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

Casey Schaufler
2009-02-22T17:54:27Z

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.
>
> Thanx, Paul
>

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.