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

[Joe Perches <joe@perches.com>]

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.