Re: [PATCH 2/6] IPC namespace - utils

[Cedric Le Goater <clg@fr.ibm.com>]

Eric W. Biederman wrote:
> Cedric Le Goater <clg@fr.ibm.com> writes:
> 
>> I've used the ipc namespace patchset in rc6-mm2. Thanks for putting this
>> together, it works pretty well ! A few questions when we clone :
>>
>> * We should do something close to what exit_sem() already does to clear the
>> sem_undo list from the task doing the clone() or unshare().
> 
> Possibly which case are you trying to prevent?

task records a list of struct sem_undo each containing a semaphore id. When
we unshare ipc namespace, we break the 'reference' between the semaphore id
and the struct sem_array because the struct sem_array are cleared and freed
in the new namespace. When the task exit, that inconstency could lead to
unexpected results in exit_sem(), task locks, BUG_ON, etc. Nope ?


>> * I don't like the idea of being able to unshare the ipc namespace and keep
>> some shared memory from the previous ipc namespace mapped in the process mm.
>> Should we forbid the unshare ?
> 
> No.  As long as the code handles that case properly we should be fine.

what is the proper way to handle that case ? the current patchset is not
protected : a process can be in one ipc namespace and use a shared segment
from a previous ipc namespace. This situation is not desirable in a
migration scenario. May be asking too much for the moment ... and I agree
this can be fixed by the way namespaces are created.

> As a general principle we should be able to keep things from other namespaces
> open if we get them.  The chroot or equivalent binary is the one that needs
> to ensure these kinds of issues don't exist if we care.
>
> Speaking of we should put together a small test application probably similar
> to chroot so people can access these features at least for testing.

are you thinking about a command unshare()ing each namespace or some kind
of create_nsproxy ?

> Ack.  For the unshare fix below.  Could you resend this one separately with
> patch in the subject so Andrew sees it and picks  up?

done.

thanks,

c.