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

Paul E. McKenney
2009-02-22T18:25:30Z

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