Re: [RFC, PATCH] fix SEM_UNDO with namespaces

["Serge E. Hallyn" <serue@us.ibm.com>]

Quoting Pavel Emelyanov (xemul@openvz.org):
> Manfred Spraul wrote:
> > Pavel Emelyanov wrote:
> >> Manfred Spraul wrote:
> >>   
> >>> Hi,
> >>>
> >>> the attached patch should fix the combination of CLONE_NEWIPC with 
> >>> shared sysv undo structures (the common case, just 
> >>> sys_unshare(CLONE_NEWIPC)):
> >>> lookup_undo() now locates the undo array based on both semid and the 
> >>> namespace pointer.
> >>>     
> >> If you start using any IPC object and then call unshare with CLONE_NEWIPC,
> >> then it's your problem, but not the kernel.
> >>   
> > The result is a kernel memory corruption, and kernel memory corruptions 
> > are always the kernel's problem.
> 
> Agree. Must be fixed, but I'm not sure we should try handling this
> case by trying to de-op semaphores for former task namespace. I think
> that destroying this list or returning -EBUSY for this case is OK.
> 
> > The code assumed that a semaphore id is globally unique. With 
> > namespaces,  this is not true anymore.
> > If two semaphore arrays exist with the same id, but different sizes, 
> > then semops will cause memory corruptions: The undo structure contains 
> > one element for each semaphore, thus the semop will write behind the end 
> > of the memory allocation.
> > 
> >> 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?

Absolutely.

-serge