linux-kernel - Re: [PATCH 1/2] lib/string: Improve strstarts() performance

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Message-ID: <202504070912.8BB9AD0AE@keescook>
Date: Mon, 7 Apr 2025 09:17:21 -0700
From: Kees Cook <kees@...nel.org>
To: Zijun Hu <zijun_hu@...oud.com>
Cc: Andy Shevchenko <andy@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
	Zijun Hu <quic_zijuhu@...cinc.com>,
	"Rob Herring (Arm)" <robh@...nel.org>
Subject: Re: [PATCH 1/2] lib/string: Improve strstarts() performance

On Mon, Apr 07, 2025 at 09:15:04PM +0800, Zijun Hu wrote:
> From: Zijun Hu <quic_zijuhu@...cinc.com>
> 
> strstarts() is frequently invoked to test if a string has another string
> as prefix, but its performance is degraded by the strlen() loop contained.
> 
> Improve its performance by eliminating the strlen() loop.

So, as Andy already said: no, this is very unlikely to be a performance
improvement, and if it is, you'll need to show the numbers (and likely
the reason _why_, in the form of assembly output, etc).

The reason this isn't going to be an improvement is because
strlen($string_constant) is optimized by the compiler into a integral
constant value. So you'd be replacing a potentially inline constant with
an explicit function call. That will be much more expensive.

With almost 300 users:
$ git grep 'strstarts' | wc -l
198

Only 38 are _not_ using a string constant:
$ git grep 'strstarts' | grep -v '"' | wc -l
38

Additionally, there is no "loop". strlen() of a runtime string would be
evaluated once.

-Kees

-- 
Kees Cook