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Date: Thu, 19 Jun 2003 13:02:06 +0400
From: Dmitry Sivachenko <demon@freebsd.org>
To: Gary Jennejohn <garyj@jennejohn.org>
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cc: hackers@freebsd.org
Subject: Re: struct ipc_perm
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On Thu, Jun 19, 2003 at 10:56:41AM +0200, Gary Jennejohn wrote:
> 
> Dmitry Sivachenko writes:
> > On Wed, Jun 18, 2003 at 06:08:37PM +0200, Gary Jennejohn wrote:
> > > 
> > > Dmitry Sivachenko writes:
> > > > Hello!
> > > > 
> > > > Is there any reason why struct ipc_perm is not protected by #ifdef _KERNE
> > L
> > > > in ipc.h?  Is it supposed to be used from userland?
> > > > 
> > > 
> > > It's needed by ipcs.
> > > 
> > 
> > Ah, I see.  It is visible via struct msqid_ds.
> > 
> > I developed a patch which requires addition of custom field to ipc_perm.
> > I am trying to imagine which problems can it cause to userland programs.
> > 
> > Any ideas?
> > 
> 
> The usual way to handle this sort of change is to put any new structure
> elements at the end so that existing applications don't get confused.
> They simply aren't aware the new elements were added.

Yes, that is exactly how I did it.


> 
> Of course, this can cause problems when the kernel does a copyout()
> using the new size but the application passed a pointer to
> storage which can only hold the old, smaller structure.

I didn't guess that.  Probably this is why X-server crashed on my machine
after that change.  Recompilation fixed the problem.

Thank you.