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

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Date:	Mon, 5 Oct 2015 15:15:24 +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:

> 
> * 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.

Hm, so GCC (v4.9.2) will only warn about this bug if -Wtype-limits is enabled 
explicitly:

 lib/string.c: In function ‘strlcpy’:
 lib/string.c:228:32: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
   if (unlikely((size_t)dst_size < 0)) {
                                 ^

... which we don't do in the kernel.

Has anyone considered enabling -Wtype-limits? It seems to catch real bugs.

I can see there are patches that enable -Wextra (which enables -Wtype-limits and 
many other warnings), but it would be more manageable to just enable one such 
warning at a time.

Thanks,

	Ingo
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