linux-kernel - Re: [PATCH v2 2/2] UML: add support for KASAN under x86

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Message-ID: <1a4e51a4d2ed51e7ae1ff55bd4da6a47fad7c0bf.camel@sipsolutions.net>
Date:   Sun, 29 May 2022 19:04:08 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     David Gow <davidgow@...gle.com>,
        Vincent Whitchurch <vincent.whitchurch@...s.com>,
        Patricia Alfonso <trishalfonso@...gle.com>,
        Jeff Dike <jdike@...toit.com>,
        Richard Weinberger <richard@....at>,
        anton.ivanov@...bridgegreys.com,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Brendan Higgins <brendanhiggins@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>
Cc:     kasan-dev <kasan-dev@...glegroups.com>,
        linux-um@...ts.infradead.org, LKML <linux-kernel@...r.kernel.org>,
        Daniel Latypov <dlatypov@...gle.com>, linux-mm@...ck.org
Subject: Re: [PATCH v2 2/2] UML: add support for KASAN under x86_64

On Fri, 2022-05-27 at 11:56 -0700, David Gow wrote:
> 
> The UML-specific KASAN initializer uses mmap to map the roughly 2.25TB

You say 2.25TB here, and
 
> +config KASAN_SHADOW_OFFSET
> +	hex
> +	depends on KASAN
> +	default 0x100000000000
> +	help
> +	  This is the offset at which the ~2.25TB of shadow memory is

here too, of course.

But I notice that I get ~16TB address space use when running,

> +/* used in kasan_mem_to_shadow to divide by 8 */
> +#define KASAN_SHADOW_SCALE_SHIFT 3
> +
> +#ifdef CONFIG_X86_64
> +#define KASAN_HOST_USER_SPACE_END_ADDR 0x00007fffffffffffUL
> +/* KASAN_SHADOW_SIZE is the size of total address space divided by 8 */
> +#define KASAN_SHADOW_SIZE ((KASAN_HOST_USER_SPACE_END_ADDR + 1) >> \
> +			KASAN_SHADOW_SCALE_SHIFT)

because this ends up being 0x100000000000, i.e. 16 TiB.

Is that intentional? Was something missed? Maybe
KASAN_HOST_USER_SPACE_END_ADDR was too big?

It doesn't really matter, but I guess then the documentation should be
updated.

johannes