Re: 2.6.23-rc6-mm1: IPC: sleeping function called ...

From: Jarek Poplawski
Date: Mon Sep 24 2007 - 02:52:06 EST

On Fri, Sep 21, 2007 at 01:03:47PM +0200, Jarek Poplawski wrote:
> I hope not! But, then it would be probably another logical trick:
> ipc_rcu_getref/putref() seems to prevent kfreeing of a structure, so
> if it's used in do_msgsnd() there should be a risk something can do
> this kfree at this moment, and it seems freeque() is the only one,
> which both: can do this and cares for this refcount. Then, e.g., if
> any of them does ipc_rcu_getref() a bit later and sees old (cached)
> value - kfree can be skipped forever. [...]

After rethinking, this scenario seems to be wrong or very unprobable
(I'm not sure of all ways "if (--container...)" could be compiled),
so there should be no such risk - double kfree/vfree is more probable,
so no danger. More likely is such refcount abuse: ipc_rcu_getref() in
do_msgsnd() done a bit after ipc_rcu_putref() in freeque() (msq
pointer acquired by do_msgsend() before freeque() started); then,
after schedule(), do_msgsnd() can work with kfreed msq_queue structure
(at least considering classic RCU).

Jarek P.
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