Re: Broken Address Dependency in mm/ksm.c::cmp_and_merge_page()

[Paul Heidekrüger]

On 13 Jan 2023, at 16:22, Alan Stern wrote:

> On Fri, Jan 13, 2023 at 12:11:25PM +0100, Paul Heidekrüger wrote:
>> Hi all,
>>
>> FWIW, here are two more broken address dependencies, both very similar to the
>> one discussed in this thread. From what I can tell, both are protected by a
>> lock, so, again, nothing to worry about right now? Would you agree?
>
> FWIW, my opinion is that in both cases the broken dependency can be
> removed entirely.
>
>> Comments marked with "AD:" were added by me for readability.
>>
>> 1. drivers/hwtracing/stm/core.c::1050 - 1085
>>
>>         /**
>>          * __stm_source_link_drop() - detach stm_source from an stm device
>>          * @src:	stm_source device
>>          * @stm:	stm device
>>          *
>>          * If @stm is @src::link, disconnect them from one another and put the
>>          * reference on the @stm device.
>>          *
>>          * Caller must hold stm::link_mutex.
>>          */
>>         static int __stm_source_link_drop(struct stm_source_device *src,
>>                                           struct stm_device *stm)
>>         {
>>                 struct stm_device *link;
>>                 int ret = 0;
>>
>>                 lockdep_assert_held(&stm->link_mutex);
>>
>>                 /* for stm::link_list modification, we hold both mutex and spinlock */
>>                 spin_lock(&stm->link_lock);
>>                 spin_lock(&src->link_lock);
>>
>>                 /* AD: Beginning of the address dependency. */
>>                 link = srcu_dereference_check(src->link, &stm_source_srcu, 1);
>>
>>                 /*
>>                  * The linked device may have changed since we last looked, because
>>                  * we weren't holding the src::link_lock back then; if this is the
>>                  * case, tell the caller to retry.
>>                  */
>>                 if (link != stm) {
>>                         ret = -EAGAIN;
>>                         goto unlock;
>>                 }
>>
>>                 /* AD: Compiler deduces that "link" and "stm" are exchangeable at this point. */
>>                 stm_output_free(link, &src->output); list_del_init(&src->link_entry);
>>
>>                 /* AD: Leads to WRITE_ONCE() to (&link->dev)->power.last_busy. */
>>                 pm_runtime_mark_last_busy(&link->dev);
>
> In both of these statements, link can safely be replaced by stm.
>
> (There's also a control dependency which the LKMM isn't aware of.  This
> makes it all the more safe.)
>
>> 2. kernel/locking/lockdep.c::6319 - 6348
>>
>>         /*
>>          * Unregister a dynamically allocated key.
>>          *
>>          * Unlike lockdep_register_key(), a search is always done to find a matching
>>          * key irrespective of debug_locks to avoid potential invalid access to freed
>>          * memory in lock_class entry.
>>          */
>>         void lockdep_unregister_key(struct lock_class_key *key)
>>         {
>>                 struct hlist_head *hash_head = keyhashentry(key);
>>                 struct lock_class_key *k;
>>                 struct pending_free *pf;
>>                 unsigned long flags;
>>                 bool found = false;
>>
>>                 might_sleep();
>>
>>                 if (WARN_ON_ONCE(static_obj(key)))
>>                         return;
>>
>>                 raw_local_irq_save(flags);
>>                 lockdep_lock();
>>
>>                 /* AD: Address dependency begins here with an rcu_dereference_raw() into k. */
>>                 hlist_for_each_entry_rcu(k, hash_head, hash_entry) {
>>                         /* AD: Compiler deduces that k and key are exchangable iff the if condition evaluates to true.
>>                         if (k == key) {
>>                                 /* AD: Leads to WRITE_ONCE() to (&k->hash_entry)->pprev. */
>>                                 hlist_del_rcu(&k->hash_entry);
>
> And here k could safely be replaced with key.  (And again there is a
> control dependency, but this is one that the LKMM would detect.)

Ha, I didn't even notice the control dependencies - of course! In that case,
this doesn't warrant a patch though, given that nothing is really breaking?

Many thanks,
Paul

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