On Mon, Feb 3, 2014 at 10:11 AM, Oleg Nesterov wrote:
> On 02/03, Andy Lutomirski wrote:
>>
>> @@ -911,6 +918,47 @@ static inline struct audit_context *audit_alloc_context(enum audit_state state)
>> return context;
>> }
>>
>> +void audit_inc_n_rules()
>> +{
>> + struct task_struct *p, *g;
>> +
>> + write_lock(&n_rules_lock);
>> +
>> + if (audit_n_rules++ != 0)
>> + goto out; /* The overall state isn't changing. */
>> +
>> + read_lock(&tasklist_lock);
>> + do_each_thread(g, p) {
>> + if (p->audit_context)
>> + set_tsk_thread_flag(p, TIF_SYSCALL_AUDIT);
>> + } while_each_thread(g, p);
>> + read_unlock(&tasklist_lock);
>
> Cosmetic, but I'd suggest to use for_each_process_thread() instead
> of do_each_thread/while_each_thread.

I didn't notice that option :)

>
> And I am not sure why n_rules_lock is rwlock_t... OK, to make
> audit_alloc() more scalable, I guess. Please see below.
>

Yes. I should probably also use the irqsave variant.

>> @@ -942,8 +995,14 @@ int audit_alloc(struct task_struct *tsk)
>> }
>> context->filterkey = key;
>>
>> + read_lock(&n_rules_lock);
>> tsk->audit_context = context;
>> - set_tsk_thread_flag(tsk, TIF_SYSCALL_AUDIT);
>> + if (audit_n_rules)
>> + set_tsk_thread_flag(tsk, TIF_SYSCALL_AUDIT);
>> + else
>> + clear_tsk_thread_flag(tsk, TIF_SYSCALL_AUDIT);
>> + read_unlock(&n_rules_lock);
>
> Perhaps this is fine, but n_rules_lock can't prevent the race with
> audit_inc/dec_n_rules(). The problem is, this is called before the
> new task is visible to for_each_process_thread().

Hmm. I missed that.

>
> If we want to fix this race, we need something like audit_sync_flags()
> called after copy_process() drops tasklist, or from tasklist_lock
> protected section (in this case it doesn't need n_rules_lock).
>
> Or perhaps audit_alloc() should not try to clear TIF_SYSCALL_AUDIT at all.
> In both cases n_rules_lock can be spinlock_t.

Here are two options:

1. Sync the flags inside tasklist_lock, as I think you're suggesting.
This seems simple.

2. Make this whole thing lazy -- always set TIF_SYSCALL_AUDIT for new
tasks, but clear it on the first syscall.

I'm leaning toward number 1.

Thanks for the instant review!

--Andy
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