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 <YTrvMSm2oB91IhuK@kroah.com>
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From:   Li Li <dualli@chromium.org>
Date:   Thu, 9 Sep 2021 23:17:42 -0700
Message-ID: <CANBPYPgEbFusdyrcV1EqridahQGTH7X=s8ufUA9SMR8SefAO2g@mail.gmail.com>
Subject: Re: [PATCH v2 1/1] binder: fix freeze race
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     Li Li <dualli@google.com>, Todd Kjos <tkjos@google.com>,
        Christian Brauner <christian@brauner.io>,
        =?UTF-8?B?QXJ2ZSBIasO4bm5ldsOlZw==?= <arve@android.com>,
        "open list:ANDROID DRIVERS" <devel@driverdev.osuosl.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Martijn Coenen <maco@google.com>,
        Hridya Valsaraju <hridya@google.com>,
        Suren Baghdasaryan <surenb@google.com>,
        "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        Android Kernel Team <kernel-team@android.com>
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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