Re: [PATCH] Smack: Simplified Mandatory Access Control Kernel

Casey Schaufler
2007-08-12T21:46:19Z

--- Andi Kleen <andi@firstfloor.org> wrote:

> On Sun, Aug 12, 2007 at 10:48:05AM -0700, Casey Schaufler wrote:
> >
> > --- Andi Kleen <andi@firstfloor.org> wrote:
> >
> > > > Entries are never deleted, although they can be modified.
> > >
> > > The modification case still seems racy then.
> >
> > Fair enough. I'll look into real list management.
>
> You don't necessarily need more list management if you don't
> plan to remove entries, but just replace them.
>
> e.g. what could work to atomically replace is:
>
> - Make the buffer a pointer to an allocated buffer that also
> contains a struct rcu_head.
> - Reader: Does rcu_read_lock() around list walk (that just disables
> preemption on preemptible kernels and is otherwise a nop).
> Also uses rcu_reference for reading the pointer.
> - Writer: Continues using the mutex to protect against other writers.
> When changing an entry allocate a new buffer + rcu_head. Initialize
> buffer. Replace pointer. Free old buffer using call_rcu()
>
> The RCU would just make sure the buffer is not freed while other
> CPUs are still accessing it. It also means they can use stale
> rules for a time, but it is a strictly bounded time
> (bounded to max time walking the list plus max time any interrupt
> handlers inbetween run [admittedly that can be very long in theory,
> but it's all logically only a single rule check])

Thank you. You have no idea how helpful that little suggestion was.

Casey Schaufler
casey@schaufler-ca.com