linux-kernel - Re: [RFC PATCH] use snprintf instead of sprintf in rcu_torture

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Date:	Thu, 19 Jun 2014 16:33:09 -0700
From:	Joe Perches <joe@...ches.com>
To:	Pranith Kumar <bobby.prani@...il.com>
Cc:	paulmck@...ux.vnet.ibm.com, LKML <linux-kernel@...r.kernel.org>,
	romanov.arya@...il.com, Josh Triplett <josh@...htriplett.org>
Subject: Re: [RFC PATCH] use snprintf instead of sprintf in
 rcu_torture_printk

On Thu, 2014-06-19 at 19:24 -0400, Pranith Kumar wrote:
> (dropping some CCs)
> 
> On 06/19/2014 05:00 PM, Paul E. McKenney wrote:
> > On Wed, Jun 18, 2014 at 12:49:42PM -0700, Joe Perches wrote:
> >>
> >> I believe the function doesn't work well.
> >>
> >> static void
> >> rcu_torture_stats_print(void)
> >> {
> >> 	int size = nr_cpu_ids * 200 + 8192;
> >> 	char *buf;
> >>
> >> 	buf = kmalloc(size, GFP_KERNEL);
> >> 	if (!buf) {
> >> 		pr_err("rcu-torture: Out of memory, need: %d\n", size);
> >> 		return;
> >> 	}
> >> 	rcu_torture_printk(buf);
> >> 	pr_alert("%s", buf);
> >> 	kfree(buf);
> >> }
> >>
> >> rcu_torture_printk simply fills buf
> >>
> >> btw: I believe the arguments should pass size and
> >>      rcu_torture_printk should use snprintf/size
> >>
> >> but all printks are limited to a maximum of 1024
> >> bytes so the large allocation is senseless and
> >> would even if it worked, would likely need to be
> >> vmalloc/vfree
> > 
> > Fair point!
> > 
> > Pranith, Romanov, if you would like part of RCU that is less touchy
> > about random hacking, this would be a good place to start.  Scripts in
> > tools/testing/selftests/rcutorture/bin do care about some of the format,
> > but the variable-length portion generated by cur_ops->stats() is as far
> > as I know only parsed by human eyes.
> > 
> 
> Here is a first run of the change. Please let me know if I am totally off. RFC. :)
> 
> Three things on Todo list:
> 
> * We need to check that we are using less than the allocated size of the buffer (used > size). (we are allocating a big buffer, so not sure if necessary)
> * Need to check with the scripts if they are working.
> * I used a loop for pr_alert(). I am not sure this is right, there should be a better way for printing large buffers
> 
> If the overall structure is ok I will go ahead and check how the scripts are handling these changes. 
[]
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
[]
> @@ -1130,17 +1134,23 @@ rcu_torture_printk(char *page)
>  static void
>  rcu_torture_stats_print(void)
>  {
[]
> +	/* printk can print only print LOG_LINE_MAX chars at a time */
> +	while (used > 0) {
> +		size_pr = pr_alert("%s", buf);
> +		used -= size_pr;
> +		buf  += size_pr;
> +	}
> +	vfree(buf);
>  }

You can't increment buf then vfree it.

Try a char *tmpbuf = buf, and increment it.

I suggest you check by strcspn(tmpbuf, "\n")
save the byte after, replace it with 0,
print, replace the 0 with saved byte, and
loop until end of buffer.


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