Subject: Re: [RFC v10][PATCH 09/13] Restore open file descriprtors
From: Dave Hansen <dave@linux.vnet.ibm.com>
To: Oren Laadan <orenl@cs.columbia.edu>
Cc: linux-api@vger.kernel.org, containers@lists.linux-foundation.org,
       linux-kernel@vger.kernel.org, linux-mm@kvack.org,
       Linus Torvalds <torvalds@osdl.org>,
       Thomas Gleixner <tglx@linutronix.de>, Al Viro <viro@ZenIV.linux.org.uk>,
       "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@elte.hu>,
       Andrew Morton <akpm@linux-foundation.org>
In-Reply-To: <49345086.4@cs.columbia.edu>
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	 <20081128112745.GR28946@ZenIV.linux.org.uk>
	 <1228159324.2971.74.camel@nimitz> <49344C11.6090204@cs.columbia.edu>
	 <1228164873.2971.95.camel@nimitz>  <49345086.4@cs.columbia.edu>
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Date: Mon, 01 Dec 2008 17:12:06 -0800
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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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