[PATCH v2 1/2] binderfs: implement "max" mount option

[Christian Brauner]

On Wed, Jan 2, 2019 at 12:32 PM Christian Brauner <christian at brauner.io> wrote:
>
> Since binderfs can be mounted by userns root in non-initial user namespaces
> some precautions are in order. First, a way to set a maximum on the number
> of binder devices that can be allocated per binderfs instance and second, a
> way to reserve a reasonable chunk of binderfs devices for the initial ipc
> namespace.
> A first approach as seen in [1] used sysctls similiar to devpts but was
> shown to be flawed (cf. [2] and [3]) since some aspects were unneeded. This
> is an alternative approach which avoids sysctls completely and instead
> switches to a single mount option.
>
> Starting with this commit binderfs instances can be mounted with a limit on
> the number of binder devices that can be allocated. The max=<count> mount
> option serves as a per-instance limit. If max=<count> is set then only
> <count> number of binder devices can be allocated in this binderfs
> instance.
>
> This allows to safely bind-mount binderfs instances into unprivileged user
> namespaces since userns root in a non-initial user namespace cannot change
> the mount option as long as it does not own the mount namespace the
> binderfs mount was created in and hence cannot drain the host of minor
> device numbers
>
> [1]: https://lore.kernel.org/lkml/20181221133909.18794-1-christian@brauner.io/
> [2]; https://lore.kernel.org/lkml/20181221163316.GA8517@kroah.com/
> [3]: https://lore.kernel.org/lkml/CAHRSSEx+gDVW4fKKK8oZNAir9G5icJLyodO8hykv3O0O1jt2FQ@mail.gmail.com/
> [4]: https://lore.kernel.org/lkml/20181221192044.5yvfnuri7gdop4rs@brauner.io/
>
> Cc: Todd Kjos <tkjos at google.com>
> Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
> Signed-off-by: Christian Brauner <christian.brauner at ubuntu.com>

Fwiw, once this lands I intend to add a binderfs.txt file to
Documentation/filesystems
so that we have proper documentation too.