linux-kernel - Re: [PATCHv9 02/16] x86/alternatives: Disable LASS when patching kernel alternatives

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <96a36e37-5558-4c91-9591-8e5a77dce850@intel.com>
Date: Thu, 31 Jul 2025 17:15:14 -0700
From: Sohil Mehta <sohil.mehta@...el.com>
To: David Laight <david.laight.linux@...il.com>, "H. Peter Anvin"
	<hpa@...or.com>, Dave Hansen <dave.hansen@...ux.intel.com>
CC: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, Andy Lutomirski
	<luto@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar
	<mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, <x86@...nel.org>, "Peter
 Zijlstra" <peterz@...radead.org>, Ard Biesheuvel <ardb@...nel.org>, "Paul E.
 McKenney" <paulmck@...nel.org>, Josh Poimboeuf <jpoimboe@...nel.org>,
	Xiongwei Song <xiongwei.song@...driver.com>, Xin Li <xin3.li@...el.com>,
	"Mike Rapoport (IBM)" <rppt@...nel.org>, Brijesh Singh
	<brijesh.singh@....com>, Michael Roth <michael.roth@....com>, Tony Luck
	<tony.luck@...el.com>, Alexey Kardashevskiy <aik@....com>, Alexander Shishkin
	<alexander.shishkin@...ux.intel.com>, Jonathan Corbet <corbet@....net>, "Ingo
 Molnar" <mingo@...nel.org>, Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
	Daniel Sneddon <daniel.sneddon@...ux.intel.com>, Kai Huang
	<kai.huang@...el.com>, Sandipan Das <sandipan.das@....com>, Breno Leitao
	<leitao@...ian.org>, Rick Edgecombe <rick.p.edgecombe@...el.com>, "Alexei
 Starovoitov" <ast@...nel.org>, Hou Tao <houtao1@...wei.com>, Juergen Gross
	<jgross@...e.com>, Vegard Nossum <vegard.nossum@...cle.com>, Kees Cook
	<kees@...nel.org>, Eric Biggers <ebiggers@...gle.com>, Jason Gunthorpe
	<jgg@...pe.ca>, "Masami Hiramatsu (Google)" <mhiramat@...nel.org>, "Andrew
 Morton" <akpm@...ux-foundation.org>, Luis Chamberlain <mcgrof@...nel.org>,
	Yuntao Wang <ytcoode@...il.com>, Rasmus Villemoes <linux@...musvillemoes.dk>,
	Christophe Leroy <christophe.leroy@...roup.eu>, Tejun Heo <tj@...nel.org>,
	Changbin Du <changbin.du@...wei.com>, Huang Shijie
	<shijie@...amperecomputing.com>, Geert Uytterhoeven
	<geert+renesas@...der.be>, Namhyung Kim <namhyung@...nel.org>, "Arnaldo
 Carvalho de Melo" <acme@...hat.com>, <linux-doc@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <linux-efi@...r.kernel.org>,
	<linux-mm@...ck.org>
Subject: Re: [PATCHv9 02/16] x86/alternatives: Disable LASS when patching
 kernel alternatives

On 7/28/2025 12:38 PM, David Laight wrote:
>>> ...
>>>
>>> Or just write a byte copy loop in C with (eg) barrier() inside it
>>> to stop gcc converting it to memcpy().
>>>
>>> 	David  
>>
>> Great. It's rep movsb without any of the performance.
> 
> And without the massive setup overhead that dominates short copies.
> Given the rest of the code I'm sure a byte copy loop won't make
> any difference to the overall performance.
>

Wouldn't it be better to introduce a generic mechanism than something
customized for this scenario?

PeterZ had suggested that inline memcpy could have more usages:
https://lore.kernel.org/lkml/20241029113611.GS14555@noisy.programming.kicks-ass.net/

Is there a concern that the inline versions might get optimized into
standard memcpy/memset calls by GCC? Wouldn't the volatile keyword
prevent that?

static __always_inline void *__inline_memcpy(void *to, const void *from,
size_t len)
{
	void *ret = to;

	asm volatile("rep movsb"
		     : "+D" (to), "+S" (from), "+c" (len)
		     : : "memory");
	return ret;
}

static __always_inline void *__inline_memset(void *s, int v, size_t n)
{
	void *ret = s;

	asm volatile("rep stosb"
		     : "+D" (s), "+c" (n)
		     : "a" ((uint8_t)v)
		     : "memory");
	return ret;
}