Re: [PATCH v11 0/6] KASAN for powerpc64 radix

[Christophe Leroy]

Le 23/03/2021 à 02:21, Daniel Axtens a écrit :
> Hi Christophe,
>
> > In the discussion we had long time ago,
> > https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20190806233827.16454-5-dja@xxxxxxxxxx/#2321067
> > , I challenged you on why it was not possible to implement things the same way as other
> > architectures, in extenso with an early mapping.
> >
> > Your first answer was that too many things were done in real mode at startup. After some discussion
> > you said that finally there was not that much things at startup but the issue was KVM.
> >
> > Now you say that instrumentation on KVM is fully disabled.
> >
> > So my question is, if KVM is not a problem anymore, why not go the standard way with an early shadow
> > ? Then you could also support inline instrumentation.
>
> Fair enough, I've had some trouble both understanding the problem myself
> and clearly articulating it. Let me try again.
>
> We need translations on to access the shadow area.
>
> We reach setup_64.c::early_setup() with translations off. At this point
> we don't know what MMU we're running under, or our CPU features.

What do you need to know ? Whether it is Hash or Radix, or more/different details ?

IIUC, today we only support KASAN on Radix. Would it make sense to say that a kernel built with 
KASAN can only run on processors having Radix capacility ? Then select CONFIG_PPC_RADIX_MMU_DEFAULT 
when KASAN is set, and accept that the kernel crashes if Radix is not available ?

> To determine our MMU and CPU features, early_setup() calls functions
> (dt_cpu_ftrs_init, early_init_devtree) that call out to generic code
> like of_scan_flat_dt. We need to do this before we turn on translations
> because we can't set up the MMU until we know what MMU we have.
>
> So this puts us in a bind:
>
>   - We can't set up an early shadow until we have translations on, which
>     requires that the MMU is set up.
>
>   - We can't set up an MMU until we call out to generic code for FDT
>     parsing.
>
> So there will be calls to generic FDT parsing code that happen before the
> early shadow is set up.

I see some logic in kernel/prom_init.c for detecting MMU. Can we get the information from there in 
order to setup the MMU ?

> The setup code also prints a bunch of information about the platform
> with printk() while translations are off, so it wouldn't even be enough
> to disable instrumentation for bits of the generic DT code on ppc64.

I'm sure the printk() stuff can be avoided or delayed without much problems, I guess the main 
problem is the DT code, isn't it ?

As far as I can see the code only use udbg_printf() before MMU is on, and this could be simply 
skipped when KASAN is selected, I see no situation where you need early printk together with KASAN.

> Does that make sense? If you can figure out how to 'square the circle'
> here I'm all ears.

Yes it is a lot more clear now, thanks you. Gave a few ideas above, does it help ?

> Other notes:
>
>   - There's a comment about printk() being 'safe' in early_setup(), that
>     refers to having a valid PACA, it doesn't mean that it's safe in any
>     other sense.
>
>   - KVM does indeed also run stuff with translations off but we can catch
>     all of that by disabling instrumentation on the real-mode handlers:
>     it doesn't seem to leak out to generic code. So you are right that
>     KVM is no longer an issue.

Christophe