Re: [stable] [PATCH 2.6.29.x - 2.6.31.1] module: fix __module_ref_addr()

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

* Steven Rostedt (rostedt@goodmis.org) wrote:
> On Mon, 2010-03-29 at 14:07 -0700, Greg KH wrote:
> > On Mon, Mar 29, 2010 at 04:09:46PM -0400, Mathieu Desnoyers wrote:
> > > * Steven Rostedt (rostedt@goodmis.org) wrote:
> > > > This patch does not apply to 2.6.34-rc, and the code in upstream looks
> > > > to have been fixed. Should this go to stable?
> > > 
> > > Yes. 2.6.34-rc does not have this issue anymore, but the patch is needed in
> > > -stable.
> > 
> > Why is this not in .34-rc2?  Can you find the specific patch in Linus's
> > tree that solves this and let stable@kernel.org know about it?
> > 
> 
> Looks like it was this commit:
> 
> commit e1783a240f491fb233f04edc042e16b18a7a79ba
> Author: Christoph Lameter <cl@linux-foundation.org>
> Date:   Tue Jan 5 15:34:50 2010 +0900
> module: Use this_cpu_xx to dynamically allocate counters
> 
> Mathieu's fix was:
> 
> -   return (local_t *) (mod->refptr + per_cpu_offset(cpu));
> +   return (local_t *) per_cpu_ptr(mod->refptr, cpu);
> 
> As Mathieu states in his change log, the bug is that the mod->refptr is
> outside the assembly obfuscation of the per_cpu_offset(). This allows
> the compiler to optimize and cause a NULL pointer dereference with the
> manipulation of per cpu data.
> 
> Christoph Lameter's change fixes this bug as a side effect:
> 
> -static inline local_t *__module_ref_addr(struct module *mod, int cpu)
> -{
> -#ifdef CONFIG_SMP
> -       return (local_t *) (mod->refptr + per_cpu_offset(cpu));
> -#else
> -       return &mod->ref;
> -#endif
> -}
> -
>  /* Sometimes we know we already have a refcount, and it's easier not
>     to handle the error case (which only happens with rmmod --wait). */
>  static inline void __module_get(struct module *module)
>  {
>         if (module) {
> -               unsigned int cpu = get_cpu();
> -               local_inc(__module_ref_addr(module, cpu));
> +               preempt_disable();
> +               __this_cpu_inc(module->refptr->count);
>                 trace_module_get(module, _THIS_IP_,
> -                                local_read(__module_ref_addr(module, cpu)));
> -               put_cpu();
> +                                __this_cpu_read(module->refptr->count));
> +               preempt_enable();
>         }
>  }
> 
> By removing the buggy code all together.

Exactly. Steven has beaten me on the start line on this one. ;)

Thanks,

Mathieu

> 
> -- Steve
> 
> 

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com