Re: [RFC, PATCH] fix SEM_UNDO with namespaces

[Manfred Spraul <manfred@colorfullife.com>]

Eric W. Biederman wrote:
> Pavel Emelyanov <xemul@openvz.org> writes:
>   
>>>> I agree, that we should probably destroy this one when the task calls 
>>>> unshare, but trying to keep this list relevant is useless.
>>>>   
>>>>         
>>> A very tricky question: Let's assume we have a process with two threads.
>>> The undo structure is shared, as per opengroup standard.
>>> Now one thread calls unshare(CLONE_NEWIPC). What should happen? We 
>>> cannot destroy the undo structure, the other thread might be still 
>>> interested in it.
>>> If we allow sys_unshare() for multithreaded processes with CLONE_NEWIPC 
>>> and without CLONE_SYSVSEM, then we must handle this case.
>>>       
>> Hm... I'd simply disable creating any new namespaces for threads.
>> I think other namespaces developers agree with me. Serge, Suka, Eric
>> what do you think?
>>     
>
> I almost agree.  sys_unshare() in a multithreaded process breaks
> all kinds of user space libs.  So you can only reasonably look at
> the problem as what we do with linux tasks that share some things
> and not others.  The posix/opengroup notion of processes and threads
> are a distraction.
>
> In this case requiring it appears that to require unsharing both
> CLONE_SYSVSEM and CLONE_NEWIPC at the same time.  (i.e. unshare
> of CLONE_SYSVSEM should fail if CLONE_NEWIPC is not also specified).
> Then to make it work we make unshare of SYSVSEM succeed when it is not shared.
>
> This looks like about a 5 line patch or two.
>
>   
Probably something like this in copy_ipcs:
       if (current->sysvsem.undo_list != NULL && 
atomic_read(current->sysvsem.undo_list->refcount) > 1)
          return -EINVAL;

I'll think about it and write a patch over the weekend.

> The effect is because we don't support unsharing of SYSVSEM currently
> we don't support a threaded process unsharing the ipc namespace.
>   
I agree.
Btw: The manpage of unshare() is IMHO a bit misleading:
 >   NAME
 >       unshare - disassociate parts of the process execution context

unshare() only operates on a single thread, shouldn't the description be 
"disassociate parts of the thread execution context"?

--
    Manfred