linux-kernel - Re: [PATCH] VFS: document what MAY

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Message-Id: <1253536193.17222.21.camel@heimdal.trondhjem.org>
Date:	Mon, 21 Sep 2009 08:29:53 -0400
From:	Trond Myklebust <trond.myklebust@....uio.no>
To:	Jamie Lokier <jamie@...reable.org>
Cc:	Eric Paris <eparis@...hat.com>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, viro@...iv.linux.org.uk,
	hch@...radead.org
Subject: Re: [PATCH] VFS: document what MAY_ACCESS means

On Mon, 2009-09-21 at 09:10 +0100, Jamie Lokier wrote
> (Btw, side issue: I was very surprised to find fchdir() to an open
> directory can fail on NFS due to change of permissions, so the pattern
> dir = open("."); chdir("foo"); fchdir(dir) can fail to restore the
> current directory).

Welcome to the world of stateless server-enforced security. Unlike the
POSIX model, a NFS server doesn't have the ability to track what
permissions have already been checked using a file descriptor. It
therefore needs to check permissions on each RPC operation you perform
using the credential you present then and there.

Cheers
  Trond

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