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Re: inode open

On Wed, Jul 16, 2008 at 05:18:47PM -0400, Steven M. Bellovin wrote:
> On Wed, 16 Jul 2008 13:34:34 -0700
> Jason Thorpe <> wrote:
> > 
> > On Jul 16, 2008, at 7:47 AM, Matt Thomas wrote:
> > 
> > > Besides the fhopen(2) previously mentioned, this isn't available  
> > > because it would break the security used by unix.
> > 
> > 
> > Other Unix-like platforms (Mac OS X) can do this without breaking
> > the Unix security model.  We should be able to, too.
> > 
> I'm curious how they do it.  Today, I can safely have a mode 666 file
> inside a 700 directory.  A setuid program can cd to that directory,
> surrender privilege, and then operate on the files.  The real user
> can't get to that directory, and hence can't touch the files -- but if
> it could open things by i-node number, it could.  (I first saw that
> technique used in an old MTA, MMDF, circa 1979.)

MacOS's open-by-inode gets the vnode, gets the path, and makes sure the 
user can access the path. At present, I believe it only tries one path, so 
you might get odd results w/ hard links. But then again hardlinks + a 
security environment where you really care about directory access 
permissions is dodgy. It might work fine, it might not. :-)

Take care,


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