linux-kernel - Re: [PATCH 3v] char: hw_random: core.c: Changed from using strncat to strlcat

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Message-ID: <20141016180517.GO17447@titan.lakedaemon.net>
Date:	Thu, 16 Oct 2014 14:05:17 -0400
From:	Jason Cooper <jason@...edaemon.net>
To:	Joe Perches <joe@...ches.com>
Cc:	Rickard Strandqvist <rickard_strandqvist@...ctrumdigital.se>,
	Matt Mackall <mpm@...enic.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Torsten Duwe <duwe@....de>, Theodore Ts'o <tytso@....edu>,
	Amit Shah <amit.shah@...hat.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Kees Cook <keescook@...omium.org>,
	Dan Carpenter <dan.carpenter@...cle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3v] char: hw_random: core.c:  Changed from using
 strncat to strlcat

On Thu, Oct 16, 2014 at 10:55:57AM -0700, Joe Perches wrote:
> On Thu, 2014-10-16 at 13:48 -0400, Jason Cooper wrote:
> > On Thu, Oct 16, 2014 at 10:25:03AM -0700, Joe Perches wrote:
> > > On Thu, 2014-10-16 at 19:15 +0200, Rickard Strandqvist wrote:
> > > > The buf is used to hold the list of hwrng devices registered.
> > > > The old code ensures we don't walk off the end of buf as we
> > > > fill it, but it's unnecessarily complicated and thus difficult
> > > > to maintain. Simplify it by using strlcat.
> > > > We also ensure the string within buf is NULL terminated
> > > > so the final strlen is ok.
> > > []
> > > > diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
> > > []
> > > > @@ -289,16 +288,13 @@ static ssize_t hwrng_attr_available_show(struct device *dev,
> > > >  		return -ERESTARTSYS;
> > > >  	buf[0] = '\0';
> > > >  	list_for_each_entry(rng, &rng_list, list) {
> > > > -		strncat(buf, rng->name, PAGE_SIZE - ret - 1);
> > > > -		ret += strlen(rng->name);
> > > > -		strncat(buf, " ", PAGE_SIZE - ret - 1);
> > > > -		ret++;
> > > > +		strlcat(buf, rng->name, PAGE_SIZE);
> > > > +		strlcat(buf, " ", PAGE_SIZE);
> > > >  	}
> > > > -	strncat(buf, "\n", PAGE_SIZE - ret - 1);
> > > > -	ret++;
> > > > +	strlcat(buf, "\n", PAGE_SIZE);
> > > >  	mutex_unlock(&rng_mutex);
> > > >  
> > > > -	return ret;
> > > > +	return strlen(buf);
> > > >  }
> > > >  
> > > >  static DEVICE_ATTR(rng_current, S_IRUGO | S_IWUSR,
> > > 
> > > Rickard, can you please use some optimizations here
> > > (and elsewhere) so that strlcat doesn't always have
> > > to strlen the first string and the return doesn't
> > > have to do the strlen too?
> > 
> > Joe, is further optimization worth the effort?  This function is only
> > called when the end user reads the sysfs file rng_available...
> 
> It's unlikely that it matters much here.
> 
> I just don't like the repeated and unnecessary strlen.
> 
> I do imagine this pattern being repeated though where
> it does matter.

Agreed.  Back to why I like to see descriptions from the author
demonstrating they grok the code they are changing :)

thx,

Jason.
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