linux-kernel - Re: [PATCH 7/7][v8] SI_USER: Masquerade si

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Date:	Thu, 19 Feb 2009 15:21:34 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...l.org>, roland@...hat.com,
	daniel@...ac.com, Containers <containers@...ts.osdl.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 7/7][v8] SI_USER: Masquerade si_pid when crossing pid ns boundary

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

> On 02/19, Eric W. Biederman wrote:
>>
>> Oleg Nesterov <oleg@...hat.com> writes:
>> >
>> > SI_FROMUSER() == T, unless we have more (hopefully not) in-kernel
>> > users which send SI_FROMUSER() signals, .si_pid must be valid?
>>
>> So the argument is that while things such as force_sig_info(SIGSEGV)
>> don't have a si_pid we don't care because from_ancestor_ns  == 0.
>>
>> Interesting.  Then I don't know if we have any kernel senders
>> that cross the namespace boundaries.
>>
>> That said I still object to this code.
>>
>> sys_kill(-pgrp, SIGUSR1)
>>   kill_something_info(SIGUSR1, &info, 0)
>>     __kill_pgrp_info(SIGUSR1, &info task_pgrp(current))
>>       group_send_sig_info(SIGUSR1, &info, tsk)
>>         __group_send_sig_info(SIGUSR1, &info, tsk)
>>           send_signal(SIGUSR1, &info, tsk, 1)
>>             __send_signal(SIGUSR1, &info, tsk, 1)
>>
>>
>> Process groups and sessions can have processes in multiple pid
>> namespaces, which is very useful for not messing up your controlling
>> terminal.
>>
>> In which case sys_kill cannot possibly set the si_pid value correct
>> and from_ancestor_ns is not enough either.
>
> (I know, I shouldn't reply today because I am already sleeping ;)
>
> Why? send_signal() should calculate the correct value of
> from_parent and pass it to __send_signal(). If it is true, then
> we clear .si_pid in the copied siginfo (which was already queued).
> We don't mangle the original siginfo.
>
> This happens for each process we send the signal.
>
> Or I misunderstood you?

Suppose I have 3 processes in a process group in three separate pid
namespaces.

Looking from the init pid namespace I have:
     pid pgrp ppid
      10 10    1
      11 10    10
      12 10    11

Looking from the pid namespace of pid 11 I have:
     pid pgrp ppid
      0  0     0
      1  0     0
      2  0     1

Looking from the pid namespace of pid 12 I have:
     pid pgrp ppid
      0  0     0
      0  0     0
      1  0     0

So if the process with pid 12 in the initial pid namespace
sends to process group 0.

pid 10 should see si_pid 12.
pid 11 should see si_pid 2.

Neither should see si_pid 0, as from_ancestor_ns will not
be true.

Eric


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