linux-kernel - Re: [PATCH v2 6/8] binder: allow freeze notification for dead nodes

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Message-ID: <CAN5Drs3TCGT1rWJjujo3FP3HxnSFUFo5hcWh=4+xhOYzDg4JqQ@mail.gmail.com>
Date: Fri, 27 Sep 2024 09:13:34 -0700
From: Yu-Ting Tseng <yutingtseng@...gle.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Carlos Llamas <cmllamas@...gle.com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	Arve Hjønnevåg <arve@...roid.com>, 
	Todd Kjos <tkjos@...roid.com>, Martijn Coenen <maco@...roid.com>, 
	Joel Fernandes <joel@...lfernandes.org>, Christian Brauner <brauner@...nel.org>, 
	Suren Baghdasaryan <surenb@...gle.com>, linux-kernel@...r.kernel.org, kernel-team@...roid.com, 
	stable@...r.kernel.org
Subject: Re: [PATCH v2 6/8] binder: allow freeze notification for dead nodes

On Fri, Sep 27, 2024 at 12:19 AM Alice Ryhl <aliceryhl@...gle.com> wrote:
>
> On Fri, Sep 27, 2024 at 1:37 AM Carlos Llamas <cmllamas@...gle.com> wrote:
> >
> > Alice points out that binder_request_freeze_notification() should not
> > return EINVAL when the relevant node is dead [1]. The node can die at
> > any point even if the user input is valid. Instead, allow the request
> > to be allocated but skip the initial notification for dead nodes. This
> > avoids propagating unnecessary errors back to userspace.
> >
> > Fixes: d579b04a52a1 ("binder: frozen notification")
> > Cc: stable@...r.kernel.org
> > Suggested-by: Alice Ryhl <aliceryhl@...gle.com>
> > Link: https://lore.kernel.org/all/CAH5fLghapZJ4PbbkC8V5A6Zay-_sgTzwVpwqk6RWWUNKKyJC_Q@mail.gmail.com/ [1]
> > Signed-off-by: Carlos Llamas <cmllamas@...gle.com>
> > ---
> >  drivers/android/binder.c | 28 +++++++++++++---------------
> >  1 file changed, 13 insertions(+), 15 deletions(-)
> >
> > diff --git a/drivers/android/binder.c b/drivers/android/binder.c
> > index 73dc6cbc1681..415fc9759249 100644
> > --- a/drivers/android/binder.c
> > +++ b/drivers/android/binder.c
> > @@ -3856,7 +3856,6 @@ binder_request_freeze_notification(struct binder_proc *proc,
> >  {
> >         struct binder_ref_freeze *freeze;
> >         struct binder_ref *ref;
> > -       bool is_frozen;
> >
> >         freeze = kzalloc(sizeof(*freeze), GFP_KERNEL);
> >         if (!freeze)
> > @@ -3872,32 +3871,31 @@ binder_request_freeze_notification(struct binder_proc *proc,
> >         }
> >
> >         binder_node_lock(ref->node);
> > -
> > -       if (ref->freeze || !ref->node->proc) {
> > -               binder_user_error("%d:%d invalid BC_REQUEST_FREEZE_NOTIFICATION %s\n",
> > -                                 proc->pid, thread->pid,
> > -                                 ref->freeze ? "already set" : "dead node");
> > +       if (ref->freeze) {
> > +               binder_user_error("%d:%d BC_REQUEST_FREEZE_NOTIFICATION already set\n",
> > +                                 proc->pid, thread->pid);
> >                 binder_node_unlock(ref->node);
> >                 binder_proc_unlock(proc);
> >                 kfree(freeze);
> >                 return -EINVAL;
> >         }
> > -       binder_inner_proc_lock(ref->node->proc);
> > -       is_frozen = ref->node->proc->is_frozen;
> > -       binder_inner_proc_unlock(ref->node->proc);
> >
> >         binder_stats_created(BINDER_STAT_FREEZE);
> >         INIT_LIST_HEAD(&freeze->work.entry);
> >         freeze->cookie = handle_cookie->cookie;
> >         freeze->work.type = BINDER_WORK_FROZEN_BINDER;
> > -       freeze->is_frozen = is_frozen;
> > -
> >         ref->freeze = freeze;
> >
> > -       binder_inner_proc_lock(proc);
> > -       binder_enqueue_work_ilocked(&ref->freeze->work, &proc->todo);
> > -       binder_wakeup_proc_ilocked(proc);
> > -       binder_inner_proc_unlock(proc);
> > +       if (ref->node->proc) {
> > +               binder_inner_proc_lock(ref->node->proc);
> > +               freeze->is_frozen = ref->node->proc->is_frozen;
> > +               binder_inner_proc_unlock(ref->node->proc);
> > +
> > +               binder_inner_proc_lock(proc);
> > +               binder_enqueue_work_ilocked(&freeze->work, &proc->todo);
> > +               binder_wakeup_proc_ilocked(proc);
> > +               binder_inner_proc_unlock(proc);
>
> This is not a problem with your change ... but, why exactly are we
> scheduling the BINDER_WORK_FROZEN_BINDER right after creating it? For
> death notications, we only schedule it immediately if the process is
> dead. So shouldn't we only schedule it if the process is not frozen?
>
> And if the answer is that frozen notifications are always sent
> immediately to notify about the current state, then we should also
> send one for a dead process ... maybe. I guess a dead process is not
> frozen?
Yes this is to immediately notify about the current state (frozen or
unfrozen). A dead process is in neither state so it feels more correct
not to send either?


>
> > +       }
> >
> >         binder_node_unlock(ref->node);
> >         binder_proc_unlock(proc);
> > --
> > 2.46.1.824.gd892dcdcdd-goog
> >