linux-kernel - Re: [RFC v10][PATCH 09/13] Restore open file descriprtors

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Message-Id: <1228180326.2971.128.camel@nimitz>
Date:	Mon, 01 Dec 2008 17:12:06 -0800
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Oren Laadan <orenl@...columbia.edu>
Cc:	linux-api@...r.kernel.org, containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Linus Torvalds <torvalds@...l.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Al Viro <viro@...IV.linux.org.uk>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC v10][PATCH 09/13] Restore open file descriprtors

On Mon, 2008-12-01 at 16:00 -0500, Oren Laadan wrote:
> Dave Hansen wrote:
> > On Mon, 2008-12-01 at 15:41 -0500, Oren Laadan wrote:
> >>>>> +   fd = cr_attach_file(file);      /* no need to cleanup 'file' below */
> >>>>> +   if (fd < 0) {
> >>>>> +           filp_close(file, NULL);
> >>>>> +           ret = fd;
> >>>>> +           goto out;
> >>>>> +   }
> >>>>> +
> >>>>> +   /* register new <objref, file> tuple in hash table */
> >>>>> +   ret = cr_obj_add_ref(ctx, file, parent, CR_OBJ_FILE, 0);
> >>>>> +   if (ret < 0)
> >>>>> +           goto out;
> >>>> Who said that file still exists at that point?
> >> Correct. This call should move higher up befor ethe call to cr_attach_file()
> > 
> > Is that sufficient?  It seems like we're depending on the fd's reference
> > to the 'struct file' to keep it valid in the hash.  If something happens
> > to the fd (like the other thread messing with it) the 'struct file' can
> > still go away.
> > 
> > Shouldn't we do another get_file() for the hash's reference?
> 
> When a shared object is inserted to the hash we automatically take another
> reference to it (according to its type) for as long as it remains in the
> hash. See:  'cr_obj_ref_grab()' and 'cr_obj_ref_drop()'.  So by moving that
> call higher up, we protect the struct file.

We also need to document that we depend on this reference in the hash to
keep the object around.  Take a look at cr_read_fd_data().  Once that
cr_attach_file() has been performed, the only thing keeping the 'file'
around is the hash reference.  If someone happened to remove it from the
hash, the vfs_llseek() below would be bogus.

I don't know how we document that the hash is one-way: writes only and
no later deletions.

-- Dave

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