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Recover from errors in ancillary chunks

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As per the third edition of the spec, decoders should recover from
errors in ancillary chunks.

In section 13.1 Error handling [1]:
> Anomalous situations other than syntax errors shall be treated as
> follows:
>  1. Encountering an unknown ancillary chunk is never an error. The
>     chunk can simply be ignored.

More specifically, in this commit, if a chunk that is detected as
ancillary, does not pass the `check_chunk_name()` function, only a
_benign_ is issued, instead of an error.

This allows libpng to fully decode images like [2] and [3]. It has been
tested by passing them to both pngtest and Gnome's image viewer. Note
that invalid-unknown-ancillary-after-IDAT.png could already be displayed
but not fully decoded.

[1] https://w3c.github.io/png/#13Decoders.Errors
[2] https://github.com/web-platform-tests/wpt/blob/master/png/errors/support/invalid-unknown-ancillary.png
[3] https://github.com/web-platform-tests/wpt/blob/master/png/errors/support/invalid-unknown-ancillary-after-IDAT.png
This commit is contained in:
Lucas CHOLLET 2025-03-03 15:06:40 -07:00
parent 44f97f08d7
commit 34005e3d3d
1 changed files with 6 additions and 2 deletions
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6
pngrutil.c
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@ -211,8 +211,12 @@ png_read_chunk_header(png_structrp png_ptr)
png_chunk_error(png_ptr, "bad header (invalid length)");
/* Check to see if chunk name is valid. */
if (!check_chunk_name(chunk_name))
if (!check_chunk_name(chunk_name)) {
if (PNG_CHUNK_ANCILLARY(png_ptr->chunk_name) == 0)
png_chunk_error(png_ptr, "bad header (invalid type)");
else
png_chunk_benign_error(png_ptr, "bad header (invalid type)");
}
#ifdef PNG_IO_STATE_SUPPORTED
png_ptr->io_state = PNG_IO_READING | PNG_IO_CHUNK_DATA;