linux-kernel - Re: [RFC][PATCH] tracing: Add trace_printk_ptr() to force non use of trace

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Message-ID: <20160629072428.GA14656@krava>
Date:	Wed, 29 Jun 2016 09:24:28 +0200
From:	Jiri Olsa <jolsa@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC][PATCH] tracing: Add trace_printk_ptr() to force non use of
 trace_bprintk()

On Tue, Jun 28, 2016 at 07:19:29PM -0400, Steven Rostedt wrote:
> trace_printk() is a very helpful tool for debugging the kernel. It adds
> lots of tricks to optimize itself to prevent any "heisenbugs". That is,
> having the addition of tracing cause the bug to change its timing and
> disappear. One of this tricks is to use trace_bprintk() when possible,
> which just stores the format and the arguments into the ring buffer to
> be processed later at the time of reading the trace output.
> 
> The issue with this is that there's some printf() fields that can do
> redirection. There's a list of "%p*" values that will dereference the
> pointer saved in the buffer. This is an issue with trace_printk()
> because the pointer could have been freed between the time the
> trace_printk() was called and the time the buffer is read. This will
> cause a bad pointer dereference.
> 
> The preferable fix is most likely to change bprintk() to recognize
> these pointers and instead of saving the pointer in the buffer to be
> processed later, it could do the conversion and save the value in the
> buffer. But this added processing kills the whole point of bprintk()
> from being fast and not doing any processing during the recording.
> Perhaps it should simply warn and/or refuse to print.
> 
> The simpler solution is to add an alternate trace_printk() that always
> uses the non optimized version that does the string processing at the
> time of the record, and saves just the string to the ring buffer.
> 
> There's been many times that I myself wanted this version. So here it
> is.
> 
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>

if we dont go with this change:
  http://marc.info/?l=linux-kernel&m=146715171527229&w=2

this patch works for me:

Tested-by: Jiri Olsa <jolsa@...nel.org>

thanks,
jirka