[Devel] Re: [PATCH] [RFC] c/r: Add UTS support

[Eric W. Biederman]

Mike Waychison <mikew at google.com> writes:


>> all of this conversation originally started.   I am happy to set the starting
>> pid to 2 to avoid confusion on that point.
>
> I wasn't really taking into consideration the notion of a 'lightweight' pid
> namespace that didn't have a 'container-init' process.

I know that was something the Vserver guys were do today.  They have something
that shows up as pid 1 but they don't really have a process there.

>> One of the other problems with changing the pid is that user space in general
>> glibc in particular can not cope with the pid of a process changing.
>>
>> My memories are foggy at the moment but I do know that on the several occasions
>> we have looked at unshare of the pid namespace it has failed due to kernel issues.
>> I also remember I was close to having resolved the issues of unsharing the pid
>> namespace if we did not change the pid of processing calling unshare.
>
> Do you have pointers to discussions about these issues?

Not better than the containers list archives.

>> You did not answer my question.  I don't quite see how you were envisioning
>> using unsharing the pid namespace as part of restart so I can't tell if my
>> proposed semantics would work for that case.
>
> Well, one way to look at doing restart with nested namespaces would be to have
> userland go off and begin by rebuilding the process tree.  While rebuilding, any
> given process being recreated would need to have the same pid in the parenting
> pid namespace (the outer most namespace in the container).  It would need to
> know if it 'got' the right pid, and if so, would then create the new child pid
> namespace.  Requiring CLONE_NEWPID set on each and every clone(2) [*] would
> certainly be possible, as long as we had some way for the task being created to
> know what it's parent namespace pid is.  I guess this could be done by a shared
> memory segment shared between the parent and child of the clone as well, though
> it doesn't seem as clear-cut to me.
>
>
>
> [*] Yes, I'm dancing around the clone_with_pid issue..

Ok.  I see what you are trying to accomplish with this and honestly I think it
is silly.

We should start the threads we need in the kernel, and if we need to run clone_pid
fine.  I am not comfortable exporting clone_with_pid to user space.

As for the implementation of allocating a struct pid with a certain set of pid values.
I expect we can do that easily enough by refactoring the pid allocator to be passed
in the min/max pid to allocate from, and have a special case that passes in a different
set of min/max values so we can allocate just the pid we need.

If the primary use for a userspace interface is restart I feel we are doing it wrong.

Eric
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