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

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Date:	Sun, 22 Feb 2009 07:31:04 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	etienne <etienne.basset@...ericable.fr>
Cc:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	casey@...aufler-ca.com, 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 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.

							Thanx, Paul
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