Re: [patch] tracing: off by one in __trace_array_vprintk()

[Steven Rostedt]

On Wed, 26 Nov 2014 18:05:41 +0300
Dan Carpenter <dan.carpenter@xxxxxxxxxx> wrote:

> On Wed, Nov 26, 2014 at 09:43:06AM -0500, Steven Rostedt wrote:
> > 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?
> 
> Heh.  vsnprintf() is going to put a NUL char at the end of the string
> even if you don't plan to use it.  :P
> 
> Another option would be to just print truncated output.  We could use
> vscnprintf() and remove the off by one limit check.

Yes, that is a better patch. The previous one wasn't really fixing
anything. But printing truncated output is better than nothing.

Want to send it officially?

Thanks,

-- Steve

> 
> regards,
> dan carpenter
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 42a822d..ab76b7b 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -2160,9 +2160,7 @@ __trace_array_vprintk(struct ring_buffer *buffer,
>  		goto out;
>  	}
>  
> -	len = vsnprintf(tbuffer, TRACE_BUF_SIZE, fmt, args);
> -	if (len > TRACE_BUF_SIZE)
> -		goto out;
> +	len = vscnprintf(tbuffer, TRACE_BUF_SIZE, fmt, args);
>  
>  	local_save_flags(flags);
>  	size = sizeof(*entry) + len + 1;
> @@ -2173,8 +2171,7 @@ __trace_array_vprintk(struct ring_buffer *buffer,
>  	entry = ring_buffer_event_data(event);
>  	entry->ip = ip;
>  
> -	memcpy(&entry->buf, tbuffer, len);
> -	entry->buf[len] = '\0';
> +	memcpy(&entry->buf, tbuffer, len + 1);
>  	if (!call_filter_check_discard(call, entry, buffer, event)) {
>  		__buffer_unlock_commit(buffer, event);
>  		ftrace_trace_stack(buffer, flags, 6, pc);

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