Bug#794947: manpages-dev: printf(3) example: possible integer overflow

["Stéphane Aulery" <saulery@legtux.org>]

retitle 794947 printf(3): possible integer overflow in make_message 
example
severity 794947 wishlist
tags 794947 + confirmed
tags 794947 + upstream
forwarded 794947 linux-man@vger.kernel.org
stop

-----

Hello Walter,

Jakub Wilk reported a possible integer overflow in make_message example 
:

> The example in the printf(3) manpages looks like this (with boring 
> parts
> omitted):
> 
> int n;
> /* ... */
>   n = vsnprintf(p, size, fmt, ap);
>    /* ... */
>    if (n < 0) {
>        /* ... */
>        return NULL;
>    }
>    /* ... */
>    size = n + 1;
> 
> 
> But vsnprintf could return INT_MAX, which would then cause "n + 1" to
> overflow.
> 
> (AFAICS, the glibc vsnprintf implementation never returns INT_MAX, but
> it could in principle.)
> 
> I'd suggest changing "n < 0" to "n < 0 || n == INT_MAX".


Since this example has been modified by you (Walter Harms), after the 
bug #794947 [1] has been reported, I wanted to ask your opinion on the 
best option.

Should we add this test to good practice, or rather a comment to mention 
that the case is not taken into account because the example uses glibc?

Regards,

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=794947

-- 
Stéphane Aulery