[PATCH] Give a useful meaning to arc4random_uniform(0);

[Theo de Raadt]

Alejandro Colomar <alx.manpages@gmail.com> wrote:

> Hi Theo,
> 
> On 12/31/22 16:13, Alejandro Colomar wrote:
> > Hi Theo,
> > On 12/31/22 15:56, Alejandro Colomar wrote:
> >>>
> >>> I do not like your proposal at all.  A function like arc4random_range()
> >>> is even more likely to be used wrong by passing backwards points
> >>> and you seem to have a lot of hubris to add a range check to it.
> > I didn't understand the entire sentence, since I'm not a native
> > English speaker.   Sorry for that.  About adding a range check, I'm
> > not against it.  But what to do in that case?  abort()?  I don't see
> > anything significantly better?  In the Linux kernel, the used
> > something BUILD_BUG, but I don't know those macros very much.
> > I'm really open to discussion about what would the the best behavior
> > when max < min.
> 
> Since there's no obvious thing to do when the bounds are reversed
> (some may want to abort(); others may prefer to set errno?; others may
> just want to ignore the possibility...
> 
> I tried to understand what it does with the obvious implementation:
> 
> 
> > Alejandro Colomar <alx.manpages@gmail.com> wrote:
> >> uint32_t
> >> arc4random_range(uint32_t min, uint32_t max)
> >> {
> >> 	return arc4random_uniform(max - min + 1) + min;
> >> }
> 
> Well, let's substitute with some actual values:
> 
> 	arc4random_range(7, 4);
> 
> This will result in:
> 
> 	arc4random_uniform(4 - 7 + 1) + 7;
> 
> which evaluates to:
> 
> 	arc4random_uniform(-2) + 7;
> 
> and is equivalent to:
> 
> 	arc4random_uniform(UINT32_MAX - 1) + 7;
> 
> Let's first ignore the +7.  The arc4random_uniform(UINT32_MAX - 1)
> call can generate 2^32 - 2 random numbers.  By offsetting to 7, the 2
> value that are excluded are 6 and 5.
> 
> So it seems that a reversed call to arc4random_range(min, max) has an
> interesting property: it generates random numbers outside of the
> non-inclusive range (min, max), that is the compementary set of
> numbers that arc4random(max, min) would produce.
> 
> If properly documented, it's not a bad behavior.

With that quality of reasoning, there are only two possible conclusions
about why this email thread is happening.  I though these conclusions
were original, but others privately made these suggestions also: 

1. you are trolling us

2. you are not sufficiently intelligent to be permitted near a C compiler.

I cannot think of a third option.

I am going to give you the benefit of the doubt and select option 1, and
trolls are deal with calling them trolls, and thennever replying again.  Bye.