linux-kernel - Re: [patch] tracing: off by one in __trace_array

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Message-ID: <20141126094306.4f465616@gandalf.local.home>
Date:	Wed, 26 Nov 2014 09:43:06 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Dan Carpenter <dan.carpenter@...cle.com>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	kernel-janitors@...r.kernel.org
Subject: Re: [patch] tracing: off by one in __trace_array_vprintk()

On Wed, 26 Nov 2014 17:37:19 +0300
Dan Carpenter <dan.carpenter@...cle.com> wrote:

> On Wed, Nov 26, 2014 at 09:27:06AM -0500, Steven Rostedt wrote:
> > On Wed, 26 Nov 2014 17:06:21 +0300
> > Dan Carpenter <dan.carpenter@...cle.com> wrote:
> > 
> > > This check says "goto out;" if we had to truncate the string.
> > > 
> > > The "tbuffer" buffer has TRACE_BUF_SIZE bytes.  The vsnprintf() function
> > > returns the number of characters (not counting the NUL char) which would
> > > have been printed if there were space.  If we we tried to print
> > > TRACE_BUF_SIZE characters, the last character would have been truncated
> > > to make space for the NUL character so we should "goto out;".
> > > 
> > 
> > Oh, and the NUL character is never copied. We copy the string into the
> > buffer with:
> > 
> > 	memcpy(&entry->buf, tbuffer, len);
> > 
> > Where len does not include NUL, and why we add it ourselves.
> 
> Yeah.  I noticed that.  Probably we should change the memcpy() to:
> 
> 	memcpy(&entry->buf, tbuffer, len + 1);
> 
> and remove separate NUL assignment?  It's a speedup!  I'll do it in v2.
> 

I highly doubt it's much of a speed up. And we do that by sacrificing
that one byte of buffer, which would lose the printk if len does equal
TRACE_BUF_SIZE.

I don't see a bug here. What exactly are you trying to fix?

-- Steve
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