Re: New GNU C Library (glibc) security flaw reported on 30 Jan 2024

[Vincent Lefevre <vincent@vinc17.net>]

On 2024-02-01 02:47:18 +0800, Xi Ruoyao wrote:
> On Wed, 2024-01-31 at 12:52 -0300, Adhemerval Zanella Netto wrote:
> 
> /* snip */
> 
> > 
> > I see this is an manual issue rather than a GNU 'extension' to qsort semantic.
> > And I think we should fix BZ#31322 by using a transitive comparison instead of
> > trying to support such cases.
> 
> To me the documentation is correct (though arguably in a very subtle
> way):
> 
>    Here is an example of a comparison function which works with an array
> of numbers of type ‘double’:
> 
>      int
>      compare_doubles (const void *a, const void *b)
>      {
>        const double *da = (const double *) a;
>        const double *db = (const double *) b;
> 
>        return (*da > *db) - (*da < *db);
>      }
> 
> It says "numbers."  But NaN literally means, "Not a Number."

Yes, the point is to sort numbers. But since NaN may occur, the code
must not yield undefined behavior in such a case. This is the goal
of NaN: avoid undefined behavior for operations that do not make any
sense, and be able to detect errors at the end.

-- 
Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)