linux-kernel - Re: Testing lxc 0.6.5 in Fedora 13

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Date:	Tue, 06 Apr 2010 08:17:21 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Matt Helsley <matthltc@...ibm.com>,
	Roland McGrath <roland@...hat.com>,
	Grzegorz Nosek <root@...aldomain.pl>,
	Sukadev Bhattiprolu <sukadev@...ibm.com>,
	containers@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: Testing lxc 0.6.5 in Fedora 13

Oleg Nesterov <oleg@...hat.com> writes:

> On 04/06, Matt Helsley wrote:
>>
>> On Mon, Apr 05, 2010 at 08:44:43PM -0700, Roland McGrath wrote:
>>
>> > tracehook_report_clone_complete() call is made when that task_struct is no
>> > longer guaranteed to be valid.
>
> Hmm. I missed this.
>
>> Also, if utrace allows multiple tracers and they each
>> exist in a different namespace then storing a pid nr isn't going to work.
>
> Yes, but utrace is simple. ptrace_report_clone() does
>
> 	ctx->eventmsg = child->pid;
>
> we should fix this line and that is all, afaics. Every tracer has a
> separate "struct ptrace_context *ctx".
>
>> So my hunch is, in the long run, we'll need to hold a reference there and
>> drop it when the last tracer detaches
>
> Without utrace only one tracer is possible.
>
> So, I think we should either change do_fork() to get the right tracee_pid_nr,
> or add get/put into do_fork() and change tracehooks as Roland suggested.

For a unicast path where the is no danger of the destination process changing
I don't see why we can't compute the userspace pid_nr.  It only get's tricky
for things like broadcast signals (pgrp, session) when we don't who the
final recipient process will be.

I think that only gets truly bad in the case of unix domain sockets.

Eric
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