[PATCH -tip v4 2/3] kasan: Treat meminstrinsic as builtins in uninstrumented files

[Marco Elver <elver@google.com>]

Where the compiler instruments meminstrinsics by generating calls to
__asan/__hwasan_ prefixed functions, let the compiler consider
memintrinsics as builtin again.

To do so, never override memset/memmove/memcpy if the compiler does the
correct instrumentation - even on !GENERIC_ENTRY architectures.

Fixes: 69d4c0d32186 ("entry, kasan, x86: Disallow overriding mem*() functions")
Signed-off-by: Marco Elver <elver@google.com>
---
v4:
* New patch.
---
 lib/Kconfig.kasan      | 9 +++++++++
 mm/kasan/shadow.c      | 5 ++++-
 scripts/Makefile.kasan | 9 +++++++++
 3 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/lib/Kconfig.kasan b/lib/Kconfig.kasan
index be6ee6020290..fdca89c05745 100644
--- a/lib/Kconfig.kasan
+++ b/lib/Kconfig.kasan
@@ -49,6 +49,15 @@ menuconfig KASAN
 
 if KASAN
 
+config CC_HAS_KASAN_MEMINTRINSIC_PREFIX
+	def_bool (CC_IS_CLANG && $(cc-option,-fsanitize=kernel-address -mllvm -asan-kernel-mem-intrinsic-prefix=1)) || \
+		 (CC_IS_GCC && $(cc-option,-fsanitize=kernel-address --param asan-kernel-mem-intrinsic-prefix=1))
+	# Don't define it if we don't need it: compilation of the test uses
+	# this variable to decide how the compiler should treat builtins.
+	depends on !KASAN_HW_TAGS
+	help
+	  The compiler is able to prefix memintrinsics with __asan or __hwasan.
+
 choice
 	prompt "KASAN mode"
 	default KASAN_GENERIC
diff --git a/mm/kasan/shadow.c b/mm/kasan/shadow.c
index f8a47cb299cb..43b6a59c8b54 100644
--- a/mm/kasan/shadow.c
+++ b/mm/kasan/shadow.c
@@ -38,11 +38,14 @@ bool __kasan_check_write(const volatile void *p, unsigned int size)
 }
 EXPORT_SYMBOL(__kasan_check_write);
 
-#ifndef CONFIG_GENERIC_ENTRY
+#if !defined(CONFIG_CC_HAS_KASAN_MEMINTRINSIC_PREFIX) && !defined(CONFIG_GENERIC_ENTRY)
 /*
  * CONFIG_GENERIC_ENTRY relies on compiler emitted mem*() calls to not be
  * instrumented. KASAN enabled toolchains should emit __asan_mem*() functions
  * for the sites they want to instrument.
+ *
+ * If we have a compiler that can instrument meminstrinsics, never override
+ * these, so that non-instrumented files can safely consider them as builtins.
  */
 #undef memset
 void *memset(void *addr, int c, size_t len)
diff --git a/scripts/Makefile.kasan b/scripts/Makefile.kasan
index fa9f836f8039..c186110ffa20 100644
--- a/scripts/Makefile.kasan
+++ b/scripts/Makefile.kasan
@@ -1,5 +1,14 @@
 # SPDX-License-Identifier: GPL-2.0
+
+ifdef CONFIG_CC_HAS_KASAN_MEMINTRINSIC_PREFIX
+# Safe for compiler to generate meminstrinsic calls in uninstrumented files.
+CFLAGS_KASAN_NOSANITIZE :=
+else
+# Don't let compiler generate memintrinsic calls in uninstrumented files
+# because they are instrumented.
 CFLAGS_KASAN_NOSANITIZE := -fno-builtin
+endif
+
 KASAN_SHADOW_OFFSET ?= $(CONFIG_KASAN_SHADOW_OFFSET)
 
 cc-param = $(call cc-option, -mllvm -$(1), $(call cc-option, --param $(1)))
-- 
2.39.2.637.g21b0678d19-goog