[Devel] Re: [PATCH 7/7][v8] SI_USER: Masquerade si_pid when crossing pid ns boundary

[Eric W. Biederman]

Roland McGrath <roland at redhat.com> writes:

>> 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.
>
> There is no "process group 0".  0 means "the sender's pgrp".

Exactly.  It just happens in this case that pid_nr_ns returns 0 for
the process group number as well as the process group the process is a
member of, that was created outside of the current pid namespace.

> One possibility is that perhaps what people really want the pid_ns to mean
> is that "the sender's pgrp" in the view of the sender does not include any
> processes outside its pid_ns scope.  That would be consistent with the
> behavior of kill (kill_something_info) on -1; it's described as "all
> processes", but in fact means "all processes within my pid_ns scope".
>
> What I mean to describe there is changing kill_something_info, so that
> e.g. killpg() inside the NS would affect only the NS init itself but e.g.
> ^Z (effectively an implicit killpg() that's always from the global NS)
> would also go to that init's "mother" pgrp in the outer NS.

> Another possibility is to decide that's just not worth having at all, and
> CLONE_NEWNS should just implicitly reset pgrp to self.  That is simple.
> But perhaps today someone has a script running a pid_ns-world whose init is
> gracefully killed by ^C of the whole script and we wouldn't want to break
> that if it is actually useful now.

It is especially useful, and this is a deliberate feature.  Having
sessions and process groups extend across pid namespace borders means
you can share a tty and job control functions correctly.  Very handy
for circumstances where you want a light weight temporary container,
and something I am actively using today.  The practical benefit is
that you can upgrade from situations where you would previous use
chroot without extra hassle.

In practice I don't care about si_pid and I doubt I care about processes
sending signals outside of their pid namespace.  But I do care about
sharing a tty and a session and having job control work.

>> pid 10 should see si_pid 12.
>> pid 11 should see si_pid 2.
>
> We indeed have this problem if we think it's useful to continue to have
> a concept of pgrp for the sub-init that can see outside its own NS.
>
>> Neither should see si_pid 0, as from_ancestor_ns will not be true.
>
> Perhaps replace from_ancestor_ns with struct pid_namespace *sender_ns?
> (I don't know if there was already a can of worms with such an idea before.)
> Then si_pid could be translated as appropriate for each recipient.
> (Or perhaps just struct pid *sender and reset si_pid from that.)

The last was my original line of thinking.  I seem to recall Oleg
figuring the code gets pretty ugly when you add in the necessary test
to see if si_pid is actually present.

There are several other cases where we also signal a process outside
of our current pid namespace, where we have a pid inside the recipients
pid namespace.  do_notify_parent is the easiest example.  However those
cases can get the value right because they are unicast signals and
know their recipient when the set the si_pid originally.

My current line of thinking is either:
a) We pass in struct pid *sender and we reset si_pid in send_signal.
b) We make the rule that send_signal must receive a valid siginfo from
   the caller and we only do the extra work for process groups.

Eric
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