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

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Message-ID: <20141126143719.GT4893@mwanda>
Date:	Wed, 26 Nov 2014 17:37:19 +0300
From:	Dan Carpenter <dan.carpenter@...cle.com>
To:	Steven Rostedt <rostedt@...dmis.org>
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, 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.

regards,
dan carpenter

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