[PATCH 4/4] powerpc: Book3S 64-bit "heavyweight" KASAN support

[Christophe Leroy]

Hi Daniel,

Le 07/08/2019 à 18:34, Christophe Leroy a écrit :
> 
> 
> Le 07/08/2019 à 01:38, Daniel Axtens a écrit :
>> KASAN support on powerpc64 is interesting:
>>
>>   - We want to be able to support inline instrumentation so as to be
>>     able to catch global and stack issues.
>>
>>   - We run a lot of code at boot in real mode. This includes stuff like
>>     printk(), so it's not feasible to just disable instrumentation
>>     around it.
> 
> Have you definitely given up the idea of doing a standard implementation 
> of KASAN like other 64 bits arches have done ?
> 
> Isn't it possible to setup an early 1:1 mapping and go in virtual mode 
> earlier ? What is so different between book3s64 and book3e64 ?
> On book3e64, we've been able to setup KASAN before printing anything 
> (except when using EARLY_DEBUG). Isn't it feasible on book3s64 too ?
> 

I looked at it once more, and cannot find that "We run a lot of code at 
boot in real mode. This includes stuff like printk()".

Can you provide exemples ?

AFAICS, there are two things which are run in real mode at boot:
1/ prom_init() in kernel/prom_init.c
2/ early_setup() in kernel/setup_64.c

1/ KASAN is already inhibited for prom_init(), and prom_init() only uses 
prom_printf() to display stuff.
2/ early_setup() only call a subset of simple functions. By regrouping 
things in a new file called early_64.c as done for PPC32 with 
early_32.c, we can easily inhibit kasan for those few stuff. printk() is 
not used there either, there is even a comment at the startup of 
early_setup() telling /* -------- printk is _NOT_ safe to use here ! 
------- */. The only things that perform display is the function 
udbg_printf(), which is called only when DEBUG is set and which is 
linked to CONFIG_PPC_EARLY_DEBUG. We already discussed that and agreed 
that CONFIG_PPC_EARLY_DEBUG could be made exclusive of CONFIG_KASAN.

Once early_setup() has run, BOOK3S64 goes in virtual mode, just like 
BOOK3E does.

What am I missing ?

Thanks
Christophe