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

Joel Fernandes
2019-06-24T20:43:38Z

On Mon, Jun 24, 2019 at 3:10 PM Jann Horn <jannh@google.com> wrote:
>
> On Mon, Jun 24, 2019 at 8:52 PM Joel Fernandes <joel@joelfernandes.org> wrote:
> > On Mon, Jun 24, 2019 at 02:45:34PM -0400, Joel Fernandes (Google) wrote:
> > > struct pid's count is an atomic_t field used as a refcount. Use
> > > refcount_t for it which is basically atomic_t but does additional
> > > checking to prevent use-after-free bugs.
> > >
> > > For memory ordering, the only change is with the following:
> > > - if ((atomic_read(&pid->count) == 1) ||
> > > - atomic_dec_and_test(&pid->count)) {
> > > + if (refcount_dec_and_test(&pid->count)) {
> > > kmem_cache_free(ns->pid_cachep, pid);
> > >
> > > Here the change is from:
> > > Fully ordered --> RELEASE + ACQUIRE (as per refcount-vs-atomic.rst)
> > > This ACQUIRE should take care of making sure the free happens after the
> > > refcount_dec_and_test().
> > >
> > > The above hunk also removes atomic_read() since it is not needed for the
> > > code to work and it is unclear how beneficial it is. The removal lets
> > > refcount_dec_and_test() check for cases where get_pid() happened before
> > > the object was freed.
> [...]
> > I had a question about refcount_inc().
> >
> > As per Documentation/core-api/refcount-vs-atomic.rst , it says:
> >
> > A control dependency (on success) for refcounters guarantees that
> > if a reference for an object was successfully obtained (reference
> > counter increment or addition happened, function returned true),
> > then further stores are ordered against this operation.
> >
> > However, in refcount_inc() I don't see any memory barriers (in the case where
> > CONFIG_REFCOUNT_FULL=n). Is the documentation wrong?
>
> 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.

Makes sense now, thank you Jann!

- Joel