svn commit: r300377 - head/sys/compat/ndis

[Conrad Meyer]

On Sat, May 21, 2016 at 10:52 AM, Pedro F. Giffuni <pfg at freebsd.org> wrote:
> Author: pfg
> Date: Sat May 21 17:52:44 2016
> New Revision: 300377
> URL: https://svnweb.freebsd.org/changeset/base/300377
>
> Log:
>   ndis(4): Avoid overflow.
>
>   This is a long standing problem: our random() function returns an
>   unsigned integer but the rand provided by ndis(4) returns an int.
>   Scale it down.
>
>   MFC after:    2 weeks
>
> Modified:
>   head/sys/compat/ndis/subr_ntoskrnl.c
>
> Modified: head/sys/compat/ndis/subr_ntoskrnl.c
> ==============================================================================
> --- head/sys/compat/ndis/subr_ntoskrnl.c        Sat May 21 17:38:43 2016        (r300376)
> +++ head/sys/compat/ndis/subr_ntoskrnl.c        Sat May 21 17:52:44 2016        (r300377)
> @@ -3189,7 +3189,7 @@ static int
>  rand(void)
>  {
>
> -       return (random());
> +       return (random() / 2 + 1);
>  }
>
>  static void
>


Won't this still return a negative integer in many cases?

random(9) returns u_long, whereas this rand() routine returns 'int'.

Even on architectures where long is the same size as ordinary
integers, the range of possible results of the 'random() / 2 + 1'
expression, before implicit cast to signed, is [1, 2^31] (inclusive).
2^31 is not representable by typical signed 32-bit integers, so this
will wrap to INT_MIN.  Also, I'm not sure why zero is excluded from
the range.

On architectures where long is larger than ordinary integers, this
expression has no hope of fitting in the non-negative range of a
signed integer.

Why not instead:

return ((u_int)random() / 2);

Best,
Conrad