netdev - Re: [PATCH iproute2 -next] tc, bpf: finalize eBPF support for cls and act front-end

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Date:	Thu, 02 Apr 2015 14:08:54 +0200
From:	Daniel Borkmann <daniel@...earbox.net>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	stephen@...workplumber.org
CC:	ast@...mgrid.com, jiri@...nulli.us, tgraf@...g.ch,
	netdev@...r.kernel.org, Jamal Hadi Salim <jhs@...atatu.com>
Subject: Re: [PATCH iproute2 -next] tc, bpf: finalize eBPF support for cls
 and act front-end

On 04/02/2015 01:30 PM, Hannes Frederic Sowa wrote:
> On Thu, Apr 2, 2015, at 12:19, Daniel Borkmann wrote:
>> On 04/02/2015 02:29 AM, Hannes Frederic Sowa wrote:
>>> On Thu, Apr 2, 2015, at 02:24, Daniel Borkmann wrote:
>>>> On 04/02/2015 02:13 AM, Hannes Frederic Sowa wrote:
>>>> ...
>>>>> Maybe a small utility programs like:
>>>>>
>>>>> bpf (--lookup|--update|--delete|--get-next-key) -fd
>>>>> filedescriptor-number (type conversion parameters here) key [value]
>>>>>
>>>>> So it can be easily used by shell scripts.
>>>>>
>>>>> For that the filedescriptor numbers would need to be exported (already
>>>>> opened) into a spawned shell and the numbers could be specified either
>>>>> in environment or just by printing text which can be sourced by shells
>>>>> (we already talked about the maybe exec 5</proc/pid/fd/1234 idea). Seems
>>>>> this can be just build ontop this current patch by extending the
>>>>> bpf-agent you already build, no?
>>>>
>>>> I was thinking about that and trying it out, but as far as I can tell,
>>>> due to the anon inodes that are currently underlying as the fd provider,
>>>> it doesn't work w/o larger kernel changes. So, the file descriptor
>>>> passing
>>>> is currently the only way to transfer control.
>>>
>>> Does receiving them via af_unix and spawning a new shell with the fds
>>> already open work?
>>
>> I'm probably missing something, would that need changes to bash?
>>
>> I mean exec could bind an fd in the shell to sockets and use that,
>> for example ...
>>
>>     exec 3<>/dev/tcp/www.slashdot.org/80
>>     echo -e "GET / HTTP/1.1\r\nhost:
>>     http://www.slashdot.org\r\nConnection: close\r\n\r\n" >&3
>>     cat <&3
>>
>> ... perhaps such a built-in fake device for retrieving bpf map fds
>> might be interesting, e.g. exec 4<>/dev/bpf/<obj-file>/<map-name> if
>> that has been given to bash?
>>
>> Anyway, I think to have some utility for shell scripts, as you
>> suggest, certainly sounds interesting!
>
> All file descriptors will be inherited by exec as long as the O_CLOEXEC
> flag wasn't specified on them. So you can retrieve the fds via af_unix
> and just exec a new shell. The file descriptors will stay open and you
> can pass the numbers of the fds via environment. This wouldn't need
> changes to bash or kernel.

Okay, I will give it a try. Thanks!
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