Re: [PATCH RFC v2] Convert struct pid count to refcount_t

Joel Fernandes
2019-06-26T21:50:46Z

On Tue, Jun 25, 2019 at 09:34:07AM +0200, Peter Zijlstra wrote:
> On Mon, Jun 24, 2019 at 09:10:15PM +0200, Jann Horn wrote:
> > That part of the documentation only talks about cases where you have a
> > control dependency on the return value of the refcount operation. But
> > refcount_inc() does not return a value, so this isn't relevant for
> > refcount_inc().
> >
> > Also, AFAIU, the control dependency mentioned in the documentation has
> > to exist *in the caller* - it's just pointing out that if you write
> > code like the following, you have a control dependency between the
> > refcount operation and the write:
> >
> > if (refcount_inc_not_zero(&obj->refcount)) {
> > WRITE_ONCE(obj->x, y);
> > }
> >
> > For more information on the details of this stuff, try reading the
> > section "CONTROL DEPENDENCIES" of Documentation/memory-barriers.txt.
>
> IIRC the argument went as follows:
>
> - if you use refcount_inc(), you've already got a stable object and
> have ACQUIRED it otherwise, typically through locking.
>
> - if you use refcount_inc_not_zero(), you have a semi stable object
> (RCU), but you still need to ensure any changes to the object happen
> after acquiring a reference, and this is where the control dependency
> comes in as Jann already explained.
>
> Specifically, it would be bad to allow STOREs to happen before we know
> the refcount isn't 0, as that would be a UaF.
>
> Also see the comment in lib/refcount.c.
>

Thanks a lot for the explanations and the pointers to the comment in
lib/refcount.c . It makes it really clearly.

Also, does this patch look good to you? If so and if ok with you, could you
Ack it? The patch is not really "RFC" but I still tagged it as such since I
wanted to have this discussion.

Thanks!

- Joel