kbuild: wire up the build rule of compile_commands.json to Makefile

Currently, you need to manually run scripts/gen_compile_commands.py
to create compile_commands.json. It parses all the .*.cmd files found
under the specified directory.

If you rebuild the kernel over again without 'make clean',
.*.cmd files from older builds will create stale entries in
compile_commands.json.

This commit wires up the compile_commands.json rule to Makefile, and
makes it parse only the .*.cmd files involved in the current build.

Pass $(KBUILD_VMLINUX_OBJS), $(KBUILD_VMLINUX_LIBS), and modules.order
to the script. The objects or archives linked to vmlinux are listed in
$(KBUILD_VMLINUX_OBJS) or $(KBUILD_VMLINUX_LIBS). All the modules are
listed in modules.order.

You can create compile_commands.json from Make:

  $ make -j$(nproc) CC=clang compile_commands.json

You can also build vmlinux, modules, and compile_commands.json all
together in a single command:

  $ make -j$(nproc) CC=clang all compile_commands.json

It works for M= builds as well. In this case, compile_commands.json
is created in the top directory of the external module.

This is convenient, but it has a drawback; the coverage of the
compile_commands.json is reduced because only the objects linked to
vmlinux or modules are handled. For example, the following C files are
not included in the compile_commands.json:

 - Decompressor source files (arch/*/boot/)
 - VDSO source files
 - C files used to generate intermediates (e.g. kernel/bounds.c)
 - Standalone host programs

I think it is fine for most developers because our main interest is
the kernel-space code.

If you want to cover all the compiled C files, please build the kernel,
then run the script manually as you did before:

  $ make clean    # if you want to remove stale .cmd files [optional]
  $ make -j$(nproc) CC=clang
  $ scripts/gen_compile_commands.py

Here is a note for out-of-tree builds. 'make compile_commands.json'
works with O= option, but please notice compile_commands.json is
created in the object tree instead of the source tree.

Some people may want to have compile_commands.json in the source tree
because Clang Tools searches for it through all parent paths of the
first input source file.

However, you cannot do this for O= builds. Kbuild should never generate
any build artifact in the source tree when O= is given because the
source tree might be read-only. Any write attempt to the source tree
is monitored and the violation may be reported. See the commit log of
8ef14c2c.

So, the only possible way is to create compile_commands.json in the
object tree, then specify '-p <build-path>' when you use clang-check,
clang-tidy, etc.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Nick Desaulniers <ndesaulniers@google.com>

Makefile

@@ -634,7 +634,7 @@ endif
 # in addition to whatever we do anyway.
 # Just "make" or "make all" shall build modules as well
 
-ifneq ($(filter all modules nsdeps,$(MAKECMDGOALS)),)
+ifneq ($(filter all modules nsdeps %compile_commands.json,$(MAKECMDGOALS)),)
   KBUILD_MODULES := 1
 endif
 
@@ -1470,7 +1470,8 @@ endif # CONFIG_MODULES
 
 # Directories & files removed with 'make clean'
 CLEAN_FILES += include/ksym vmlinux.symvers \
-	       modules.builtin modules.builtin.modinfo modules.nsdeps
+	       modules.builtin modules.builtin.modinfo modules.nsdeps \
+	       compile_commands.json
 
 # Directories & files removed with 'make mrproper'
 MRPROPER_FILES += include/config include/generated          \
@@ -1704,9 +1705,12 @@ KBUILD_MODULES := 1
 
 build-dirs := $(KBUILD_EXTMOD)
 PHONY += modules
-modules: descend
+modules: $(MODORDER)
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modpost
 
+$(MODORDER): descend
+	@:
+
 PHONY += modules_install
 modules_install: _emodinst_ _emodinst_post
 
@@ -1720,8 +1724,12 @@ PHONY += _emodinst_post
 _emodinst_post: _emodinst_
 	$(call cmd,depmod)
 
+compile_commands.json: $(extmod-prefix)compile_commands.json
+PHONY += compile_commands.json
+
 clean-dirs := $(KBUILD_EXTMOD)
-clean: rm-files := $(KBUILD_EXTMOD)/Module.symvers $(KBUILD_EXTMOD)/modules.nsdeps
+clean: rm-files := $(KBUILD_EXTMOD)/Module.symvers $(KBUILD_EXTMOD)/modules.nsdeps \
+	$(KBUILD_EXTMOD)/compile_commands.json
 
 PHONY += help
 help:
@@ -1834,6 +1842,19 @@ nsdeps: export KBUILD_NSDEPS=1
 nsdeps: modules
 	$(Q)$(CONFIG_SHELL) $(srctree)/scripts/nsdeps
 
+# Clang Tooling
+# ---------------------------------------------------------------------------
+
+quiet_cmd_gen_compile_commands = GEN     $@
+      cmd_gen_compile_commands = $(PYTHON3) $< -a $(AR) -o $@ $(filter-out $<, $(real-prereqs))
+
+$(extmod-prefix)compile_commands.json: scripts/gen_compile_commands.py \
+	$(if $(KBUILD_EXTMOD),,$(KBUILD_VMLINUX_OBJS) $(KBUILD_VMLINUX_LIBS)) \
+	$(if $(CONFIG_MODULES), $(MODORDER)) FORCE
+	$(call if_changed,gen_compile_commands)
+
+targets += $(extmod-prefix)compile_commands.json
+
 # Scripts to check various things for consistency
 # ---------------------------------------------------------------------------