For annoying architectural reasons, it's very difficult to support inline
instrumentation on powerpc64.*

Add a Kconfig flag to allow an arch to disable inline. (It's a bit
annoying to be 'backwards', but I'm not aware of any way to have
an arch force a symbol to be 'n', rather than 'y'.)

We also disable stack instrumentation in this case as it does things that
are functionally equivalent to inline instrumentation, namely adding
code that touches the shadow directly without going through a C helper.

* on ppc64 atm, the shadow lives in virtual memory and isn't accessible in
real mode. However, before we turn on virtual memory, we parse the device
tree to determine which platform and MMU we're running under. That calls
generic DT code, which is instrumented. Inline instrumentation in DT would
unconditionally attempt to touch the shadow region, which we won't have
set up yet, and would crash. We can make outline mode wait for the arch to
be ready, but we can't change what the compiler inserts for inline mode.

Reviewed-by: Marco Elver <elver@google.com>
Signed-off-by: Daniel Axtens <redacted>
---
 lib/Kconfig.kasan | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/lib/Kconfig.kasan b/lib/Kconfig.kasan
index cffc2ebbf185..cb5e02d09e11 100644
--- a/lib/Kconfig.kasan
+++ b/lib/Kconfig.kasan

@@ -12,6 +12,15 @@ config HAVE_ARCH_KASAN_HW_TAGS
 config HAVE_ARCH_KASAN_VMALLOC
        bool

+config ARCH_DISABLE_KASAN_INLINE
+       bool
+       help
+         Sometimes an architecture might not be able to support inline
+         instrumentation but might be able to support outline instrumentation.
+         This option allows an architecture to prevent inline and stack
+         instrumentation from being enabled.

 config CC_HAS_KASAN_GENERIC
        def_bool $(cc-option, -fsanitize=kernel-address)

@@ -130,6 +139,7 @@ config KASAN_OUTLINE

 config KASAN_INLINE
        bool "Inline instrumentation"
+       depends on !ARCH_DISABLE_KASAN_INLINE
        help
          Compiler directly inserts code checking shadow memory before
          memory accesses. This is faster than outline (in some workloads

@@ -141,6 +151,7 @@ endchoice
 config KASAN_STACK
        bool "Enable stack instrumentation (unsafe)" if CC_IS_CLANG && !COMPILE_TEST
        depends on KASAN_GENERIC || KASAN_SW_TAGS
+       depends on !ARCH_DISABLE_KASAN_INLINE
        default y if CC_IS_GCC
        help
          The LLVM stack address sanitizer has a know problem that

@@ -154,6 +165,9 @@ config KASAN_STACK
          but clang users can still enable it for builds without
          CONFIG_COMPILE_TEST.  On gcc it is assumed to always be safe
          to use and enabled by default.
+         If the architecture disables inline instrumentation, this is