Re: [RFC][PATCH 2/7] kref: Add kref_read()

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Nov 15, 2016 at 12:53:35PM -0800, Kees Cook wrote:
> On Tue, Nov 15, 2016 at 12:03 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Tue, Nov 15, 2016 at 08:33:22AM +0100, Greg KH wrote:
> >> On Mon, Nov 14, 2016 at 06:39:48PM +0100, Peter Zijlstra wrote:
> >
> >> > --- a/drivers/block/drbd/drbd_req.c
> >> > +++ b/drivers/block/drbd/drbd_req.c
> >> > @@ -520,7 +520,7 @@ static void mod_rq_state(struct drbd_req
> >> >             /* Completion does it's own kref_put.  If we are going to
> >> >              * kref_sub below, we need req to be still around then. */
> >> >             int at_least = k_put + !!c_put;
> >> > -           int refcount = atomic_read(&req->kref.refcount);
> >> > +           int refcount = kref_read(&req->kref);
> >> >             if (refcount < at_least)
> >> >                     drbd_err(device,
> >> >                             "mod_rq_state: Logic BUG: %x -> %x: refcount = %d, should be >= %d\n",
> >>
> >> As proof of "things you should never do", here is one such example.
> >>
> >> ugh.
> >>
> >>
> >> > --- a/drivers/block/virtio_blk.c
> >> > +++ b/drivers/block/virtio_blk.c
> >> > @@ -767,7 +767,7 @@ static void virtblk_remove(struct virtio
> >> >     /* Stop all the virtqueues. */
> >> >     vdev->config->reset(vdev);
> >> >
> >> > -   refc = atomic_read(&disk_to_dev(vblk->disk)->kobj.kref.refcount);
> >> > +   refc = kref_read(&disk_to_dev(vblk->disk)->kobj.kref);
> >> >     put_disk(vblk->disk);
> >> >     vdev->config->del_vqs(vdev);
> >> >     kfree(vblk->vqs);
> >>
> >> And this too, ugh, that's a huge abuse and is probably totally wrong...
> >>
> >> thanks again for digging through this crap.  I wonder if we need to name
> >> the kref reference variable "do_not_touch_this_ever" or some such thing
> >> to catch all of the people who try to be "too smart".
> >
> > There's unimaginable bong hits involved in this stuff, in the end I
> > resorted to brute force and scripts to convert all this.
> 
> What should we do about things like this (bpf_prog_put() and callbacks
> from kernel/bpf/syscall.c):
> 
> 
> static void bpf_prog_uncharge_memlock(struct bpf_prog *prog)
> {
>         struct user_struct *user = prog->aux->user;
> 
>         atomic_long_sub(prog->pages, &user->locked_vm);

Oh that's scary.  Let's just make one reference count rely on another
one and not check things...

>         free_uid(user);
> }
> 
> static void __bpf_prog_put_rcu(struct rcu_head *rcu)
> {
>         struct bpf_prog_aux *aux = container_of(rcu, struct bpf_prog_aux, rcu);
> 
>         free_used_maps(aux);
>         bpf_prog_uncharge_memlock(aux->prog);
>         bpf_prog_free(aux->prog);
> }
> 
> void bpf_prog_put(struct bpf_prog *prog)
> {
>         if (atomic_dec_and_test(&prog->aux->refcnt))
>                 call_rcu(&prog->aux->rcu, __bpf_prog_put_rcu);
> }
> 
> 
> Not only do we want to protect prog->aux->refcnt, but I think we want
> to protect user->locked_vm too ... I don't think it's sane for
> user->locked_vm to be a stats_t ?

I don't think this is sane code...

greg k-h