[webkit-reviews] review requested: [Bug 15778] Public GIF image decoder can corrupt memory on malformed GIFs : [Attachment 16968] patch v1

[bugzilla-daemon at webkit.org]

Peter Kasting <zerodpx at gmail.com> has asked  for review:
Bug 15778: Public GIF image decoder can corrupt memory on malformed GIFs
http://bugs.webkit.org/show_bug.cgi?id=15778

Attachment 16968: patch v1
http://bugs.webkit.org/attachment.cgi?id=16968&action=edit

------- Additional Comments from Peter Kasting <zerodpx at gmail.com>
This patch does a couple of things:
* Explicitly ignores rows past the end of the image in
GIFImageDecoder::haveDecodedRow().  This is similar to the behavior of the
Mozilla image decoder sources, which allow a frame decoder to provide whatever
data it wants, and then simply ignore the excess data when compositing/drawing
the image.
* Removes some broken code in GIFImageReader::output_row() that looks like it
was trying to prevent this case, but didn't really succeed.  This code isn't
present in the Mozilla GIF decoder this file is based on, doesn't work right,
and is significantly more complex than the simple added conditional described
above.
* Fixes some code in GIFImageDecoder::read() that would be hit in this case to
set local variables more correctly.  This doesn't make much difference, but
it's much closer to the original Mozilla code -- it looks like when the file
was ported to WebKit, someone originally misplaced a close brace in here, so
e.g. screen_height would only get reset if screen_width also needed to be
reset.

The current Mozilla sources avoid the "rowbuf" struct entirely, but updating to
that change is both a large patch and not really related to this bug.