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

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Message-ID: <20100419182643.GG32347@kroah.com>
Date:	Mon, 19 Apr 2010 11:26:43 -0700
From:	Greg KH <greg@...ah.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	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, Mar 29, 2010 at 10:22:38PM -0400, Mathieu Desnoyers wrote:
> * Steven Rostedt (rostedt@...dmis.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@...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.
> 
> Exactly. Steven has beaten me on the start line on this one. ;)

Ok, I'm totally confused now :(

What patch should I apply to a stable release, and which stable release?

thanks,

greg k-h
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