Re: [PATCH v2 2/2] UML: add support for KASAN under x86_64

[David Gow]

On Mon, May 30, 2022 at 1:04 AM Johannes Berg <johannes@xxxxxxxxxxxxxxxx> 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@xxxxxxxxxx/

-- David

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