Re: [PATCH 1/2] tracing: Prevent unloadable modules from usingtrace_bprintk()

[Steven Rostedt]

On Thu, 2010-10-21 at 10:16 +0200, Ingo Molnar wrote:
> * Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> 
> > From: Steven Rostedt <srostedt@xxxxxxxxxx>
> > 
> > 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@xxxxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > ---
> >  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 ...

Heh, OK.

> 
> > +# 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.

Just added them.

One thing that bothers me about this patch is the negative of
FORCE_TRACEPRINTK.  I think it would be a nicer patch if I changed that
to a positive TRACE_BPRINTK_ALLOWED, then it would be:

+	if (__builtin_constant_p(fmt) && TRACE_BPRINTK_ALLOWED) {

+#if TRACE_BPRINTK_ALLOWED
  EXPORT_SYMBOL_GPL(__trace_bprintk);
+#endif
 

The patch is still a bit ugly, but this change makes it a little nicer
to read.

Thanks,

-- Steve


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