kbuild: change working directory to external module directory with M=

Currently, Kbuild always operates in the output directory of the kernel,
even when building external modules. This increases the risk of external
module Makefiles attempting to write to the kernel directory.

This commit switches the working directory to the external module
directory, allowing the removal of the $(KBUILD_EXTMOD)/ prefix from
some build artifacts.

The command for building external modules maintains backward
compatibility, but Makefiles that rely on working in the kernel
directory may break. In such cases, $(objtree) and $(srctree) should
be used to refer to the output and source directories of the kernel.

The appearance of the build log will change as follows:

[Before]

  $ make -C /path/to/my/linux M=/path/to/my/externel/module
  make: Entering directory '/path/to/my/linux'
    CC [M]  /path/to/my/externel/module/helloworld.o
    MODPOST /path/to/my/externel/module/Module.symvers
    CC [M]  /path/to/my/externel/module/helloworld.mod.o
    CC [M]  /path/to/my/externel/module/.module-common.o
    LD [M]  /path/to/my/externel/module/helloworld.ko
  make: Leaving directory '/path/to/my/linux'

[After]

  $ make -C /path/to/my/linux M=/path/to/my/externel/module
  make: Entering directory '/path/to/my/linux'
  make[1]: Entering directory '/path/to/my/externel/module'
    CC [M]  helloworld.o
    MODPOST Module.symvers
    CC [M]  helloworld.mod.o
    CC [M]  .module-common.o
    LD [M]  helloworld.ko
  make[1]: Leaving directory '/path/to/my/externel/module'
  make: Leaving directory '/path/to/my/linux'

Printing "Entering directory" twice is cumbersome. This will be
addressed later.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>

1 files changed, 49 insertions, 33 deletions

diff --git a/Makefile b/Makefile
index cf1d55560ae2..2718d2037f5e 100644
--- a/Makefile
+++ b/Makefile
@@ -180,7 +180,24 @@ ifeq ("$(origin O)", "command line")
   KBUILD_OUTPUT := $(O)
 endif
 
-output := $(KBUILD_OUTPUT)
+ifdef KBUILD_EXTMOD
+    ifdef KBUILD_OUTPUT
+        objtree := $(realpath $(KBUILD_OUTPUT))
+        $(if $(objtree),,$(error specified kernel directory "$(KBUILD_OUTPUT)" does not exist))
+    else
+        objtree := $(CURDIR)
+    endif
+    output := $(KBUILD_EXTMOD)
+    # KBUILD_EXTMOD might be a relative path. Remember its absolute path before
+    # Make changes the working directory.
+    srcroot := $(realpath $(KBUILD_EXTMOD))
+    $(if $(srcroot),,$(error specified external module directory "$(KBUILD_EXTMOD)" does not exist))
+else
+    objtree := .
+    output := $(KBUILD_OUTPUT)
+endif
+
+export objtree srcroot
 
 # Do we want to change the working directory?
 ifneq ($(output),)
@@ -230,36 +247,34 @@ else # need-sub-make
 
 # We process the rest of the Makefile if this is the final invocation of make
 
-ifeq ($(abs_srctree),$(CURDIR))
-        # building in the source tree
-        srctree := .
-	building_out_of_srctree :=
-else
-        ifeq ($(abs_srctree)/,$(dir $(CURDIR)))
-                # building in a subdirectory of the source tree
-                srctree := ..
-        else
-                srctree := $(abs_srctree)
-        endif
-	building_out_of_srctree := 1
+ifndef KBUILD_EXTMOD
+srcroot := $(abs_srctree)
 endif
 
-ifneq ($(KBUILD_ABS_SRCTREE),)
-srctree := $(abs_srctree)
+ifeq ($(srcroot),$(CURDIR))
+building_out_of_srctree :=
+else
+export building_out_of_srctree := 1
 endif
 
-objtree		:= .
+ifdef KBUILD_ABS_SRCTREE
+    # Do nothing. Use the absolute path.
+else ifeq ($(srcroot),$(CURDIR))
+    # Building in the source.
+    srcroot := .
+else ifeq ($(srcroot)/,$(dir $(CURDIR)))
+    # Building in a subdirectory of the source.
+    srcroot := ..
+endif
 
-VPATH		:=
+export srctree := $(if $(KBUILD_EXTMOD),$(abs_srctree),$(srcroot))
 
-ifeq ($(KBUILD_EXTMOD),)
 ifdef building_out_of_srctree
-VPATH		:= $(srctree)
-endif
+export VPATH := $(srcroot)
+else
+VPATH :=
 endif
 
-export building_out_of_srctree srctree objtree VPATH
-
 # To make sure we do not include .config for any of the *config targets
 # catch them early, and hand them over to scripts/kconfig/Makefile
 # It is allowed to specify more targets when calling make, including
@@ -540,7 +555,7 @@ USERINCLUDE    := \
 LINUXINCLUDE    := \
 		-I$(srctree)/arch/$(SRCARCH)/include \
 		-I$(objtree)/arch/$(SRCARCH)/include/generated \
-		$(if $(building_out_of_srctree),-I$(srctree)/include) \
+		-I$(srctree)/include \
 		-I$(objtree)/include \
 		$(USERINCLUDE)
 
@@ -711,7 +726,7 @@ endif
 # in addition to whatever we do anyway.
 # Just "make" or "make all" shall build modules as well
 
-ifneq ($(filter all modules nsdeps %compile_commands.json clang-%,$(MAKECMDGOALS)),)
+ifneq ($(filter all modules nsdeps compile_commands.json clang-%,$(MAKECMDGOALS)),)
   KBUILD_MODULES := 1
 endif
 
@@ -1107,7 +1122,7 @@ export MODLIB
 
 PHONY += prepare0
 
-export extmod_prefix = $(if $(KBUILD_EXTMOD),$(KBUILD_EXTMOD)/)
+export extmod_prefix =
 export MODORDER := $(extmod_prefix)modules.order
 export MODULES_NSDEPS := $(extmod_prefix)modules.nsdeps
 
@@ -1799,14 +1814,10 @@ filechk_kernel.release = echo $(KERNELRELEASE)
 KBUILD_BUILTIN :=
 KBUILD_MODULES := 1
 
-build-dir := $(KBUILD_EXTMOD)
+build-dir := .
 
-compile_commands.json: $(extmod_prefix)compile_commands.json
-PHONY += compile_commands.json
-
-clean-dirs := $(KBUILD_EXTMOD)
-clean: private rm-files := $(KBUILD_EXTMOD)/Module.symvers $(KBUILD_EXTMOD)/modules.nsdeps \
-	$(KBUILD_EXTMOD)/compile_commands.json
+clean-dirs := .
+clean: private rm-files := Module.symvers modules.nsdeps compile_commands.json
 
 PHONY += prepare
 # now expand this into a simple variable to reduce the cost of shell evaluations
@@ -1948,7 +1959,7 @@ $(clean-dirs):
 
 clean: $(clean-dirs)
 	$(call cmd,rmfiles)
-	@find $(or $(KBUILD_EXTMOD), .) $(RCS_FIND_IGNORE) \
+	@find . $(RCS_FIND_IGNORE) \
 		\( -name '*.[aios]' -o -name '*.rsi' -o -name '*.ko' -o -name '.*.cmd' \
 		-o -name '*.ko.*' \
 		-o -name '*.dtb' -o -name '*.dtbo' \
@@ -1981,7 +1992,12 @@ tags TAGS cscope gtags: FORCE
 PHONY += rust-analyzer
 rust-analyzer:
 	+$(Q)$(CONFIG_SHELL) $(srctree)/scripts/rust_is_available.sh
+ifdef KBUILD_EXTMOD
+# FIXME: external modules must not descend into a sub-directory of the kernel
+	$(Q)$(MAKE) $(build)=$(objtree)/rust src=$(srctree)/rust $@
+else
 	$(Q)$(MAKE) $(build)=rust $@
+endif
 
 # Script to generate missing namespace dependencies
 # ---------------------------------------------------------------------------