beauty of a cast example

[Dean Gaudet <dg...@arctic.org>]

This came out of a long thread on linux-kernel that happened right around
the same time that egcs started giving signed/unsigned warnings as part of
-Wall... much like msc is doing to us right now. 

Dean

> From: Theodore Y. Ts'o (tytso@MIT.EDU)
> Date: Wed, 24 Sep 1997 13:08:19 -0400 
> 
> In my experience it means that programmers start to figure C is
> so dumb they have to explicitly cast *everything* - which not
> only makes it impossible to read their code but ensures neither
> the compiler nor the human has the faintest idea whether the
> programmer *really* intended a type conversion or if there is
> a bug lurking. Not good.
> 
> My favorite example of this was from a version of a very well known
> piece of security software, by a famous software company that was
> purchased by an even larger computer company in the past year or so.
> 
> One early version of the software had a piece of code that looked
> something like this:
> 
> struct key {
>     unsigned int length;
>     char *data;
> }
> 
> unsigned char get_random_byte();
> 
> init_random_key(struct key *key)
> {
>     int i;
> 
>     for (i=0; i < key->length; i++)
>         key->data[i] = (char) get_random_byte;
> }
> 
> Now ---- trick question ---- what's wrong with the above code?
> 
> The result was the public key was initialized to constant value, and the
> fact that a cast was used completely hid the problem from the compiler.
> As you might imagine, this had a pretty horrific impact on the security
> of the program! The way it was caught was purely by luck --- a
> developer was looking at the actual value of the key while debugging a
> completely unrelated problem, and thought it odd that every single byte
> of the key was identical....
> 
> Moral of the story? Casts are evil, and should be avoided whenever
> possible. If you must use a cast, ***think*** before throwing it in.
> A cast bypasses all of the compiler's type warnings, and in some cases
> leave you with some very subtle bugs.
> 
> - Ted