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

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Message-ID: <CAFo99gbGrGXz+gYQn0hAPKxN73BLLWhaUPVN0DVq6-3PiFzOSw@mail.gmail.com>
Date:	Thu, 16 Oct 2014 19:41:55 +0200
From:	Rickard Strandqvist <rickard_strandqvist@...ctrumdigital.se>
To:	Joe Perches <joe@...ches.com>
Cc:	Matt Mackall <mpm@...enic.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Torsten Duwe <duwe@....de>, "Theodore Ts'o" <tytso@....edu>,
	Jason Cooper <jason@...edaemon.net>,
	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 19:25 GMT+02:00 Joe Perches <joe@...ches.com>:
> 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?
>
> You could use a temporary for the returned length
> of the strlcat so that if it's shorter than
> the buffer, the next strlcat can start at the
> appropriate known position instead of having
> to do the initial strlen again and again.
>


Hi Joe

Yes that did not take advantage last strlcat was foolish.

But the others, I am very hesitant about. Because strlcat like
snprintf and strlcpy returns the length that would be copied rather
than what is actually copied. Hence such a code to be even more
complex than before.


Kind regards
Rickard Strandqvist
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