From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753050Ab0JUIRD (ORCPT ); Thu, 21 Oct 2010 04:17:03 -0400 Received: from mx2.mail.elte.hu ([157.181.151.9]:46829 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752309Ab0JUIQ7 (ORCPT ); Thu, 21 Oct 2010 04:16:59 -0400 Date: Thu, 21 Oct 2010 10:16:48 +0200 From: Ingo Molnar To: Steven Rostedt Cc: linux-kernel@vger.kernel.org, Andrew Morton , Frederic Weisbecker , Thomas Gleixner Subject: Re: [PATCH 1/2] tracing: Prevent unloadable modules from using trace_bprintk() Message-ID: <20101021081648.GC8775@elte.hu> References: <20101021024233.530237054@goodmis.org> <20101021024304.827826848@goodmis.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20101021024304.827826848@goodmis.org> User-Agent: Mutt/1.5.20 (2009-08-17) X-ELTE-SpamScore: -2.0 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-2.0 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.5 -2.0 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Steven Rostedt wrote: > From: Steven Rostedt > > 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 > Cc: Thomas Gleixner > Signed-off-by: Steven Rostedt > --- > 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) && \ > + (defined(CONFIG_MODULE_UNLOAD) || !defined(CONFIG_MODVERSIONS)) Erm. Ignore checkpatch when the solution is to mess up the code ... > +# define FORCE_TRACEPRINTK 1 > +#else > +# define FORCE_TRACEPRINTK 0 > +#endif > + > /** > * trace_printk - printf formatting in the ftrace buffer > * @fmt: the printf format for printing > @@ -558,14 +575,14 @@ do { \ > #define trace_printk(fmt, args...) \ > do { \ > __trace_printk_check_format(fmt, ##args); \ > - if (__builtin_constant_p(fmt)) { \ > + if (__builtin_constant_p(fmt) && !FORCE_TRACEPRINTK) { \ > static const char *trace_printk_fmt \ > __attribute__((section("__trace_printk_fmt"))) = \ > __builtin_constant_p(fmt) ? fmt : NULL; \ > \ > __trace_bprintk(_THIS_IP_, trace_printk_fmt, ##args); \ > } else \ > - __trace_printk(_THIS_IP_, fmt, ##args); \ > + __trace_printk(_THIS_IP_, fmt, ##args); \ > } while (0) > > extern int > diff --git a/kernel/trace/trace_printk.c b/kernel/trace/trace_printk.c > index 2547d88..c4a5db6 100644 > --- a/kernel/trace/trace_printk.c > +++ b/kernel/trace/trace_printk.c > @@ -115,7 +115,9 @@ int __trace_bprintk(unsigned long ip, const char *fmt, ...) > va_end(ap); > return ret; > } > +#if !FORCE_TRACEPRINTK > EXPORT_SYMBOL_GPL(__trace_bprintk); > +#endif Looks quite ugly all around. Cannot suggest anything better though straight away - so please Cc: it more widely and get an ack from the module folks: Rusty, Linus, akpm. Thanks, Ingo