Re: [RFC][PATCH -mm] IPC: fix error checking in all new xxx_lock() functions

[Nadia Derbey <Nadia.Derbey@bull.net>]

Pierre Peiffer wrote:
> In the new implementation of the [sem|shm|msg]_lock[_check]() routines,
> we use the return value of ipc_lock() in container_of() without any check.
> But ipc_lock may return a errcode. The use of this errcode in container_of()
> may alter this errcode, and we don't want this.
> 
> Today, there is no problem because the member used in these container_of()
> is the first member of its container (offset == 0), the errcode isn't changed
> then. But in the general case, we can't count on this assumption and this
> may lead later to a real bug if we don't correct this.
> 
> In fact, the proposed solution is simple and correct. But it has the drawback
> of adding one more check ('if' statement) in the chain: we do a first check in
> ipc_lock(), now in xxx_lock() and then one later in the caller of xxx_lock()
> That's why I send this as RFC, may be another approach could be considered.
> 

This is really what disturbs me this solution: the same check will be 
done several times. But is true that we have to do something.
So why not simply adding a BIG COMMENT before the msg_queue, sem_array 
and shmid_ds stating that the kern_ipc_perm should stay at the beinnign 
of the structure?

Will try to look for another solution.

Regards,
Nadia