Re: [RFC PATCH] UML: add support for KASAN under x86_64

[Dmitry Vyukov]

On Wed, Feb 12, 2020 at 12:48 AM Patricia Alfonso
<trishalfonso@xxxxxxxxxx> wrote:
>
> On Thu, Jan 16, 2020 at 12:44 AM Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> >
> > On Wed, Jan 15, 2020 at 7:28 PM Patricia Alfonso
> > <trishalfonso@xxxxxxxxxx> wrote:
> > > +config KASAN_SHADOW_OFFSET
> > > +       hex
> > > +       depends on KASAN
> > > +       default 0x100000000000
> > > +       help
> > > +         This is the offset at which the ~2.25TB of shadow memory is
> > > +         initialized and used by KASAN for memory debugging. The default
> > > +         is 0x100000000000.
> >
> > What are restrictions on this value?
> The only restriction is that there is enough space there to map all of
> the KASAN shadow memory without conflicting with anything else.
>
> > In user-space we use 0x7fff8000 as a base (just below 2GB) and it's
> > extremely profitable wrt codegen since it fits into immediate of most
> > instructions.
> > We can load and add the base with a short instruction:
> >     2d8c: 48 81 c2 00 80 ff 7f    add    $0x7fff8000,%rdx
> > Or even add base, load shadow and check it with a single 7-byte instruction:
> >      1e4: 80 b8 00 80 ff 7f 00    cmpb   $0x0,0x7fff8000(%rax)
> >
> I just tested with 0x7fff8000 as the KASAN_SHADOW_OFFSET and it worked
> so I can make that the default if it will be more efficient.

I think it's the right thing to do if it works.