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Date: Mon, 5 Nov 2007 14:29:38 +0100
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: cygwin stable and cvs snapshot - fork() bug
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On Nov  5 07:43, Lev Bishop wrote:
> On 11/5/07, Corinna Vinschen  wrote:
> > Ouch, ouch, ouch.  shmctl(IPC_RMID) closed the handle to the shared
> > memory, but neglected to remove the actual mappings as well as the
> > bookkeeping structure.  The result is that after a fork the child thinks
> > there are still mappings which have to be duplicated into its own
> > memory.  But the handle has already been closed in the parent, so the
> > MapViewOfFile call fails with "invalid handle".
> >
> > Unfortunately not many applications use shmctl(IPC_RMID) before creating
> > a child process since usually the shared memory is meant to be... well,
> > shared.  That's why this didn't crop up more often, obviously.
> 
> Are you sure that you're interpreting IPC_RMID correctly? My
> understanding is that you can still share the memory until you
> actually remove the mapping. (Sort of like how you can unlink() a temp
> file immediately after you open it, and continue to use it). I assumed
> this was the reason for the create-map-remove pattern used by mpd.
> 
> From the linux man page:
>        IPC_RMID    Mark the segment to be destroyed.  The  segment  will  only
>                    actually  be  destroyed  after the last process detaches it
>                    (i.e., when the shm_nattch member of the associated  strucā
>                    ture  shmid_ds  is  zero).  The caller must be the owner or
>                    creator, or be privileged.  If a segment  has  been  marked
>                    for  destruction,  then the (non-standard) SHM_DEST flag of
>                    the shm_perm.mode field in the  associated  data  structure
>                    retrieved by IPC_STAT will be set.

Well, when I created the patch, I had read the SUSv3 man page
http://www.opengroup.org/onlinepubs/009695399/functions/shmctl.html.
It says something different:

  IPC_RMID
    Remove the shared memory identifier specified by shmid from the
    system and destroy the shared memory segment and shmid_ds data
    structure associated with it. IPC_RMID can only be executed by a
    process that has an effective user ID equal to either that of a
    process with appropriate privileges or to the value of shm_perm.cuid
    or shm_perm.uid in the shmid_ds data structure associated with
    shmid.

OTOH, the implementation within cygserver is more along the lines of
the Linux manpage.  Oh well.  I guess I'll have a look into implementing
it more Linux-like.


Corinna

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

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