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

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Message-ID: <49345086.4@cs.columbia.edu>
Date:	Mon, 01 Dec 2008 16:00:54 -0500
From:	Oren Laadan <orenl@...columbia.edu>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
CC:	linux-api@...r.kernel.org, containers@...ts.linux-foundation.org,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Linus Torvalds <torvalds@...l.org>,
	Al Viro <viro@...IV.linux.org.uk>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC v10][PATCH 09/13] Restore open file descriprtors



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.

Oren.

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