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

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Date:	Fri, 3 Jun 2011 13:34:51 -0500
From:	Timur Tabi <timur@...escale.com>
To:	Alexey Dobriyan <adobriyan@...il.com>
CC:	<alan@...rguk.ukuu.org.uk>, <linux-kernel@...r.kernel.org>,
	<scottwood@...escale.com>, <akpm@...ux-foundation.org>
Subject: Re: [PATCH] lib: introduce strdup_from_user

Alexey Dobriyan wrote:

> If mm is shared, data can or will change under you.

Ah, that's a good point.  The only side-effect I can see of that is that it will
copy the string incorrectly, but it won't overwrite memory.

Would it be better if I did this:

	str = kzalloc(max, GFP_KERNEL);
	if (!str)
		return ERR_PTR(-ENOMEM);

	if (copy_from_user(str, ustr, max - 1)) {
		kfree(str);
		return ERR_PTR(-EFAULT);
	}

	return krealloc(str, strlen(str) + 1, GFP_KERNEL);

Maybe the krealloc() is overkill.

>> > How else is it supposed to know how much to allocate
>> > without using strlen first?
> I don't know.
> What I know is that your function doesn't guarantee NUL-termination.

That's a bug.  The copy_from_user() should look like this:

	if (copy_from_user(str, ustr, len - 1)) {

-- 
Timur Tabi
Linux kernel developer at Freescale

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