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

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Date:	Mon, 29 Mar 2010 15:21:32 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Randy Dunlap <randy.dunlap@...cle.com>,
	Eric Dumazet <dada1@...mosbay.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	linux-kernel@...r.kernel.org, stable <stable@...nel.org>
Subject: Re: [PATCH 2.6.29.x - 2.6.31.1] module: fix __module_ref_addr()

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?

-- Steve


On Sat, 2010-03-27 at 10:31 -0400, Mathieu Desnoyers wrote:
> __module_ref_addr() should use per_cpu_ptr() to obfuscate the pointer
> (RELOC_HIDE is needed for per cpu pointers).
> 
> This non-standard per-cpu pointer use has been introduced by commit
> 720eba31f47aeade8ec130ca7f4353223c49170f
> 
> It causes a NULL pointer exception on some configurations when CONFIG_TRACING is
> enabled on 2.6.33. This patch fixes the problem (acknowledged by Randy who
> reported the bug).
> 
> It did not appear to hurt previously because most of the accesses were done
> through local_inc, which probably obfuscated the access enough that no compiler
> optimizations were done. But with local_read() done when CONFIG_TRACING is
> active, this becomes a problem. Non-CONFIG_TRACING is probably affected as well
> (module.c contains local_set and local_read that use __module_ref_addr()), but I
> guess nobody noticed because we've been lucky enough that the compiler did not
> generate the inappropriate optimization pattern there.
> 
> This patch should be queued for the 2.6.29.x through 2.6.33.x stable branches.
> (tested on 2.6.33.1 x86_64)
> 
> The __module_ref_addr() problem disappears in 2.6.34-rc kernels because these
> percpu accesses were re-factored.
> 
> It makes me wonder about other non-standard uses of per_cpu_offset: there is one
> in module.c and two in lockdep.c, which are still in 2.6.34-rc. This should
> probably be fixed by the code authors in separate patches.
> 
> lockdep.c: commit 8e18257d29238311e82085152741f0c3aa18b74d
> module.c: commit 6b588c18f8dacfa6d7957c33c5ff832096e752d3
> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Tested-by: Randy Dunlap <randy.dunlap@...cle.com>
> CC: Eric Dumazet <dada1@...mosbay.com>
> CC: Rusty Russell <rusty@...tcorp.com.au>
> CC: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> CC: Tejun Heo <tj@...nel.org>
> CC: Ingo Molnar <mingo@...e.hu>
> CC: Andrew Morton <akpm@...ux-foundation.org>
> CC: Linus Torvalds <torvalds@...ux-foundation.org>
> CC: Greg Kroah-Hartman <gregkh@...e.de>
> CC: Steven Rostedt <rostedt@...dmis.org>
> ---
>  include/linux/module.h |    2 +-
>  kernel/module.c        |    2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> Index: linux-2.6-lttng/include/linux/module.h
> ===================================================================
> --- linux-2.6-lttng.orig/include/linux/module.h	2010-03-25 11:01:53.000000000 -0400
> +++ linux-2.6-lttng/include/linux/module.h	2010-03-25 11:01:59.000000000 -0400
> @@ -467,7 +467,7 @@ void symbol_put_addr(void *addr);
>  static inline local_t *__module_ref_addr(struct module *mod, int cpu)
>  {
>  #ifdef CONFIG_SMP
> -	return (local_t *) (mod->refptr + per_cpu_offset(cpu));
> +	return (local_t *) per_cpu_ptr(mod->refptr, cpu);
>  #else
>  	return &mod->ref;
>  #endif
> 


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