linux-kernel - Re: [PATCH][SMACK] convert smack rule list to linux list

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Date:	Sun, 22 Feb 2009 10:25:15 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Casey Schaufler <casey@...aufler-ca.com>
Cc:	etienne <etienne.basset@...ericable.fr>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	paul.moore@...com, linux-security-module@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH][SMACK] convert smack rule list to linux list

On Sun, Feb 22, 2009 at 09:54:00AM -0800, Casey Schaufler wrote:
> Paul E. McKenney wrote:
> > On Sun, Feb 22, 2009 at 02:14:38PM +0100, etienne wrote:
> >   
> >> Tetsuo Handa wrote:
> >>     
> >>> etienne wrote:
> >>>       
> >>>> diff --git a/security/smack/smack_access.c b/security/smack/smack_access.c
> >>>> index 2e0b83e..3dc312d 100644
> >>>> --- a/security/smack/smack_access.c
> >>>> +++ b/security/smack/smack_access.c
> >>>> @@ -87,7 +87,6 @@ static u32 smack_next_secid = 10;
> >>>>  int smk_access(char *subject_label, char *object_label, int request)
> >>>>  {
> >>>>  	u32 may = MAY_NOT;
> >>>> -	struct smk_list_entry *sp;
> >>>>  	struct smack_rule *srp;
> >>>>  
> >>>>  	/*
> >>>> @@ -139,8 +138,8 @@ int smk_access(char *subject_label, char *object_label, int request)
> >>>>  	 * access (e.g. read is included in readwrite) it's
> >>>>  	 * good.
> >>>>  	 */
> >>>> -	for (sp = smack_list; sp != NULL; sp = sp->smk_next) {
> >>>> -		srp = &sp->smk_rule;
> >>>> +
> >>>> +	list_for_each_entry(srp, &smack_rule_list, list) {
> >>>>  
> >>>>  		if (srp->smk_subject == subject_label ||
> >>>>  		    strcmp(srp->smk_subject, subject_label) == 0) {
> >>>>         
> >>> Use of standard doubly linked list requires a lock, doesn't it?
> >>> What lock protects smack_rule_list?
> >>>
> >>>       
> >> you're right; 
> >>
> >> what's the best way, using a rcu variant for "list_for_each, container_of ...etc" ?
> >> (concurrent list insertion are already protected with a mutex, so rcu must the good idea for the read side) 
> >>     
> >
> > You want list_for_each_entry_rcu() above.  You will need list_add_rcu()
> > when adding elements to the list.
> >
> > Again, if these elements are ever removed, you will need rcu_read_lock()
> > and rcu_read_unlock() surrounding their use.  Otherwise, an element can
> > be freed out from under a reader who is still referencing it.
> 
> You'll also need to be very careful that the locking is safe to use
> in the networking hooks, in particular smack_socket_sock_rcv_skb. The
> amount of care required to get the locking correct is a major factor
> in the current list implementation.

I must defer to you on this one!

							Thanx, Paul
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