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

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Date:	Wed, 26 Nov 2014 17:34:04 +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 09:25:37AM -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;".
> > 
> > My other concern here was that a few lines later we do:
> > 
> > 	entry->buf[len] = '\0';
> > 
> > I worried that maybe we were putting the NUL char past the end of the
> > array but I wasn't smart enough to figure out the size of entry->buf[].
> 
> entry is of type struct print_entry *, which is defined by macro magic
> (sorry),

I figure this bit out using make devel/kernel/trace/trace.i.

> and would look like this:
> 
> struct print_entry {
> 	unsigned long		ip;
> 	char			buf[];
> };
> 
> But then it is allocated like so:
> 
> 	size = sizeof(*entry) + len + 1;
> 	event = trace_buffer_lock_reserve(buffer, TRACE_PRINT, size,
> 					  flags, pc);
> 
> 	entry = ring_buffer_event_data(event);

I was so close to figuring this out on my own...  Let me send a v2 with
an amended changelog.

regards,
dan carpenter

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