Re: [RFC PATCH v3] UML: add support for KASAN under x86_64

[Johannes Berg]

On Fri, 2022-05-27 at 07:31 +0200, Dmitry Vyukov wrote:
> > - This doesn't seem to work when CONFIG_STATIC_LINK is enabled (because
> >   libc crt0 code calls memory functions, which expect the shadow memory
> >   to already exist, due to multiple symbols being resolved.
> >   - I think we should just make this depend on dynamic UML.
> >   - For that matter, I think static UML is actually broken at the
> >     moment. I'll send a patch out tomorrow.
> 
> I don't know how important the static build is for UML.

Depends who you ask, I guess.

IMHO just making KASAN depend on !STATIC_LINK is fine, until somebody
actually wants to do what you describe:

> Generally I prefer to build things statically b/c e.g. if a testing
> system builds on one machine but runs tests on another, dynamic link
> may be a problem. Or, say, if a testing system provides binary
> artifacts, and then nobody can run it locally.
> 
> One potential way to fix it is to require outline KASAN
> instrumentation for static build and then make kasan_arch_is_ready()
> return false until the shadow is mapped. I see kasan_arch_is_ready()
> is checked at the beginning of all KASAN runtime entry points.
> But it would be nice if the dynamic build also supports inline and
> does not add kasan_arch_is_ready() check overhead.

which sounds fine too, but ... trade-offs.

> > +       if (IS_ENABLED(CONFIG_UML)) {
> > +               __memset(kasan_mem_to_shadow((void *)addr), KASAN_VMALLOC_INVALID, shadow_end - shadow_start);
> 
> "kasan_mem_to_shadow((void *)addr)" can be replaced with shadow_start.

and then the memset line isn't so long anymore :)

> 
> 
> > +               return 0;
> > +       }
> > +
> > +       shadow_start = ALIGN_DOWN(shadow_start, PAGE_SIZE);
> >         shadow_end = ALIGN(shadow_end, PAGE_SIZE);
> 
> There is no new fancy PAGE_ALIGN macro for this. And I've seen people

s/no/now the/ I guess, but it's also existing code.

johannes