Re: [PATCH net-next 3/4] bpf: add support for persistent maps/progs

[Daniel Borkmann]

On 10/19/2015 09:36 AM, Hannes Frederic Sowa wrote:
> Hi,
>
> On Sun, Oct 18, 2015, at 22:59, Alexei Starovoitov wrote:
> > On 10/18/15 9:49 AM, Daniel Borkmann wrote:
> > > Okay, I have pushed some rough working proof of concept here:
> > >
> > > https://git.breakpoint.cc/cgit/dborkman/net-next.git/log/?h=ebpf-fds-final5
> > >
> > > So the idea eventually had to be slightly modified after giving this
> > > further
> > > thoughts and is the following:
> > >
> > > We have 3 commands (BPF_DEV_CREATE, BPF_DEV_DESTROY, BPF_DEV_CONNECT), and
> > > related to that a bpf_attr extension with only a single __u32 fd member
> > > in it.
> > ...
> > > The nice thing about it is that you can create/unlink as many as you
> > > want, but
> > > when you remove the real device from an application via
> > > bpf_dev_destroy(fd),
> > > then all links disappear with it. Just like in the case of a normal
> > > device driver.
> >
> > interesting idea!
> > What happens if user app creates a dev via bpf_dev_create(), exits and
> > then admin does rm of that dev ?
> > Looks like map/prog will leak ?
> > So the only proper way to delete such cdevs is via bpf_dev_destroy ?
>
> The mknod is not the holder but rather the kobject which should be
> represented in sysfs will be. So you can still get the map major:minor
> by looking up the /dev file in the correspdonding sysfs directory or I
> think we should provide a 'unbind' file, which will drop the kobject if
> the user writes a '1' to it.

I agree, this could still be done.

> > > On device creation, the kernel will return the minor number via bpf(2),
> > > so you
> > > can access the file easily, f.e. /dev/bpf/bpf_map<minor> resp.
> > > /dev/bpf/bpf_prog<minor>,
> > > and then move on with mknod(2) or symlink(2) from there if wished.
> >
> > what if admin mknod in that dir with some arbitrary minor ?
>
> Basically, -EIO. :)
>
> > mknod will succeed, but it won't hold anything?
>
> That is right now true for basically all mknod operations, which udev
> creates.
>
> > looks like bpf_dev_connect will handle it gracefully.
> > So these cdevs should only be created and destroyed via bpf syscall
> > and only sensible operations on them is open() to get fd and pass
> > to bpf_dev_connect and symlink. Anything else admin should be
> > careful not to do. Right?
>
> Besides maybe some statistics and other stuff in sysfs directory, no,
> that is all.
>
> Bye,
> Hannes
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