Re: [RFC v2 6/6] android: binder: Add a buffer flag to relinquish ownership of fds - Greg Kroah-Hartman

From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: Suren Baghdasaryan <surenb@google.com>
Cc: "T.J. Mercier" <tjmercier@google.com>,
	"Maarten Lankhorst" <maarten.lankhorst@linux.intel.com>,
	"Maxime Ripard" <mripard@kernel.org>,
	"Thomas Zimmermann" <tzimmermann@suse.de>,
	"David Airlie" <airlied@linux.ie>,
	"Daniel Vetter" <daniel@ffwll.ch>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Arve Hjønnevåg" <arve@android.com>,
	"Todd Kjos" <tkjos@android.com>,
	"Martijn Coenen" <maco@android.com>,
	"Joel Fernandes" <joel@joelfernandes.org>,
	"Christian Brauner" <brauner@kernel.org>,
	"Hridya Valsaraju" <hridya@google.com>,
	"Sumit Semwal" <sumit.semwal@linaro.org>,
	"Christian König" <christian.koenig@amd.com>,
	"Benjamin Gaignard" <benjamin.gaignard@linaro.org>,
	"Liam Mark" <lmark@codeaurora.org>,
	"Laura Abbott" <labbott@redhat.com>,
	"Brian Starkey" <Brian.Starkey@arm.com>,
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Subject: Re: [RFC v2 6/6] android: binder: Add a buffer flag to relinquish ownership of fds
Date: Tue, 15 Feb 2022 08:30:43 +0100	[thread overview]
Message-ID: <YgtWo+6UeAU4/CvT@kroah.com> (raw)
In-Reply-To: <CAJuCfpG0V2qZVVUPRmw3fZP-bQmp+w36nOUe9iHtgmuHb7PemQ@mail.gmail.com>

On Mon, Feb 14, 2022 at 11:19:35PM -0800, Suren Baghdasaryan wrote:
> On Mon, Feb 14, 2022 at 11:01 PM Greg Kroah-Hartman
> <gregkh@linuxfoundation.org> wrote:
> >
> > On Mon, Feb 14, 2022 at 02:25:47PM -0800, T.J. Mercier wrote:
> > > On Fri, Feb 11, 2022 at 11:19 PM Greg Kroah-Hartman
> > > > > --- a/include/uapi/linux/android/binder.h
> > > > > +++ b/include/uapi/linux/android/binder.h
> > > > > @@ -137,6 +137,7 @@ struct binder_buffer_object {
> > > > >
> > > > >  enum {
> > > > >       BINDER_BUFFER_FLAG_HAS_PARENT = 0x01,
> > > > > +     BINDER_BUFFER_FLAG_SENDER_NO_NEED = 0x02,
> > > > >  };
> > > > >
> > > > >  /* struct binder_fd_array_object - object describing an array of fds in a buffer
> > > > > --
> > > > > 2.35.1.265.g69c8d7142f-goog
> > > > >
> > > >
> > > > How does userspace know that binder supports this new flag?
> > >
> > > Sorry, I don't completely follow even after Todd's comment. Doesn't
> > > the presence of BINDER_BUFFER_FLAG_SENDER_NO_NEED in the header do
> > > this?
> >
> > There is no "header" when running a new kernel on an old userspace,
> > right?  How about the other way around, old kernel, new userspace?
> 
> 1. new kernel + old userspace = kernel supports the feature but
> userspace does not use it. The old userspace won't even mount the new
> cgroup controller, accounting is not performed, charge is not
> transferred.
> 2. old kernel + new userspace = the new cgroup controller is not
> supported by the kernel, accounting is not performed, charge is not
> transferred.
> 3. old kernel + old userspace = same as #2
> 4. new kernel + new userspace = cgroup is mounted, feature is
> supported and used.
> Does that work or do we need a separate indication of whether binder
> driver supports the charge transfer feature?

Ok, if that's all working, this is fine, it just seemed odd to add a new
type like this.  Perhaps this can go into the changelog text...

thanks,

greg k-h