[FFmpeg-devel] [PATCH] apedec: add ability to check CRC

[Lynne]

Apr 4, 2019, 10:30 AM by michael at niedermayer.cc:

> On Wed, Mar 06, 2019 at 02:47:37PM +0100, Lynne wrote:
>
>> 6 Mar 2019, 11:22 by >> dev at lynne.ee <mailto:dev at lynne.ee>>> :
>>
>> > The CRC flag is only signalled once every few minutes but CRC is still
>> > always present so the patch uses the file version instead.
>> > CRC on 24-bit files wants non-padded samples so skip such files.
>> > Some corrupt samples may have been output before the final check
>> > depending on the -max_samples setting.
>> >
>> v2 attached
>>
>> apedec.c |   26 +++++++++++++++++++++++++-
>>  1 file changed, 25 insertions(+), 1 deletion(-)
>> c0cc550d77927ee0349094a4976f73d0ef671ff6  0001-apedec-add-ability-to-check-CRC-v2.patch
>> From 68c25bb026761eacda3c276148e2beb34e8929fd Mon Sep 17 00:00:00 2001
>> From: Lynne <>> dev at lynne.ee <mailto:dev at lynne.ee>>> >
>> Date: Wed, 6 Mar 2019 11:01:01 +0000
>> Subject: [PATCH v2] apedec: add ability to check CRC
>>
>> The CRC flag is only signalled once every few minutes but CRC is still
>> always present so the patch uses the file version instead.
>> CRC on 24-bit files wants non-padded samples so skip such files.
>> Some corrupt samples may have been output before the final check
>> depending on the -max_samples setting.
>> ---
>>  libavcodec/apedec.c | 26 +++++++++++++++++++++++++-
>>  1 file changed, 25 insertions(+), 1 deletion(-)
>>
>> diff --git a/libavcodec/apedec.c b/libavcodec/apedec.c
>> index 15eb416ba4..a20d884606 100644
>> --- a/libavcodec/apedec.c
>> +++ b/libavcodec/apedec.c
>> @@ -24,6 +24,7 @@
>>  
>>  #include "libavutil/avassert.h"
>>  #include "libavutil/channel_layout.h"
>> +#include "libavutil/crc.h"
>>  #include "libavutil/opt.h"
>>  #include "lossless_audiodsp.h"
>>  #include "avcodec.h"
>> @@ -147,7 +148,8 @@ typedef struct APEContext {
>>  int fset;                                ///< which filter set to use (calculated from compression level)
>>  int flags;                               ///< global decoder flags
>>  
>> -    uint32_t CRC;                            ///< frame CRC
>> +    uint32_t CRC;                            ///< signalled frame CRC
>> +    uint32_t CRC_state;                      ///< accumulated CRC
>>  int frameflags;                          ///< frame flags
>>  APEPredictor predictor;                  ///< predictor used for final reconstruction
>>  
>> @@ -730,6 +732,7 @@ static int init_entropy_decoder(APEContext *ctx)
>>  
>>  /* Read the frame flags if they exist */
>>  ctx->frameflags = 0;
>> +    ctx->CRC_state = UINT32_MAX;
>>  if ((ctx->fileversion > 3820) && (ctx->CRC & 0x80000000)) {
>>  ctx->CRC &= ~0x80000000;
>>  
>> @@ -1548,6 +1551,27 @@ static int ape_decode_frame(AVCodecContext *avctx, void *data,
>>  
>>  s->samples -= blockstodecode;
>>  
>> +    if ((avctx->err_recognition & AV_EF_CRCCHECK) &&
>>
>> +        (s->fileversion >= 3900) && (s->bps < 24)) {
>>
>
> what about file versions other than this ? do they have no crc ?
>
They have a different CRC version which is poorly documented.



>
> what about othet bps ?
>
24 bit CRC needs to ignore the zeroes padding each sample in 32 bits. I couldn't find any 24-bit files so I left that for a future patch.



>> +        uint32_t crc = s->CRC_state;
>> +        const AVCRC *crc_tab = av_crc_get_table(AV_CRC_32_IEEE_LE);
>> +        for (i = 0; i < blockstodecode; i++) {
>> +            for (ch = 0; ch < s->channels; ch++) {
>> +                uint8_t *smp = frame->data[ch] + (i*(s->bps >> 3));
>> +                crc = av_crc(crc_tab, crc, smp, s->bps >> 3);
>> +            }
>> +        }
>> +
>> +        if (!s->samples && ((~crc >> 1) ^ s->CRC)) {
>>
>> +            av_log(avctx, AV_LOG_ERROR, "CRC mismatch! Previously decoded "
>> +                   "frames may have been affected as well.\n");
>>
>
> What is the usecase for this ?
> the implementation does only check CRC for a subset of files and it could
> report them at a time different from where they occur
>
The use case is to be able to verify the integrity of files. Other lossless codecs allow to do that, like wavpack and tta.
The subset of files is increasing and is large as the version of the encoder producing them isn't new.
The time of reporting depends on the -max_samples setting or the encoder's choice of samples in a packet. For an integrity check the report time shouldn't matter.