Re: [PATCH] binder: implement binderfs

[Christian Brauner <christian@brauner.io>]

On Thu, Dec 06, 2018 at 03:04:03PM +0100, Greg KH wrote:
> On Wed, Dec 05, 2018 at 10:42:06PM +0100, Christian Brauner wrote:
> > On Wed, Dec 05, 2018 at 09:01:45PM +0100, Greg KH wrote:
> > > > /* binder-control */
> > > > Each new binderfs instance comes with a binder-control device. No other
> > > > devices will be present at first. The binder-control device can be used to
> > > > dynamically allocate binder devices. All requests operate on the binderfs
> > > > mount the binder-control device resides in:
> > > > - BINDER_CTL_ADD
> > > >   Allocate a new binder device.
> > > > Assuming a new instance of binderfs has been mounted at /dev/binderfs via
> > > > mount -t binderfs binderfs /dev/binderfs. Then a request to create a new
> > > > binder device can be made via:
> > > > 
> > > > struct binderfs_device device = {0};
> > > > int fd = open("/dev/binderfs/binder-control", O_RDWR);
> > > > ioctl(fd, BINDER_CTL_ADD, &device);
> > > > 
> > > > The struct binderfs_device will be used to return the major and minor
> > > > number, as well as the index used as the new name for the device.
> > > > Binderfs devices can simply be removed via unlink().
> > > 
> > > I think you should provide a name in the BINDER_CTL_ADD command.  That
> > > way you can easily emulate the existing binder queues, and it saves you
> > > a create/rename sequence that you will be forced to do otherwise.  Why
> > > not do it just in a single command?
> > 
> > Sounds reasonable. How do you feel about capping the name length at 255
> > bytes aka the standard Linux file name length (e.g. xfs, ext4 etc.)?
> > 
> > #define BINDERFS_NAME_MAX 255
> > 
> > struct binderfs_device {
> >         char name[BINDERFS_NAME_MAX + 1];
> 
> __u8 is the proper type to cross the user/kernel boundry :)

Will switch. :)

> 
> > 	__u32 major;
> > 	__u32 minor;
> > }
> 
> Yes, limiting it to 255 is fine with me.

Perfect!

> 
> > > That way also you don't need to care about the major/minor number at
> > > all.  Userspace should never need to worry about that, use a name,
> > > that's the best thing.  Also, it allows you to drop the use of the idr,
> > > making the kernel code simpler overall.
> > > 
> > > > /* Implementation details */
> > > > - When binderfs is registered as a new filesystem it will dynamically
> > > >   allocate a new major number. The allocated major number will be returned
> > > >   in struct binderfs_device when a new binder device is allocated.
> > > 
> > > Why does userspace care about major/minor numbers at all?  You should
> > 
> > Userspace cares for the sake of the devices cgroup which operates on
> > device numnbers to restrict access to devices. Since binderfs doesn't
> > have a static major number returning that information is helpful.
> 
> Ugh, ok, that makes sense.  If we really want to make the kernel
> interface simpler, drop the major/minor and then have userspace do the
> stat(2) to see what the major/minor number they care about is.
> 
> But yeah, keeping it here makes everyone's life simpler, the kernel
> already knows this, and it's trivial to pass it back to userspace this
> way.
> 
> Care to make this change and resend?

For sure. I have a long-haul flight for ~15h so by the time I land I
have a new version I can send out. :)

Thanks!
Christian