Re: fix off-by-one in dup2

[Corinna Vinschen <corinna-cygwin@cygwin.com>]

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On Dec  4 10:32, Corinna Vinschen wrote:
> Hi guys,
> 
> 
> I'm not quite sure yet *why* this happens, but this change in
> dtable::find_unused_handle...
> 
> On Sep 25 17:26, Eric Blake wrote:
> > [...]
> > diff --git i/winsup/cygwin/dtable.cc w/winsup/cygwin/dtable.cc
> > index 2501a26..c2982a8 100644
> > --- i/winsup/cygwin/dtable.cc
> > +++ w/winsup/cygwin/dtable.cc
> > @@ -233,7 +233,7 @@ dtable::find_unused_handle (int start)
> >  	if (fds[i] == NULL)
> >  	  return i;
> >      }
> > -  while (extend (NOFILE_INCR));
> > +  while (extend (MAX (NOFILE_INCR, start - size)));
> >    return -1;
> >  }
> 
> ...introduced the problem reported in
> http://cygwin.com/ml/cygwin/2013-12/msg00072.html
> 
> The problem is still present in the current sources.
> 
> If I apply this change...
> 
> Index: dtable.cc
> ===================================================================
> RCS file: /cvs/src/src/winsup/cygwin/dtable.cc,v
> retrieving revision 1.275
> diff -u -p -r1.275 dtable.cc
> --- dtable.cc	1 Dec 2013 19:17:56 -0000	1.275
> +++ dtable.cc	4 Dec 2013 09:26:01 -0000
> @@ -223,7 +223,8 @@ dtable::delete_archetype (fhandler_base 
>  int
>  dtable::find_unused_handle (size_t start)
>  {
> -  size_t extendby = (start >= size) ? 1 + start - size : NOFILE_INCR;
> +  //size_t extendby = (start >= size) ? 1 + start - size : NOFILE_INCR;
> +  size_t extendby = NOFILE_INCR;
>  
>    /* This do loop should only ever execute twice. */
>    int res = -1;
> 
> 
> ..., which essentially reverts the original change from Eric, the
> problem is fixed.
> 
> Off the top of my head I don't understand why Eric's as well as cgf's
> solution (which are not equivalent) both introduce this problem, but
> always using NOFILE_INCR works, so I publish it here for discussion.
> 
> I'm off for a doc appointment now, maybe I have some clue while sitting
> in the anteroom.

Not really.  Btw., this helps to fix the problem as well:

  size_t extendby = (start >= size) ? MAX (1 + start - size, NOFILE_INCR)
				    : NOFILE_INCR;


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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