linux-kernel - Re: [PATCH 1/2] tracing: Prevent unloadable modules from using trace

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Date:	Thu, 21 Oct 2010 05:42:58 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/2] tracing: Prevent unloadable modules from using
	trace_bprintk()

On Wed, Oct 20, 2010 at 10:42:34PM -0400, Steven Rostedt wrote:
> From: Steven Rostedt <srostedt@...hat.com>
> 
> While debugging a module, I found that unloading the module and
> then reading the ring buffer can cause strange side effects, including
> a kernel crash.
> 
> This is due to the trace_bprintk(). The trace_bprintk() is a faster
> version of trace_printk(). The difference is that trace_bprintk()
> only copies the arguments and a pointer to the format string into
> the ring buffer.
> 
> If a module uses this function and is unloaded, the pointer back to
> the format string in the module is still around. If the trace file
> is read, then the pointer is referenced and this can cause a kernel
> oops.
> 
> The simple solution is to not let modules use trace_bprintk() and
> instead it will use the slower version of this.
> 
> When talking with Frederic Weisbecker about it, he suggested not to
> punish modules that can not be unloaded since they do not have
> this side effect. Modules that can not be unloaded can still use
> trace_bprintk(). We added a check for MODVERSIONS to be set to make
> sure that the module and kernel have the same options. If you
> run without MODVERSIONS set, and you load a module that was compiled
> differently, then that's just your tough luck.
> 
> Cc: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> ---
>  include/linux/kernel.h      |   21 +++++++++++++++++++--
>  kernel/trace/trace_printk.c |    2 ++
>  2 files changed, 21 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index 2b0a35e..1003476 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -538,6 +538,23 @@ do {									\
>  		____trace_printk_check_format(fmt, ##args);		\
>  } while (0)
>  
> +/*
> + * Module code must not use trace_bprintk, because if it is unloaded
> + * then we leave a pointer back to the module code inside
> + * the ring buffer, and then reading the ring buffer may cause a bug.
> + *
> + * We do allow for modules to use it if the kernel does not allow
> + * unloading of modules, and MODVERSIONS is set (to make sure kernel
> + * and module are the same). If you load modules without MODVERSIONS
> + * set, then you deserve what you get.
> + */
> +#if defined(MODULE) &&							\



Did you mean CONFIG_MODULE may be?

Thanks.

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