Re: [PATCH RFC 01/10] mm: add Kernel Electric-Fence infrastructure

[Alexander Potapenko]

On Thu, Sep 10, 2020 at 5:43 PM Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:


> > +       /* Calculate address for this allocation. */
> > +       if (right)
> > +               meta->addr += PAGE_SIZE - size;
> > +       meta->addr = ALIGN_DOWN(meta->addr, cache->align);
>
> I would move this ALIGN_DOWN under the (right) if.
> Do I understand it correctly that it will work, but we expect it to do
> nothing for !right? If cache align is >PAGE_SIZE, nothing good will
> happen anyway, right?
> The previous 2 lines look like part of the same calculation -- "figure
> out the addr for the right case".

Yes, makes sense.

> > +
> > +       schedule_delayed_work(&kfence_timer, 0);
> > +       WRITE_ONCE(kfence_enabled, true);
>
> Can toggle_allocation_gate run before we set kfence_enabled? If yes,
> it can break. If not, it's still somewhat confusing.

Correct, it should go after we enable KFENCE. We'll fix that in v2.

> > +void __kfence_free(void *addr)
> > +{
> > +       struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
> > +
> > +       if (unlikely(meta->cache->flags & SLAB_TYPESAFE_BY_RCU))
>
> This may deserve a comment as to why we apply rcu on object level
> whereas SLAB_TYPESAFE_BY_RCU means slab level only.

Sorry, what do you mean by "slab level"?
SLAB_TYPESAFE_BY_RCU means we have to wait for possible RCU accesses
in flight before freeing objects from that slab - that's basically
what we are doing here below:

> > +               call_rcu(&meta->rcu_head, rcu_guarded_free);
> > +       else
> > +               kfence_guarded_free(addr, meta);
> > +}


> > +void kfence_print_object(struct seq_file *seq, const struct kfence_metadata *meta)
> > +{
> > +       const int size = abs(meta->size);
>
> This negative encoding is somewhat confusing. We do lots of abs, but
> do we even look at the sign anywhere? I can't find any use that is not
> abs.

I think initially there was a reason for this, but now we don't seem
to use it anywhere. Nice catch!

Alex