linux-kernel - Re: [PATCH v4 14/14] of: Improve __of_add_property

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Message-ID: <lpd4bay4mv4qh2ax6ywdwe3hb47bd7ppcxn3uv32fsrzo3e6lj@yyocdgrpwnkq>
Date: Mon, 13 Jan 2025 09:33:32 +0100
From: Krzysztof Kozlowski <krzk@...nel.org>
To: Zijun Hu <zijun_hu@...oud.com>
Cc: Rob Herring <robh@...nel.org>, Saravana Kannan <saravanak@...gle.com>, 
	Maxime Ripard <mripard@...nel.org>, Robin Murphy <robin.murphy@....com>, 
	Grant Likely <grant.likely@...retlab.ca>, Marc Zyngier <maz@...nel.org>, 
	Andreas Herrmann <andreas.herrmann@...xeda.com>, Marek Szyprowski <m.szyprowski@...sung.com>, 
	Catalin Marinas <catalin.marinas@....com>, Mike Rapoport <rppt@...nel.org>, 
	Oreoluwa Babatunde <quic_obabatun@...cinc.com>, devicetree@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Zijun Hu <quic_zijuhu@...cinc.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Kees Cook <kees@...nel.org>
Subject: Re: [PATCH v4 14/14] of: Improve __of_add_property_sysfs()
 readability

On Sat, Jan 11, 2025 at 08:44:34PM +0800, Zijun Hu wrote:
> On 2025/1/11 17:17, Krzysztof Kozlowski wrote:
> > On Sat, Jan 11, 2025 at 07:48:48AM +0800, Zijun Hu wrote:
> >> On 2025/1/11 04:41, Rob Herring wrote:
> >>> On Thu, Jan 09, 2025 at 09:27:05PM +0800, Zijun Hu wrote:
> >>>> From: Zijun Hu <quic_zijuhu@...cinc.com>
> >>>>
> >>>> __of_add_property_sysfs() hard codes string "security-" length as 9, but
> >>>> that is not obvious for readers.
> >>>>
> >>>> Improve its readability by using strlen().
> >>>
> >>> Does the compiler optimize the strlen call away? Maybe, maybe not. If 
> >>> not, that's N calls to strlen() where N is the number of properties in 
> >>> your DT. That's in the 1000s easily.
> >>>
> >>> Do you really want to go test enough compiler versions we support to 
> >>> feel confident this is optimized away. I don't.
> >>>
> >>
> >> i understand your concern about performance.
> >> perhaps, such impact about performance may be ignored for linux OS.
> >>
> >> what about below solution ?
> >>
> >> const char security_prefix[] = "security-";
> >> use 'sizeof(security_prefix) - 1' for the length of string.
> > 
> > Code is still not equivalent - just de-assemble it and you will see
> > some overhead.
> > 
> 
> Thank you Krzysztof Kozlowski for code review
> 
> the overhead may be that use of stack is increased a bit.
> it may not almost impact performance since 'sizeof(security_prefix) - 1'
> may be still a compile time constant.

No, I was talking about initialization. Look how it is done.

> 
> > Maybe just introduce builtin_strlen() to string.h and use such? It would
> > be the pretty obvious code.
> > 
> 
> it is a good idea to introduce API for this common issue, it seems the
> main strncmp() usages in current kernel tree is to check if a string
> begin with the other string a shown below:
> 
> // "s2_string" is wrote twice, and also impact performance
> strncmp(s1, "s2_string", strlen("s2_string"))
> 
> // it is not obvious for readers that "s2_string" length is 9
> strncmp(s1, "s2_string", 9)
> 
> //"s2_string" is wrote twice, same as strcmp(), may be wrong
> strncmp(s1, "s2_string", sizeof("s2_string"))
> 
> //"s2_string" is wrote twice
> strncmp(s1, "s2_string", sizeof("s2_string") - 1)
> 
> what about a new API such as ?
> 
> // check if @s1 begins with @s2
> bool str_begin_with(const char *cs, const char *ct)

Nothing related to my proposal, so no clue why you ask me. Anyway, as it
is one more unsafe str-like function, I don't see how it makes things
much better.

Best regards,
Krzysztof