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Revision as of 01:56, 10 June 2007 by UowNlw (talk | contribs)
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I've come across a subtle bug that could be in many places in the sources. Basically, any comparrisons of booleans might be unsafe.

I first ran across it in sp_document_set_undo_sensitive in document-undo.c. In it, the param 'sensitive' is of type gboolean. At one point it is compared with doc->priv->sensitive. Ok, things seem fine so far... however...

The specific types in question come to contribute to the problem. Since the code is C and not C , it uses gboolean instead of bool (of course, since bool doesn't really exist in plain C). gboolean, in turn, lives in glib's includes (a good place to get things from) and is typedef'd as gint.

That's where the problems come in. Since it's an int, not a true bool, any non-zero value is taken as true, and any zero value is taken as false. So far that seems OK, but let's take a quick look at some possible values:

0 == false
1 == true
-1 == true
$ffffffff == true
~0 == true
4 == true

Now, given all that, this next line of code clearly has some issues:

if (sensitive == doc->priv->sensitive) return;

Let's say that 'sensitive' is set to '~0' (all bits set. a nice safe non-false value and often used for TRUE), and that doc->priv->sensitive is set to 1. Now both integer values represent 'true', however they are not actually equal to each other, and that check will fail even when both variables are not false.


Therefore, anywhere two boolean-ish variables are compared for equality or inequality they will not always evaluate properly. Additionally, since these are just int values of one type or another, they could be all over the codebase yet not noticed. (There's not something simple and definitive that I can grep all of the code for, otherwise I would have tried to find all of them myself).

So the solution would seem to be to have all eyes watching for potential problem cases and then fixing them as they are encountered.

However, there is some good news. Doing a check of a single gboolean variable is safe, since all non-zero values will be treated as true, code will work as expected. It's only the case where two values are compared that can have problems.

if ( sensitive ) // works
if ( !sensitive ) // works
if ( sensitive