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From: Masahiro Yamada <masahiroy@kernel.org>
To: linux-kbuild@vger.kernel.org
Cc: linux-arch@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	Masahiro Yamada <masahiroy@kernel.org>
Subject: [PATCH 1/3] kbuild: provide reasonable defaults for tool coverage
Date: Mon,  6 May 2024 22:35:42 +0900
Message-Id: <20240506133544.2861555-2-masahiroy@kernel.org>
In-Reply-To: <20240506133544.2861555-1-masahiroy@kernel.org>
References: <20240506133544.2861555-1-masahiroy@kernel.org>
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The objtool, sanitizers (KASAN, UBSAN, etc.), and profilers (GCOV, etc.)
are intended for kernel space objects. To exclude objects from their
coverage, you need to set variables such as OBJECT_FILES_NON_STNDARD=y,
KASAN_SANITIZE=n, etc.

For instance, the following are not kernel objects, and therefore should
opt out of coverage:

  - vDSO
  - purgatory
  - bootloader (arch/*/boot/)

Kbuild can detect these cases without relying on such variables because
objects not directly linked to vmlinux or modules are considered
"non-standard objects".

Detecting objects linked to vmlinux or modules is straightforward:

  - objects added to obj-y are linked to vmlinux
  - objects added to lib-y are linked to vmlinux
  - objects added to obj-m are linked to modules

In the past, there was yet another case:

  - object paths added to head-y were linked to vmlinux

Commit ce697ccee1a8 ("kbuild: remove head-y syntax") eliminated this.

There are still some exceptions. For example, arch/s390/boot/Makefile
needlessly uses obj-y for the bootloader objects, which can be fixed
later.

Going forward, objects that are not listed in obj-y, lib-y, or obj-m
will opt out of objtool, sanitizers, and profilers by default.

You can still override the Kbuild decision by explicitly specifying
OBJECT_FILES_NON_STANDARD, KASAN_SANITIZE, etc. but most of such Make
variables can be removed.

The next commit will clean up redundant variables.

Note:

The coverage for some objects will be changed:

  - exclude .vmlinux.export.o from UBSAN, KCOV
  - exclude arch/csky/kernel/vdso/vgettimeofday.o from UBSAN
  - exclude arch/parisc/kernel/vdso32/vdso32.so from UBSAN
  - exclude arch/parisc/kernel/vdso64/vdso64.so from UBSAN
  - exclude arch/x86/um/vdso/um_vdso.o from UBSAN
  - exclude drivers/misc/lkdtm/rodata.o from UBSAN, KCOV
  - exclude init/version-timestamp.o from UBSAN, KCOV
  - exclude lib/test_fortify/*.o from all santizers and profilers

I believe these are positive effects.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
---

 scripts/Makefile.build |  2 +-
 scripts/Makefile.lib   | 20 ++++++++++++--------
 2 files changed, 13 insertions(+), 9 deletions(-)

diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index c9c07a6144eb..56bacd992a09 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -214,7 +214,7 @@ endif # CONFIG_FTRACE_MCOUNT_USE_RECORDMCOUNT
 # 'OBJECT_FILES_NON_STANDARD_foo.o := 'y': skip objtool checking for a file
 # 'OBJECT_FILES_NON_STANDARD_foo.o := 'n': override directory skip for a file
 
-is-standard-object = $(if $(filter-out y%, $(OBJECT_FILES_NON_STANDARD_$(target-stem).o)$(OBJECT_FILES_NON_STANDARD)n),y)
+is-standard-object = $(if $(filter-out y%, $(OBJECT_FILES_NON_STANDARD_$(target-stem).o)$(OBJECT_FILES_NON_STANDARD)n),$(is-kernel-object))
 
 $(obj)/%.o: private objtool-enabled = $(if $(is-standard-object),$(if $(delay-objtool),$(is-single-obj-m),y))
 
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 5972ec4ee29b..d3180182af47 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -154,7 +154,7 @@ _cpp_flags     = $(KBUILD_CPPFLAGS) $(cppflags-y) $(CPPFLAGS_$(target-stem).lds)
 #
 ifeq ($(CONFIG_GCOV_KERNEL),y)
 _c_flags += $(if $(patsubst n%,, \
-		$(GCOV_PROFILE_$(target-stem).o)$(GCOV_PROFILE)$(CONFIG_GCOV_PROFILE_ALL)), \
+		$(GCOV_PROFILE_$(target-stem).o)$(GCOV_PROFILE)$(if $(is-kernel-object),$(CONFIG_GCOV_PROFILE_ALL))), \
 		$(CFLAGS_GCOV))
 endif
 
@@ -165,32 +165,32 @@ endif
 ifeq ($(CONFIG_KASAN),y)
 ifneq ($(CONFIG_KASAN_HW_TAGS),y)
 _c_flags += $(if $(patsubst n%,, \
-		$(KASAN_SANITIZE_$(target-stem).o)$(KASAN_SANITIZE)y), \
+		$(KASAN_SANITIZE_$(target-stem).o)$(KASAN_SANITIZE)$(is-kernel-object)), \
 		$(CFLAGS_KASAN), $(CFLAGS_KASAN_NOSANITIZE))
 endif
 endif
 
 ifeq ($(CONFIG_KMSAN),y)
 _c_flags += $(if $(patsubst n%,, \
-		$(KMSAN_SANITIZE_$(target-stem).o)$(KMSAN_SANITIZE)y), \
+		$(KMSAN_SANITIZE_$(target-stem).o)$(KMSAN_SANITIZE)$(is-kernel-object)), \
 		$(CFLAGS_KMSAN))
 _c_flags += $(if $(patsubst n%,, \
-		$(KMSAN_ENABLE_CHECKS_$(target-stem).o)$(KMSAN_ENABLE_CHECKS)y), \
+		$(KMSAN_ENABLE_CHECKS_$(target-stem).o)$(KMSAN_ENABLE_CHECKS)$(is-kernel-object)), \
 		, -mllvm -msan-disable-checks=1)
 endif
 
 ifeq ($(CONFIG_UBSAN),y)
 _c_flags += $(if $(patsubst n%,, \
-		$(UBSAN_SANITIZE_$(target-stem).o)$(UBSAN_SANITIZE)y), \
+		$(UBSAN_SANITIZE_$(target-stem).o)$(UBSAN_SANITIZE)$(is-kernel-object)), \
 		$(CFLAGS_UBSAN))
 _c_flags += $(if $(patsubst n%,, \
-		$(UBSAN_SIGNED_WRAP_$(target-stem).o)$(UBSAN_SANITIZE_$(target-stem).o)$(UBSAN_SIGNED_WRAP)$(UBSAN_SANITIZE)y), \
+		$(UBSAN_SIGNED_WRAP_$(target-stem).o)$(UBSAN_SANITIZE_$(target-stem).o)$(UBSAN_SIGNED_WRAP)$(UBSAN_SANITIZE)$(is-kernel-object)), \
 		$(CFLAGS_UBSAN_SIGNED_WRAP))
 endif
 
 ifeq ($(CONFIG_KCOV),y)
 _c_flags += $(if $(patsubst n%,, \
-	$(KCOV_INSTRUMENT_$(target-stem).o)$(KCOV_INSTRUMENT)$(CONFIG_KCOV_INSTRUMENT_ALL)), \
+	$(KCOV_INSTRUMENT_$(target-stem).o)$(KCOV_INSTRUMENT)$(if $(is-kernel-object),$(CONFIG_KCOV_INSTRUMENT_ALL))), \
 	$(CFLAGS_KCOV))
 endif
 
@@ -200,7 +200,7 @@ endif
 #
 ifeq ($(CONFIG_KCSAN),y)
 _c_flags += $(if $(patsubst n%,, \
-	$(KCSAN_SANITIZE_$(target-stem).o)$(KCSAN_SANITIZE)y), \
+	$(KCSAN_SANITIZE_$(target-stem).o)$(KCSAN_SANITIZE)$(is-kernel-object)), \
 	$(CFLAGS_KCSAN))
 # Some uninstrumented files provide implied barriers required to avoid false
 # positives: set KCSAN_INSTRUMENT_BARRIERS for barrier instrumentation only.
@@ -219,6 +219,10 @@ _cpp_flags += $(addprefix -I, $(src) $(obj))
 endif
 endif
 
+# If $(is-kernel-object) is 'y', this object will be linked to vmlinux or modules
+is-kernel-object = $(or $(part-of-builtin),$(part-of-module))
+
+part-of-builtin = $(if $(filter $(basename $@).o, $(real-obj-y) $(lib-y)),y)
 part-of-module = $(if $(filter $(basename $@).o, $(real-obj-m)),y)
 quiet_modtag = $(if $(part-of-module),[M],   )
 
-- 
2.40.1