[FFmpeg-trac] #11012(avformat:new): APNG decoder incorrectly assumes fcTL chunk must be immediately followed by fdAT or IDAT

[FFmpeg]

#11012: APNG decoder incorrectly assumes fcTL chunk must be immediately followed by
fdAT or IDAT
----------------------------------+---------------------------------------
             Reporter:  tobbez    |                     Type:  defect
               Status:  new       |                 Priority:  normal
            Component:  avformat  |                  Version:  unspecified
             Keywords:            |               Blocked By:
             Blocking:            |  Reproduced by developer:  0
Analyzed by developer:  0         |
----------------------------------+---------------------------------------
 The APNG decoder (more specifically `apng_read_packet` in
 libavformat/apngdec.c) incorrectly assumes an `fcTL` chunk must be
 ''immediately'' followed by either an `fdAT` or an `IDAT` chunk.

 In addition, when the `fdAT`/`IDAT` check fails, the input stream is not
 rewinded to the start of the chunk that follows the `fcTL` chunk, causing
 misleading log messages: "tag=ware (0x65726177) len=1399809652" in the
 output below ought to be "tag=tEXt (0x74584574) len=16".

 An example file can be created using exiftool on an existing APNG file
 (`exiftool -Software=example apng.png`). This produces a file where the
 first `fcTL` chunk is followed by a `tEXt` chunk. Such an example will be
 attached.

 How to reproduce:
 {{{
 % ffmpeg -i example.png -f null /dev/null
 ffmpeg version N-115183-g02c032abcd Copyright (c) 2000-2024 the FFmpeg
 developers
   built with gcc 13 (13.2.1_p20240503 p15)
   configuration:
   libavutil      59. 17.100 / 59. 17.100
   libavcodec     61.  5.103 / 61.  5.103
   libavformat    61.  3.103 / 61.  3.103
   libavdevice    61.  2.100 / 61.  2.100
   libavfilter    10.  2.101 / 10.  2.101
   libswscale      8.  2.100 /  8.  2.100
   libswresample   5.  2.100 /  5.  2.100
 [apng @ 0x55841a01a7c0] Could not find codec parameters for stream 0
 (Video: apng, none, 64x64): unspecified pixel format
 Consider increasing the value for the 'analyzeduration' (0) and
 'probesize' (5000000) options
 Input #0, apng, from 'example.png':
   Duration: N/A, bitrate: N/A
   Stream #0:0: Video: apng, none, 64x64, 100k tbr, 100k tbn
 Stream mapping:
   Stream #0:0 -> #0:0 (apng (native) -> wrapped_avframe (native))
 Press [q] to stop, [?] for help
 [apng @ 0x55841a01a7c0] In-stream tag=ware (0x65726177) len=1399809652 is
 not implemented. Update your FFmpeg version to the newest one from Git. If
 the problem still occurs, it means that your file has a feature which has
 not been implemented.
 [apng @ 0x55841a01a7c0] If you want to help, upload a sample of this file
 to https://streams.videolan.org/upload/ and contact the ffmpeg-devel
 mailing list. (ffmpeg-devel at ffmpeg.org)
 [in#0/apng @ 0x55841a01a500] Error during demuxing: Not yet implemented in
 FFmpeg, patches welcome
 Cannot determine format of input 0:0 after EOF
 [vf#0:0 @ 0x55841a01cb80] Task finished with error code: -1094995529
 (Invalid data found when processing input)
 [vf#0:0 @ 0x55841a01cb80] Terminating thread with return code -1094995529
 (Invalid data found when processing input)
 [vost#0:0/wrapped_avframe @ 0x55841a01cfc0] Could not open encoder before
 EOF
 [vost#0:0/wrapped_avframe @ 0x55841a01cfc0] Task finished with error code:
 -22 (Invalid argument)
 [vost#0:0/wrapped_avframe @ 0x55841a01cfc0] Terminating thread with return
 code -22 (Invalid argument)
 [out#0/null @ 0x55841a01c940] Nothing was written into output file,
 because at least one of its streams received no packets.
 frame=    0 fps=0.0 q=0.0 Lsize=       0KiB time=N/A bitrate=N/A speed=N/A
 Conversion failed!
 }}}
-- 
Ticket URL: <https://trac.ffmpeg.org/ticket/11012>
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