On Thu, Sep 09, 2021 at 11:17:42PM -0700, Li Li wrote:
> On Thu, Sep 9, 2021 at 10:38 PM Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> 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.
Ah, can you add that to the changelog text to make it more obvious?
thanks,
greg k-h
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