Re: [PATCH v12 1/6] kasan: allow an architecture to disable inline instrumentation

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.

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..935814f332a7 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

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

+# ppc64 turns on virtual memory late in boot, after calling into generic code
+# like the device-tree parser, so it uses this in conjuntion with a hook in
+# outline mode to avoid invalid access early in boot.

+config ARCH_DISABLE_KASAN_INLINE
+       bool
+
 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
+         also disabled as it adds inline-style instrumentation that
+         is run unconditionally.

 config KASAN_SW_TAGS_IDENTIFY
        bool "Enable memory corruption identification"
--

2.27.0

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