Re: [PATCH] Use malloca instead alloca

[Rich Felker <dalias at aerifal dot cx>]

On Fri, Dec 28, 2012 at 06:38:23PM +0100, OndÅej BÃlka wrote:
>   /* malloca(N) is a safe variant of alloca(N).  It allocates N bytes of
>      memory allocated on the stack or heap for large requests.
>      It must be freed using freea() before
>      the function returns.  Upon failure, it returns NULL.  */
> 
> #if 1
> #define malloca(n) ({\
>   void *__r__ = NULL;\
>   if (n < 4096 - 8)\

This comparison is performed without promoting n to size_t. Although
in most correct usages it should not matter, I think this should be
fixed; things like malloca(-1) should fail (or allocate 4gb-1
successfully) rather than succeeding and then causing memory
corruption. Note that n is also being evaluated more than once, so
just storing it in a variable of type size_t first would avoid this
issue too.

Rich