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

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Message-ID: <8737xnbqtn.fsf@rasmusvillemoes.dk>
Date:	Wed, 07 Oct 2015 00:00:20 +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 Tue, Oct 06 2015, Ingo Molnar <mingo@...nel.org> wrote:

> * Rasmus Villemoes <linux@...musvillemoes.dk> wrote:
>
>> 
>> I'm not against making strlcpy more robust, but I think the theoretical race is 
>> far more likely to manifest through a member of the printf family.
>
> So the printf family is generally less frequently used in ABI output than string 
> copies,

Huh? snprintf and friends are often used just to copy or concatenate
strings (essentially, any format string containing no specifiers other
than %s does exactly that) - even if a str*() function could do the same
thing. I see no reason to believe this wouldn't also be done in cases
where the final string ends up being presented to userspace.

> but yeah, since there are 15,000 s[n]printf() calls in the kernel it's 
> more likely to be an issue not just by virtue of timing, but also by sheer mass of 
> usage, statistically.

Yes.

> So I'd improve it all in the following order:
>
>   - fix the strscpy() uninitialized use
>
>   - base strlcpy() on strscpy() via the patch I sent. This makes all users faster 
>     and eliminates the theoretical race.

I'm not so sure about that part. I'd strongly suspect that the vast
majority of strings handled by strlcpy (and in the future strscpy) are
shorter than 32 bytes, so is all the word_at_a_time and pre/post
alignment yoga really worth it? strscpy is 299 bytes - that's a lot of
instruction cache lines, and it will almost always be cache cold. This
isn't an argument against basing strlcpy on strscpy (that would likely
just make the former a little smaller), but I'd like to see numbers
(cycle counts, distribution of input lengths, ...)  before I believe in
the performance argument.

>   - phase out 'simple' strlcpy() uses via an automated patch. This gets rid of 
>     2,000 strlcpy() call sites in a single pass.

That seems to be exactly the kind of mass-conversion Linus referred to
above. Also, you can't really do it if strscpy keeps it __must_check
annotation, as you'd then introduce 2000+ warnings...

Rasmus
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