netdev - Re: [patch] isdn: fix information leak

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Date:	Thu, 5 Aug 2010 21:24:46 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	Dan Carpenter <error27@...il.com>
Cc:	Karsten Keil <isdn@...ux-pingi.de>, netdev@...r.kernel.org,
	kernel-janitors@...r.kernel.org
Subject: Re: [patch] isdn: fix information leak

On Thu, Aug 5, 2010 at 9:18 PM, Changli Gao <xiaosuo@...il.com> wrote:
> On Thu, Aug 5, 2010 at 7:37 PM, Dan Carpenter <error27@...il.com> wrote:
>>
>> Both strncpy() and strlcpy() take a limitter.  The difference is that
>> strlcpy() always takes on a terminator and strncpy() only adds a
>> terminator if there is space.
>>
>> strlcpy() is a BSD function that never caught on in Linux.  The glibc
>> maintainers think that if you accidentally chop off the last part of a
>> word that makes you an idiot.  They think you should known the length of
>> your data at all times and use memcpy() or a proper string library.
>>
>> I prefer strlcpy() to strncpy().  Some people do stuff like:
>>        strncpy(bar, foo, n);
>>        bar[n] = '\0';
>> You have to read through the code to find if n is "sizeof(bar)" or
>> "sizeof(bar) - 1".  Which is a pain in the arse.  strlcpy() is explicit
>> and it's just one line of code instead of two.
>>
>> The other tricky thing you should remember about strncpy() is that the
>> posix version writes NUL chars from the end of the string to the
>> limitter but the kernel version only copies one NUL character.
>>
>
> You should spend some time on reading the source code of strlcpy() and
> strncpy().
>
> the example use of them is:
>
> char dst[24];
> char *src = "test";
>
> strncpy(dst, src, sizeof(dst) - 1);

Oh, Sorry, I made a mistake here. As you said, the code should be
 strncpy(dst, src, sizeof(dst));
 dst[sizeof(dst) - 1] = '\0';

However, if you use strlcpy(), you really don't need to zero the dst buffer.

> strlcpy(dst, src, sizeof(dst));
>
> both of them don't need to zero dst, and they don't need to pad zero
> at then end of the dst.
>



-- 
Regards,
Changli Gao(xiaosuo@...il.com)
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