Messages in this thread | | | From | Li Li <> | Date | Thu, 9 Sep 2021 23:17:42 -0700 | Subject | Re: [PATCH v2 1/1] binder: fix freeze race |
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On Thu, Sep 9, 2021 at 10:38 PM Greg KH <gregkh@linuxfoundation.org> wrote: > > On Thu, Sep 09, 2021 at 08:53:16PM -0700, Li Li wrote: > > struct binder_frozen_status_info { > > __u32 pid; > > + > > + /* process received sync transactions since last frozen > > + * bit 0: received sync transaction after being frozen > > + * bit 1: new pending sync transaction during freezing > > + */ > > __u32 sync_recv; > > You just changed a user/kernel api here, did you just break existing > userspace applications? If not, how did that not happen? >
That's a good question. This design does keep backward compatibility.
The existing userspace applications call ioctl(BINDER_GET_FROZEN_INFO) to check if there's sync or async binder transactions sent to a frozen process.
If the existing userspace app runs on a new kernel, a sync binder call still sets bit 1 of sync_recv (as it's a bool variable) so the ioctl will return the expected value (TRUE). The app just doesn't check bit 1 intentionally so it doesn't have the ability to tell if there's a race - this behavior is aligned with what happens on an old kernel as the old kernel doesn't have bit 1 set at all.
The bit 1 of sync_recv enables new userspace app the ability to tell 1) there's a sync binder transaction happened when being frozen - same as before; and 2) if that sync binder transaction happens exactly when there's a race - a new information for rollback decision.
> thanks, > > greg k-h
Thanks, Li
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