644
--- a/libavcodec/dovi_rpu.c
+++ b/libavcodec/dovi_rpu.c
@@ -22,6 +22,7 @@
   */

#include "libavutil/buffer.h"

+#include "libavutil/crc.h"

#include "dovi_rpu.h"

  #include "golomb.h"
@@ -191,13 +192,17 @@ static inline int64_t get_se_coef(GetBitContext *gb, 
const AVDOVIRpuDataHeader *
          }                                                                     
  \
      } while (0)

-int ff_dovi_rpu_parse(DOVIContext *s, const uint8_t *rpu, size_t rpu_size)

+int ff_dovi_rpu_parse(DOVIContext *s, const uint8_t *rpu, size_t rpu_size,
+                      int err_recognition)
  {
      AVDOVIRpuDataHeader *hdr = &s->header;
      GetBitContext *gb = &(GetBitContext){0};
      DOVIVdrRef *vdr;
      int ret;

+ size_t actual_rpu_size;

+    uint8_t trailing_zeroes = 0;
+
      uint8_t nal_prefix;
      uint8_t rpu_type;
      uint8_t vdr_seq_info_present;
@@ -205,7 +210,22 @@ int ff_dovi_rpu_parse(DOVIContext *s, const uint8_t *rpu, 
size_t rpu_size)
      uint8_t use_prev_vdr_rpu;
      uint8_t use_nlq;
      uint8_t profile;
-    if ((ret = init_get_bits8(gb, rpu, rpu_size)) < 0)
+
+    uint32_t rpu_data_crc32;
+    uint32_t computed_crc32;
+
+    for (int i = rpu_size - 1; i > 0; i--) {
+        if (!rpu[i]) {
+            trailing_zeroes++;
+        } else {
+            break;
+        }
+    }
+
+    actual_rpu_size = rpu_size - trailing_zeroes;
+
+    /* Exclude trailing byte (0x80) from reader */
+    if ((ret = init_get_bits8(gb, rpu, actual_rpu_size - 1)) < 0)
          return ret;

/* RPU header, common values */

@@ -440,7 +460,27 @@ int ff_dovi_rpu_parse(DOVIContext *s, const uint8_t *rpu, 
size_t rpu_size)
          color->source_diagonal = get_bits(gb, 10);
      }

- /* FIXME: verify CRC32, requires implementation of AV_CRC_32_MPEG_2 */

+    if (!(err_recognition & AV_EF_CRCCHECK))
+        return 0;
+
+    /* Skip unsupported until CRC32 */
+    skip_bits_long(gb, get_bits_left(gb) - 32);
+
+    rpu_data_crc32 = get_bits_long(gb, 32);
+
+    /* Verify CRC32, buffer excludes the prefix, CRC32 and trailing byte */
+    computed_crc32 = av_bswap32(av_crc(av_crc_get_table(AV_CRC_32_IEEE),
+                                       -1, rpu + 1, actual_rpu_size - 6));
+
+    if (rpu_data_crc32 != computed_crc32) {
+        av_log(s->logctx, AV_LOG_ERROR,
+               "RPU CRC mismatch! Expected %"PRIu32", received %"PRIu32"\n",
+               rpu_data_crc32, computed_crc32);
+
+        if (err_recognition & AV_EF_EXPLODE)
+            goto fail;
+    }

(correctly designed) CRCs have the beautifull symmetry that you can merge
the crc32 value into the crc computation and then a 0 means no CRC missmatch
(there are many other cool properties but this one allows to simplify the code)

This works too: (and is simpler)

     /* Skip unsupported until CRC32 */
     skip_bits_long(gb, get_bits_left(gb));

     /* Verify CRC32, buffer excludes the prefix, CRC32 and trailing byte */
     computed_crc32 = av_bswap32(av_crc(av_crc_get_table(AV_CRC_32_IEEE),
                                        -1, rpu + 1, actual_rpu_size - 2));

     if (computed_crc32) {
         av_log(s->logctx, AV_LOG_ERROR, "RPU CRC mismatch! %"PRIX32"\n",
                computed_crc32);

         if (err_recognition & AV_EF_EXPLODE)
             goto fail;
     }

Hi Michael. I like the idea and it's a cool property.

However the then printed CRC on mismatch is not a useful value, so I'm
unsure if it's better to simplify here.
I like having the expected CRC logged here.

Thanks.

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