Re: Possible bug report - nmap scan elapsed time changes into negative time

[David Fifield <david () bamsoftware com>]

On Sun, Mar 27, 2011 at 09:36:18PM -0500, Daniel Miller wrote:
> On 3/27/11, David Fifield <david () bamsoftware com> wrote:
> > On Fri, Mar 25, 2011 at 09:01:47AM -0500, Daniel Miller wrote:
> > > I looked into this some more. The particular function that affects
> > > that output line is NmapOps::TimeSinceStartMS, which returns an int
> > > generated with the TIMEVAL_MSEC_SUBTRACT macro #defined in
> > > nbase/nbase.h:
> > >
> > > #define TIMEVAL_MSEC_SUBTRACT(a,b) ((((a).tv_sec - (b).tv_sec) * 1000)
> > > + ((a).tv_usec - (b).tv_usec) / 1000)
> > >
> > > The overflow happens when the difference in seconds is multiplied by
> > > 1000. Interestingly, the TIMEVAL_SEC_SUBTRACT macro:
> > >
> > > #define TIMEVAL_SEC_SUBTRACT(a,b) ((a).tv_sec - (b).tv_sec +
> > > (((a).tv_usec < (b).tv_usec) ? - 1 : 0))
> > >
> > > returns seconds, and does not suffer from this overflow. A quick grep
> > > through the source for calls to TimeSinceStartMS:
> > >
> > > $ find . \( -name .svn -prune -false \) -o -print0 | xargs -0 grep -H
> > > --color TimeSinceStartMS
> > >
> > > shows that in only one case is the return value NOT divided by 1000.0
> > > (which casts it to a float). The one case is an assignment to the
> > > starttimems attribute of the OsScanInfo class, which is only used one
> > > place (divided by 1000.0).
> > >
> > > I'm attaching a patch that adds a function, NmapOps::TimeSinceStartS,
> > > which returns a float. It does this using a new macro,
> > > TIMEVAL_FSEC_SUBTRACT, that results in a floating-point difference in
> > > seconds, which is the most common case. After converting all the calls
> > > to use this function, I can no longer find any calls to
> > > TimeSinceStartMS, but I left it in anyway. I tested this patch with a
> > > -A -T5 -vv -dd scan of my /24 subnet, and saw no issues.
> >
> > Thanks Daniel, this is a good idea. Make a few changes and I'll commit
> > it:
> >     Rename TimeSinceStartS to TimeSinceStart.
> >     Rename starttimes to starttime.
> >     Remove TimeSinceStartMS.
> >
> > David Fifield
> I made the changes you suggested, and attached the patch.

Thanks, committed just now. Good work.

David Fifield
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