fix off-by-one in dup2

Corinna Vinschen corinna-cygwin@cygwin.com
Wed Dec 4 09:32:00 GMT 2013


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.


Corinna

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