linux-kernel - Re: [stable] [PATCH 2.6.29.x - 2.6.31.1] module: fix __module_ref

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Date:	Mon, 29 Mar 2010 21:08:00 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Greg KH <greg@...ah.com>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	stable <stable@...nel.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org, Eric Dumazet <dada1@...mosbay.com>,
	Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [stable] [PATCH 2.6.29.x - 2.6.31.1] module: fix
 __module_ref_addr()

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@...dmis.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@...nel.org know about it?
> 

Looks like it was this commit:

commit e1783a240f491fb233f04edc042e16b18a7a79ba
Author: Christoph Lameter <cl@...ux-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.

-- Steve


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