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

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Message-ID: <CABVgOSmcwGa4tw8EvuxbeOvrTLh8d7V3mFxGyj2spnqqWkFp9Q@mail.gmail.com>
Date:   Thu, 30 Jun 2022 15:53:22 +0800
From:   David Gow <davidgow@...gle.com>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     Vincent Whitchurch <vincent.whitchurch@...s.com>,
        Patricia Alfonso <trishalfonso@...gle.com>,
        Jeff Dike <jdike@...toit.com>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <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>,
        kasan-dev <kasan-dev@...glegroups.com>,
        linux-um <linux-um@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Daniel Latypov <dlatypov@...gle.com>,
        Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [PATCH v2 2/2] UML: add support for KASAN under x86_64

On Mon, May 30, 2022 at 1:04 AM Johannes Berg <johannes@...solutions.net> wrote:
>
> 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.

Whoops, the amount of shadow memory allocated was changed for v1 of
this patch (it was ~2.25 TB in the original RFC). I've updated these
comments in v3:
https://lore.kernel.org/lkml/20220630074757.2739000-2-davidgow@google.com/

-- David

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