Re: [PATCH] close race condition in shared memory mapping/unmapping

[Chris Wright <chrisw@osdl.org>]

* Neil Horman (nhorman@redhat.com) wrote:
> Hey all-
> 	Found this the other day poking through the ipc code.  There appears to 
> be a race condition in the counter that records how many processes are 
> accessing a given shared memory segment.  In most places the shm_nattch 
> variable is protected by the shm_ids.sem semaphore, but there are a few 
> openings which appear to be able to allow a corruption of this variable 
> when run on SMP systems.  I've attached a patch to 2.6.9-rc2 for review. 
>   The locking may be a little over-aggressive (I was following examples 
> from other points in this file), but I figure better safe than sorry :).

Are you sure you've got this right?  I thought that the shmid_kernel
struct protects shm_nattch with a local (per structure) lock which is
embedded in kern_ipc_perm.  Did you find shm_nattch changes w/out
shm_lock/shm_unlock around it?  I believe shm_ids.sem is protecting the
id allocation, not per object data such as shm_nattch.

thanks,
-chris
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