linux-kernel - Re: [PATCH] string: Improve the generic strlcpy() implementation

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Message-ID: <87r3krowqt.fsf@rasmusvillemoes.dk>
Date:	Mon, 19 Oct 2015 14:42:34 +0200
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Chris Metcalf <cmetcalf@...hip.com>,
	open list <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH] string: Improve the generic strlcpy() implementation

On Mon, Oct 05 2015, Ingo Molnar <mingo@...nel.org> wrote:

> * Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>
>> On Thu, Sep 10, 2015 at 8:43 PM, Chris Metcalf <cmetcalf@...hip.com> wrote:
>> >
>> > Please pull the following changes for 4.3 from:
>> >
>> >   git://git.kernel.org/pub/scm/linux/kernel/git/cmetcalf/linux-tile.git strscpy
>> 
>> So I finally pulled it. I like the patch, I like the new interface,
>> but despite that I wasn't really sure if I wanted to pull it in - thus
>> the long delay of me just seeing this in my list of pending pulls for
>> almost a month, but never really getting to the point where I decided
>> I want to commit to it.
>
> Interesting. I noticed that strscpy() says this in its comments:
>
>  * In addition, the implementation is robust to the string changing out
>  * from underneath it, unlike the current strlcpy() implementation.

Apologies for beating a dead horse, but:

		c = *(unsigned long *)(src+res);
		if (has_zero(c, &data, &constants)) {
			data = prep_zero_mask(c, data, &constants);
			data = create_zero_mask(data);
			*(unsigned long *)(dest+res) = c & zero_bytemask(data);
			return res + find_zero(data);
		}
		*(unsigned long *)(dest+res) = c;

I wonder whether an insane compiler might actually reload c before
storing to dest+res, so that we'd have exactly the same problem of
embedded nul characters? And similarly in the byte-by-byte case:

		char c;

		c = src[res];
		dest[res] = c;
		if (!c)
			return res;

(yes, there's a store between the load and the test, and the compiler
probably can't know or prove that src and dest don't alias - but we're
storing c, so a sufficiently clever _and_ insane compiler could test
src[res]). Would READ_ONCE be justified?

Rasmus
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