Re: [PATCH] powerpc/Kconfig: Force THREAD_SHIFT to at least 14 with KASAN

Michael Ellerman
2022-05-31T06:16:30Z

Christophe Leroy <christophe.leroy@csgroup.eu> writes:
> Allthough 14 is the default THREAD_SHIFT when KASAN is selected,
> taking an old config may keep 13 when CONFIG_EXPERT is selected.
>
> Force it to 14 as a minimum when KASAN is selected.
>
> Also default to 15 when KASAN on PPC64.
>
> Reported-by: Erhard Furtner <erhard_f@mailbox.org>
> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> ---
> arch/powerpc/Kconfig | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)

I was thinking of doing it in C, similar to the way arm64 handles it.

Something like below. It means we always double the stack size when
KASAN is enabled. I think it's preferable, as it will always work
regardless of whether the user has an old .config (or is bisectting)?

cheers

diff --git a/arch/powerpc/include/asm/thread_info.h b/arch/powerpc/include/asm/thread_info.h
index 125328d1b980..c9735f93f8e6 100644
--- a/arch/powerpc/include/asm/thread_info.h
+++ b/arch/powerpc/include/asm/thread_info.h
@@ -14,12 +14,17 @@

#ifdef __KERNEL__

-#if defined(CONFIG_VMAP_STACK) && CONFIG_THREAD_SHIFT < PAGE_SHIFT
-#define THREAD_SHIFT PAGE_SHIFT
+#ifdef CONFIG_KASAN
+#define THREAD_SHIFT (CONFIG_THREAD_SHIFT + 1)
#else
#define THREAD_SHIFT CONFIG_THREAD_SHIFT
#endif

+#if defined(CONFIG_VMAP_STACK) && THREAD_SHIFT < PAGE_SHIFT
+#undef THREAD_SHIFT
+#define THREAD_SHIFT PAGE_SHIFT
+#endif
+
#define THREAD_SIZE (1 << THREAD_SHIFT)

/*