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

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Date:	Wed, 26 Nov 2014 09:25:37 -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 17:06:21 +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), 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);

Now that size is the key. The "len + 1" covers the string. Which means

	entry->buf[len] = '\0';

is fine.

-- Steve


> 
> Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 42a822d..22af2ae 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -2161,7 +2161,7 @@ __trace_array_vprintk(struct ring_buffer *buffer,
>  	}
>  
>  	len = vsnprintf(tbuffer, TRACE_BUF_SIZE, fmt, args);
> -	if (len > TRACE_BUF_SIZE)
> +	if (len >= TRACE_BUF_SIZE)
>  		goto out;
>  
>  	local_save_flags(flags);

--
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