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[Gnash-dev] What to do when a dynamic cast fail in librender?

From: Petter Reinholdtsen
Subject: [Gnash-dev] What to do when a dynamic cast fail in librender?
Date: Sun, 20 Apr 2014 08:58:01 +0200
User-agent: Mutt/1.5.20 (2009-12-10)


The only Coverity issue reported in librendrer is a potential null
pointer dereference.  I thought I should try to bring down the number
of defects in this module to zero, but I am unable to figure out how
to fix it.  The problem is in the function cairo_pattern_t*
operator()(const BitmapFill& f) const.  There, the dynamic_cast might
fail, but there is no check before using the result.  As far as I can
tell, there is no alternative to throwing an exception to report the
error, but as far as I can see no other part of librendrer throw any
exceptions.  Is it safe to introduce throwing to report the problem?

Here is a sketch for a fix, to give you an idea about the location in
the code:

diff --git a/librender/cairo/Renderer_cairo.cpp 
index c597dd4..2e95686 100644
--- a/librender/cairo/Renderer_cairo.cpp
+++ b/librender/cairo/Renderer_cairo.cpp
@@ -299,6 +299,9 @@ struct StyleHandler : 
         const bitmap_info_cairo* binfo =
             dynamic_cast<const bitmap_info_cairo*>(bm);
+       if (NULL == binfo) { // CID 1154657, binfo used below
+           throw std::bad_cast();
+       }
         cairo_matrix_t mat;
         init_cairo_matrix(&mat, m);       

Happy hacking
Petter Reinholdtsen

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