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Re: inode open

Bill Stouder-Studenmund wrote:
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,

Trying to look at MacOS right now, but can't find a function to open a file by inode number. What's the name of the function?
(I'm just curious...)


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