linux-kernel - Re: [RFC v6 03/40] vfs: Add MAY_DELETE_SELF and MAY_DELETE

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Date:	Fri, 28 Aug 2015 17:57:58 -0400
From:	bfields@...ldses.org (J. Bruce Fields)
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Andreas Gruenbacher <andreas.gruenbacher@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	linux-nfs@...r.kernel.org, Linux API <linux-api@...r.kernel.org>,
	linux-cifs@...r.kernel.org,
	LSM List <linux-security-module@...r.kernel.org>,
	Andreas Gruenbacher <agruenba@...hat.com>
Subject: Re: [RFC v6 03/40] vfs: Add MAY_DELETE_SELF and MAY_DELETE_CHILD
 permission flags

On Fri, Aug 28, 2015 at 02:36:15PM -0700, Andy Lutomirski wrote:
> On Tue, Aug 4, 2015 at 4:53 AM, Andreas Gruenbacher
> <andreas.gruenbacher@...il.com> wrote:
> > Normally, deleting a file requires write and execute access to the parent
> > directory.  With Richacls, a process with MAY_DELETE_SELF access to a file
> > may delete the file even without write access to the parent directory.
> >
> > To support that, pass the MAY_DELETE_CHILD mask flag to inode_permission()
> > when checking for delete access inside a directory, and MAY_DELETE_SELF
> > when checking for delete access to a file itelf.
> >
> > The MAY_DELETE_SELF permission does not override the sticky directory
> > check.  It probably should.
> 
> Silly question from the peanut gallery: is there any such thing as
> opening an fd pointing at a file such that the "open file description"
> (i.e. the struct file) captures the right to delete the file?
> 
> IOW do we need FMODE_DELETE_SELF?

I guess FMODE_READ and _WRITE make sense because we pass file
descriptors to read() and write().  But we don't have a way to pass a
file descriptor to an operation that deletes a file.

(I think Windows may be different in both respects, it might be
interesting to compare, but I really don't understand how it works...).

--b.
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