Re: [PATCH v2][GIT PULL] tracing: Prevent unloadable modules fromusing trace_bprintk()

[Steven Rostedt]

On Fri, 2010-10-22 at 13:30 +0800, Li Zefan wrote:

> In fact tracepoint is free from this bug, because we'll empty the ring
> buffer if the unloading module has tracepoints in it.

Hehe, and I should know, I wrote that code :-)


> 
> So for trace_bprintk, why can't we do the same thing? If a module has
> trace_bprintk calls in it, just empty the ring buffer when unloading
> module.
> 
> And that's as simple as something like this:
> 
> diff --git a/kernel/trace/trace_printk.c b/kernel/trace/trace_printk.c
> index 2547d88..103987f 100644
> --- a/kernel/trace/trace_printk.c
> +++ b/kernel/trace/trace_printk.c
> @@ -80,6 +80,13 @@ static int module_trace_bprintk_format_notify(struct notifier_block *self,
>  
>  		if (val == MODULE_STATE_COMING)
>  			hold_module_trace_bprintk_format(start, end);
> +		else if (val == MODULE_STATE_GOING) {
> +			/*
> +			 * It is safest to reset the ring buffer if the
> +			 * module being unloaded uses trace_bprintk.
> +			 */
> +			tracing_reset_current_online_cpus();
> +		}
>  	}
>  	return 0;
>  }

This could definitely work.

But this prevents developers using trace_printk() in their exit() code.
We should probably add a trace_mod_printk() or something that just
forces the slow version that does not require flushing the ring buffer
on removal. As long as there's not a single trace_printk() in the
module, and it only uses the alternative, then this should work.

-- Steve


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