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

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Date:	Mon, 19 Oct 2015 11:51:08 +0200
From:	Daniel Borkmann <daniel@...earbox.net>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Alexei Starovoitov <ast@...mgrid.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	davem@...emloft.net, viro@...IV.linux.org.uk, tgraf@...g.ch,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Alexei Starovoitov <ast@...nel.org>
Subject: Re: [PATCH net-next 3/4] bpf: add support for persistent maps/progs

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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