linux-kernel - Re: [PATCH] lib: introduce strdup_from

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Message-ID: <BANLkTikmVncMdJhK0Hi6uWeiE_S9TtdhTQ@mail.gmail.com>
Date:	Fri, 3 Jun 2011 22:12:37 +0300
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	Scott Wood <scottwood@...escale.com>
Cc:	Timur Tabi <timur@...escale.com>, alan@...rguk.ukuu.org.uk,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [PATCH] lib: introduce strdup_from_user

On Fri, Jun 3, 2011 at 9:53 PM, Scott Wood <scottwood@...escale.com> wrote:
> On Fri, 3 Jun 2011 21:39:28 +0300
> Alexey Dobriyan <adobriyan@...il.com> wrote:
>
>> On Fri, Jun 3, 2011 at 9:34 PM, Timur Tabi <timur@...escale.com> wrote:
>> > Would it be better if I did this:
>>
>> The point is data should cross kernelspace/userspace boundary only once.
>>
>
> Why does it matter, as long as it doesn't hurt the kernel if userspace
> plays games (i.e. take care of the NUL termination), and it's not a
> performance problem?

Because now you're lucky C strings are NUL-terminated.
If this "idiom" applies to some other case like "validate + copy",
we have a bug.

We copy data to kernelspace THEN validate or copy or whatever.
This is obviously correct and safe.
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