Date:	Fri, 20 Feb 2015 12:33:31 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Andrew Vagin <avagin@...allels.com>
Cc:	Pavel Emelyanov <xemul@...allels.com>,
	Roger Luethi <rl@...lgate.ch>, Oleg Nesterov <oleg@...hat.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux API <linux-api@...r.kernel.org>,
	Andrey Vagin <avagin@...nvz.org>
Subject: Re: [PATCH 0/7] [RFC] kernel: add a netlink interface to get
 information about processes

On Thu, Feb 19, 2015 at 1:39 PM, Andrew Vagin <avagin@...allels.com> wrote:
> On Wed, Feb 18, 2015 at 05:18:38PM -0800, Andy Lutomirski wrote:
>> > > I don't suppose this could use real syscalls instead of netlink.  If
>> > > nothing else, netlink seems to conflate pid and net namespaces.
>> >
>> > What do you mean by "conflate pid and net namespaces"?
>>
>> A netlink socket is bound to a network namespace, but you should be
>> returning data specific to a pid namespace.
>
> Here is a good question. When we mount a procfs instance, the current
> pidns is saved on a superblock. Then if we read data from
> this procfs from another pidns, we will see pid-s from the pidns where
> this procfs has been mounted.
>
> $ unshare -p -- bash -c '(bash)'
> $ cat /proc/self/status | grep ^Pid:
> Pid:    15770
> $ echo $$
> 1
>
> A similar situation with socket_diag. A socket_diag socket is bound to a
> network namespace. If we open a socket_diag socket and change a network
> namespace, it will return infromation about the initial netns.
>
> In this version I always use a current pid namespace.
> But to be consistant with other kernel logic, a socket diag has to be
> linked with a pidns where it has been created.
>

Attaching a pidns to every freshly created netlink socket seems odd,
but I don't see a better solution that still uses netlink.

>>
>> On a related note, how does this interact with hidepid?  More
>
> Currently it always work as procfs with hidepid = 2 (highest level of
> security).
>
>> generally, what privileges are you requiring to obtain what data?
>
> It dumps information only if ptrace_may_access(tsk, PTRACE_MODE_READ) returns true

Sounds good to me.

>
>>
>> >
>> > >
>> > > Also, using an asynchronous interface (send, poll?, recv) for
>> > > something that's inherently synchronous (as the kernel a local
>> > > question) seems awkward to me.
>> >
>> > Actually all requests are handled synchronously. We call sendmsg to send
>> > a request and it is handled in this syscall.
>> >  2)               |  netlink_sendmsg() {
>> >  2)               |    netlink_unicast() {
>> >  2)               |      taskdiag_doit() {
>> >  2)   2.153 us    |        task_diag_fill();
>> >  2)               |        netlink_unicast() {
>> >  2)   0.185 us    |          netlink_attachskb();
>> >  2)   0.291 us    |          __netlink_sendskb();
>> >  2)   2.452 us    |        }
>> >  2) + 33.625 us   |      }
>> >  2) + 54.611 us   |    }
>> >  2) + 76.370 us   |  }
>> >  2)               |  netlink_recvmsg() {
>> >  2)   1.178 us    |    skb_recv_datagram();
>> >  2) + 46.953 us   |  }
>> >
>> > If we request information for a group of tasks (NLM_F_DUMP), a first
>> > portion of data is filled from the sendmsg syscall. And then when we read
>> > it, the kernel fills the next portion.
>> >
>> >  3)               |  netlink_sendmsg() {
>> >  3)               |    __netlink_dump_start() {
>> >  3)               |      netlink_dump() {
>> >  3)               |        taskdiag_dumpid() {
>> >  3)   0.685 us    |          task_diag_fill();
>> > ...
>> >  3)   0.224 us    |          task_diag_fill();
>> >  3) + 74.028 us   |        }
>> >  3) + 88.757 us   |      }
>> >  3) + 89.296 us   |    }
>> >  3) + 98.705 us   |  }
>> >  3)               |  netlink_recvmsg() {
>> >  3)               |    netlink_dump() {
>> >  3)               |      taskdiag_dumpid() {
>> >  3)   0.594 us    |        task_diag_fill();
>> > ...
>> >  3)   0.242 us    |        task_diag_fill();
>> >  3) + 60.634 us   |      }
>> >  3) + 72.803 us   |    }
>> >  3) + 88.005 us   |  }
>> >  3)               |  netlink_recvmsg() {
>> >  3)               |    netlink_dump() {
>> >  3)   2.403 us    |      taskdiag_dumpid();
>> >  3) + 26.236 us   |    }
>> >  3) + 40.522 us   |  }
>> >  0) + 20.407 us   |  netlink_recvmsg();
>> >
>> >
>> > netlink is really good for this type of tasks.  It allows to create an
>> > extendable interface which can be easy customized for different needs.
>> >
>> > I don't think that we would want to create another similar interface
>> > just to be independent from network subsystem.
>>
>> I guess this is a bit streamy in that you ask one question and get
>> multiple answers.
>
> It's like seq_file in procfs. The kernel allocates a buffer then fills
> it, copies it into userspace, fills it again, ... repeats these actions.
> And we can read data from file by portions.
>
> Actually here is one more analogy. When we open a file in procfs,
> we sends a request to the kernel and a file path is a request body in
> this case. But in case of procfs, we can't construct requests, we only
> have a set of predefined requests.

Fair enough.  Procfs is also a bit absurd and only makes sense because
it's compatible with lots of tools.  In a totally sane world, I would
argue that you should issue one syscall asking questions about a bit
and you should get answers immediately.

--Andy
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