fortify: refactor test_fortify Makefile to fix some build problems

There are some issues in the test_fortify Makefile code.

Problem 1: cc-disable-warning invokes compiler dozens of times

To see how many times the cc-disable-warning is evaluated, change
this code:

  $(call cc-disable-warning,fortify-source)

to:

  $(call cc-disable-warning,$(shell touch /tmp/fortify-$$$$)fortify-source)

Then, build the kernel with CONFIG_FORTIFY_SOURCE=y. You will see a
large number of '/tmp/fortify-<PID>' files created:

  $ ls -1 /tmp/fortify-* | wc
       80      80    1600

This means the compiler was invoked 80 times just for checking the
-Wno-fortify-source flag support.

$(call cc-disable-warning,fortify-source) should be added to a simple
variable instead of a recursive variable.

Problem 2: do not recompile string.o when the test code is updated

The test cases are independent of the kernel. However, when the test
code is updated, $(obj)/string.o is rebuilt and vmlinux is relinked
due to this dependency:

  $(obj)/string.o: $(obj)/$(TEST_FORTIFY_LOG)

always-y is suitable for building the log files.

Problem 3: redundant code

  clean-files += $(addsuffix .o, $(TEST_FORTIFY_LOGS))

... is unneeded because the top Makefile globally cleans *.o files.

This commit fixes these issues and makes the code readable.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20240727150302.1823750-2-masahiroy@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>

Author: masahir0y

lib/.gitignore

@@ -5,5 +5,3 @@
 /gen_crc32table
 /gen_crc64table
 /oid_registry_data.c
-/test_fortify.log
-/test_fortify/*.log

lib/Makefile

@@ -393,40 +393,4 @@ obj-$(CONFIG_GENERIC_LIB_DEVMEM_IS_ALLOWED) += devmem_is_allowed.o
 
 obj-$(CONFIG_FIRMWARE_TABLE) += fw_table.o
 
-# FORTIFY_SOURCE compile-time behavior tests
-TEST_FORTIFY_SRCS = $(wildcard $(src)/test_fortify/*-*.c)
-TEST_FORTIFY_LOGS = $(patsubst $(src)/%.c, %.log, $(TEST_FORTIFY_SRCS))
-TEST_FORTIFY_LOG = test_fortify.log
-
-quiet_cmd_test_fortify = TEST    $@
-      cmd_test_fortify = $(CONFIG_SHELL) $(srctree)/scripts/test_fortify.sh \
-			$< $@ "$(NM)" $(CC) $(c_flags) \
-			$(call cc-disable-warning,fortify-source) \
-			-DKBUILD_EXTRA_WARN1
-
-targets += $(TEST_FORTIFY_LOGS)
-clean-files += $(TEST_FORTIFY_LOGS)
-clean-files += $(addsuffix .o, $(TEST_FORTIFY_LOGS))
-$(obj)/test_fortify/%.log: $(src)/test_fortify/%.c \
-			   $(src)/test_fortify/test_fortify.h \
-			   $(srctree)/include/linux/fortify-string.h \
-			   $(srctree)/scripts/test_fortify.sh \
-			   FORCE
-	$(call if_changed,test_fortify)
-
-quiet_cmd_gen_fortify_log = GEN     $@
-      cmd_gen_fortify_log = cat </dev/null $(filter-out FORCE,$^) 2>/dev/null > $@ || true
-
-targets += $(TEST_FORTIFY_LOG)
-clean-files += $(TEST_FORTIFY_LOG)
-$(obj)/$(TEST_FORTIFY_LOG): $(addprefix $(obj)/, $(TEST_FORTIFY_LOGS)) FORCE
-	$(call if_changed,gen_fortify_log)
-
-# Fake dependency to trigger the fortify tests.
-ifeq ($(CONFIG_FORTIFY_SOURCE),y)
-$(obj)/string.o: $(obj)/$(TEST_FORTIFY_LOG)
-endif
-
-# Some architectures define __NO_FORTIFY if __SANITIZE_ADDRESS__ is undefined.
-# Pass CFLAGS_KASAN to avoid warnings.
-$(foreach x, $(patsubst %.log,%.o,$(TEST_FORTIFY_LOGS)), $(eval KASAN_SANITIZE_$(x) := y))
+subdir-$(CONFIG_FORTIFY_SOURCE) += test_fortify

lib/test_fortify/.gitignore

@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0-only
+/*.log

lib/test_fortify/Makefile

@@ -0,0 +1,28 @@
+# SPDX-License-Identifier: GPL-2.0
+
+ccflags-y := $(call cc-disable-warning,fortify-source)
+
+quiet_cmd_test_fortify = TEST    $@
+      cmd_test_fortify = $(CONFIG_SHELL) $(srctree)/scripts/test_fortify.sh \
+			$< $@ "$(NM)" $(CC) $(c_flags) -DKBUILD_EXTRA_WARN1
+
+$(obj)/%.log: $(src)/%.c $(srctree)/scripts/test_fortify.sh \
+	      $(src)/test_fortify.h \
+	      $(srctree)/include/linux/fortify-string.h \
+	      FORCE
+	$(call if_changed,test_fortify)
+
+logs = $(patsubst $(src)/%.c, %.log, $(wildcard $(src)/*-*.c))
+targets += $(logs)
+
+quiet_cmd_gen_fortify_log = CAT     $@
+      cmd_gen_fortify_log = cat $(or $(real-prereqs),/dev/null) > $@
+
+$(obj)/test_fortify.log: $(addprefix $(obj)/, $(logs)) FORCE
+	$(call if_changed,gen_fortify_log)
+
+always-y += test_fortify.log
+
+# Some architectures define __NO_FORTIFY if __SANITIZE_ADDRESS__ is undefined.
+# Pass CFLAGS_KASAN to avoid warnings.
+KASAN_SANITIZE := y

scripts/remove-stale-files

@@ -21,3 +21,5 @@ set -e
 # then will be really dead and removed from the code base entirely.
 
 rm -f *.spec
+
+rm -f lib/test_fortify.log