linux-kernel - Re: [PATCH v5 36/37] s390/kmsan: Implement the architecture-specific functions

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Date: Thu, 20 Jun 2024 11:25:11 +0200
From: Alexander Gordeev <agordeev@...ux.ibm.com>
To: Ilya Leoshkevich <iii@...ux.ibm.com>
Cc: Alexander Potapenko <glider@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>, David Rientjes <rientjes@...gle.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, Marco Elver <elver@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Pekka Enberg <penberg@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Vasily Gorbik <gor@...ux.ibm.com>, Vlastimil Babka <vbabka@...e.cz>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>, kasan-dev@...glegroups.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-s390@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        Mark Rutland <mark.rutland@....com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Sven Schnelle <svens@...ux.ibm.com>
Subject: Re: [PATCH v5 36/37] s390/kmsan: Implement the architecture-specific
 functions

On Wed, Jun 19, 2024 at 05:44:11PM +0200, Ilya Leoshkevich wrote:

Hi Ilya,

> +static inline bool is_lowcore_addr(void *addr)
> +{
> +	return addr >= (void *)&S390_lowcore &&
> +	       addr < (void *)(&S390_lowcore + 1);
> +}
> +
> +static inline void *arch_kmsan_get_meta_or_null(void *addr, bool is_origin)
> +{
> +	if (is_lowcore_addr(addr)) {
> +		/*
> +		 * Different lowcores accessed via S390_lowcore are described
> +		 * by the same struct page. Resolve the prefix manually in
> +		 * order to get a distinct struct page.
> +		 */

> +		addr += (void *)lowcore_ptr[raw_smp_processor_id()] -
> +			(void *)&S390_lowcore;

If I am not mistaken neither raw_smp_processor_id() itself, nor
lowcore_ptr[raw_smp_processor_id()] are atomic. Should the preemption
be disabled while the addr is calculated?

But then the question arises - how meaningful the returned value is?
AFAICT kmsan_get_metadata() is called from a preemptable context.
So if the CPU is changed - how useful the previous CPU lowcore meta is?

Is it a memory block that needs to be ignored instead?

> +		if (WARN_ON_ONCE(is_lowcore_addr(addr)))
> +			return NULL;

lowcore_ptr[] pointing into S390_lowcore is rather a bug.

> +		return kmsan_get_metadata(addr, is_origin);
> +	}
> +	return NULL;
> +}

Thanks!