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

[Casey Schaufler <casey@schaufler-ca.com>]

--- 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