Re: [patch] fix divide by zero in random/taviso()

On Tue, Jan 21, 2014 at 11:03:43AM -0800, Andrew Honig wrote:
 
 > >  > In random.c the taviso() routine uses "rand() % rand()" which
 > >  > can cause a floating point exception if rand() returns 0.
 > >  > Add some tests to make sure that doesn't happen.
 > >  >
 > >  > Signed-off-by: Vince Weaver <[email protected]>
 > >  >
 > >  > diff --git a/random.c b/random.c
 > >  > index 7def9ff..a71fcea 100644
 > >  > --- a/random.c
 > >  > +++ b/random.c
 > >  > @@ -42,6 +42,7 @@ static unsigned long randbits(int limit)
 > >  >  static unsigned long taviso(void)
 > >  >  {
 > >  >      unsigned long r = 0;
 > >  > +    unsigned long temp;
 > >  >
 > >  >      switch (rand() % 4) {
 > >  >      case 0: r = rand() & rand();
 > >  > @@ -51,10 +52,14 @@ static unsigned long taviso(void)
 > >  >  #endif
 > >  >              break;
 > >  >
 > >  > -    case 1: r = rand() % rand();
 > >  > +    case 1: temp = rand();
 > >  > +            r = rand();
 > >
 > 
 > Perhaps, I'm reading this wrong, but this looks backwards to me.  This will
 > perform the mod operation only when temp is 0.  Shouldn't this be
 > if (temp) r %= temp;

clang seems to agree.

random.c:57:16: warning: Division by zero
                if (!temp) r %= temp;
                           ~~^~~~~~~
random.c:62:26: warning: Division by zero
                if (!temp) r |= rand() % temp;
                                ~~~~~~~^~~~~~

Vince, want to send a follow-up fix ?

	Dave