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

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Message-ID: <20151005115355.GA27073@gmail.com>
Date:	Mon, 5 Oct 2015 13:53:55 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	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


* Ingo Molnar <mingo@...nel.org> wrote:

> 2)
> 
> Another problem is that strlcpy() will also happily do bad stuff if we pass
> it a negative size. Instead of that we will from now on print a (one time)
> warning and return safely.

Hm, so this check is buggy, as 'size_t' is unsigned - and for some reason GCC 
didn't warn about the never-met comparison and the resulting unreachable dead
code here:

> +	/* Overflow check: */
> +	if (unlikely(dest_size < 0)) {
> +		WARN_ONCE(1, "strlcpy(): dest_size < 0 underflow!");
> +		return strlen(src);
> +	}

which is annoying.

Would people object to something like:

> +	/* Overflow check: */
> +	if (unlikely((ssize_t)dest_size < 0)) {
> +		WARN_ONCE(1, "strlcpy(): dest_size < 0 underflow!");
> +		return strlen(src);
> +	}

?

As I doubt it's legit to have larger than 2GB strings.

Also, I'm wondering why GCC didn't warn.

Thanks,

	Ingo
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