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

[Lynne]

7 Mar 2019, 21:22 by michael at niedermayer.cc:

> On Thu, Mar 07, 2019 at 07:26:32PM +0100, Lynne wrote:
>
>> By default now, if AV_EF_CRCCHECK or AV_EF_IGNORE_ERR are enabled the decoder
>> will skip the chunk and carry on with the next one. This should make the       
>> decoder able to decode more corrupt files because the functions which decode
>> individual chunks will very likely error out if fed invalid data and stop the
>> decoding of the entire image.
>> Should this be made default? CRC verification doesn't take long even for very
>> large files.                                                      
>> Also fix the length check for chunk size. It needs to take into account the
>> 4 byte tag as well as the 4 byte CRC.
>>
>> pngdec.c |   19 ++++++++++++++++++-
>>  1 file changed, 18 insertions(+), 1 deletion(-)
>> 4255c91468cee2bc2fa757fae69762ff5ee5774a  0001-pngdec-add-ability-to-check-chunk-CRC.patch
>> From 7aff99d12faf557753c5ee860a9672c7a09a26e3 Mon Sep 17 00:00:00 2001
>> From: Lynne <>> dev at lynne.ee <mailto:dev at lynne.ee>>> >
>> Date: Thu, 7 Mar 2019 18:15:23 +0000
>> Subject: [PATCH] pngdec: add ability to check chunk CRC
>>
>> By default now, if AV_EF_CRCCHECK or AV_EF_IGNORE_ERR are enabled the decoder
>> will skip the chunk and carry on with the next one. This should make the
>> decoder able to decode more corrupt files because the functions which decode
>> individual chunks will very likely error out if fed invalid data and stop the
>> decoding of the entire image.
>> Should this be made default? CRC verification doesn't take long even for very
>> large files.
>>
>
> i would tend toward enabling it by default but maybe first post some
> numbers of how much this changes decode time 
>

For the largest png I found: https://vk.com/doc218587497_437472325?hash=51300ca9ba40f462ac&dl=1bcf9a57b0d989da1f <https://vk.com/doc218587497_437472325?hash=51300ca9ba40f462ac&dl=1bcf9a57b0d989da1f>

There was no increase in decoding time, it took 2.3 seconds on my machine
with and without -err_detect crccheck.

With -err_detect crccheck perf reported 2.52% spent in av_crc


>> Also fix the length check for chunk size. It needs to take into account the
>> 4 byte tag as well as the 4 byte CRC.
>>
>
> this should be a seperate patch as its unrelated
>

removed and attached new patch file

Maybe always enabling the CRC check isn't worth it since if you download from
the internet you could either get a full error-free file or an incomplete one
rather than a corrupt one. Maybe only for torrents with huge png files where
not all chunks have been downloaded yet, or broken hard drives, but the ignore_err
flag could be manually enabled in those cases.
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