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

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Date:	Thu, 16 Oct 2014 20:11:45 +0200
From:	Rickard Strandqvist <rickard_strandqvist@...ctrumdigital.se>
To:	Jason Cooper <jason@...edaemon.net>
Cc:	Joe Perches <joe@...ches.com>, 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" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3v] char: hw_random: core.c: Changed from using strncat to strlcat

2014-10-16 20:05 GMT+02:00 Jason Cooper <jason@...edaemon.net>:
> 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 :)
>


Hi

I will submit a new patch without return strlen.


All these features are more or less stupid really.
You can also read what Linus thinks about this here.

https://plus.google.com/111049168280159033135/posts/1amLbuhWbh5


I keep on trying to get a patch that solves one of these stupid
functions, but the strncpy in that case.


Kind regards
Rickard Strandqvist
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