kasan: simplify and clarify Makefile

When KASAN support was being added to the Linux kernel, GCC did not yet
support all of the KASAN-related compiler options.  Thus, the KASAN
Makefile had to probe the compiler for supported options.

Nowadays, the Linux kernel GCC version requirement is 5.1+, and thus we
don't need the probing of the -fasan-shadow-offset parameter: it exists in
all 5.1+ GCCs.

Simplify the KASAN Makefile to drop CFLAGS_KASAN_MINIMAL.

Also add a few more comments and unify the indentation.

[andreyknvl@gmail.com: comments fixes per Miguel]
  Link: https://lkml.kernel.org/r/20240814161052.10374-1-andrey.konovalov@linux.dev
Link: https://lkml.kernel.org/r/20240813224027.84503-1-andrey.konovalov@linux.devSigned-off-by: Andrey Konovalov <andreyknvl@gmail.com>
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Acked-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

scripts/Makefile.kasan

@@ -22,30 +22,31 @@ endif
 ifdef CONFIG_KASAN_GENERIC
 
 ifdef CONFIG_KASAN_INLINE
+	# When the number of memory accesses in a function is less than this
+	# call threshold number, the compiler will use inline instrumentation.
+	# 10000 is chosen offhand as a sufficiently large number to make all
+	# kernel functions to be instrumented inline.
 	call_threshold := 10000
 else
 	call_threshold := 0
 endif
 
-CFLAGS_KASAN_MINIMAL := -fsanitize=kernel-address
-
-# -fasan-shadow-offset fails without -fsanitize
-CFLAGS_KASAN_SHADOW := $(call cc-option, -fsanitize=kernel-address \
-			-fasan-shadow-offset=$(KASAN_SHADOW_OFFSET), \
-			$(call cc-option, -fsanitize=kernel-address \
-			-mllvm -asan-mapping-offset=$(KASAN_SHADOW_OFFSET)))
-
-ifeq ($(strip $(CFLAGS_KASAN_SHADOW)),)
-	CFLAGS_KASAN := $(CFLAGS_KASAN_MINIMAL)
-else
-	# Now add all the compiler specific options that are valid standalone
-	CFLAGS_KASAN := $(CFLAGS_KASAN_SHADOW) \
-	 $(call cc-param,asan-globals=1) \
-	 $(call cc-param,asan-instrumentation-with-call-threshold=$(call_threshold)) \
-	 $(call cc-param,asan-instrument-allocas=1)
-endif
-
-CFLAGS_KASAN += $(call cc-param,asan-stack=$(stack_enable))
+# First, enable -fsanitize=kernel-address together with providing the shadow
+# mapping offset, as for GCC, -fasan-shadow-offset fails without -fsanitize
+# (GCC accepts the shadow mapping offset via -fasan-shadow-offset instead of
+# a --param like the other KASAN parameters).
+# Instead of ifdef-checking the compiler, rely on cc-option.
+CFLAGS_KASAN := $(call cc-option, -fsanitize=kernel-address \
+		-fasan-shadow-offset=$(KASAN_SHADOW_OFFSET), \
+		$(call cc-option, -fsanitize=kernel-address \
+		-mllvm -asan-mapping-offset=$(KASAN_SHADOW_OFFSET)))
+
+# Now, add other parameters enabled similarly in both GCC and Clang.
+# As some of them are not supported by older compilers, use cc-param.
+CFLAGS_KASAN += $(call cc-param,asan-instrumentation-with-call-threshold=$(call_threshold)) \
+		$(call cc-param,asan-stack=$(stack_enable)) \
+		$(call cc-param,asan-instrument-allocas=1) \
+		$(call cc-param,asan-globals=1)
 
 # Instrument memcpy/memset/memmove calls by using instrumented __asan_mem*()
 # instead. With compilers that don't support this option, compiler-inserted
@@ -57,9 +58,9 @@ endif # CONFIG_KASAN_GENERIC
 ifdef CONFIG_KASAN_SW_TAGS
 
 ifdef CONFIG_KASAN_INLINE
-    instrumentation_flags := $(call cc-param,hwasan-mapping-offset=$(KASAN_SHADOW_OFFSET))
+	instrumentation_flags := $(call cc-param,hwasan-mapping-offset=$(KASAN_SHADOW_OFFSET))
 else
-    instrumentation_flags := $(call cc-param,hwasan-instrument-with-calls=1)
+	instrumentation_flags := $(call cc-param,hwasan-instrument-with-calls=1)
 endif
 
 CFLAGS_KASAN := -fsanitize=kernel-hwaddress \
@@ -70,7 +71,7 @@ CFLAGS_KASAN := -fsanitize=kernel-hwaddress \
 
 # Instrument memcpy/memset/memmove calls by using instrumented __hwasan_mem*().
 ifeq ($(call clang-min-version, 150000)$(call gcc-min-version, 130000),y)
-CFLAGS_KASAN += $(call cc-param,hwasan-kernel-mem-intrinsic-prefix=1)
+	CFLAGS_KASAN += $(call cc-param,hwasan-kernel-mem-intrinsic-prefix=1)
 endif
 
 endif # CONFIG_KASAN_SW_TAGS