gcc-plugins: Add support for plugin subdirectories

This adds support for building more complex gcc plugins that live in a
subdirectory instead of just in a single source file.
Reported-by: PaX Team <pageexec@freemail.hu>
Signed-off-by: Emese Revfy <re.emese@gmail.com>
[kees: clarified commit message]
Signed-off-by: Kees Cook <keescook@chromium.org>

scripts/Makefile.gcc-plugins

@@ -21,7 +21,7 @@ ifdef CONFIG_GCC_PLUGINS
 
   GCC_PLUGINS_CFLAGS := $(strip $(addprefix -fplugin=$(objtree)/scripts/gcc-plugins/, $(gcc-plugin-y)) $(gcc-plugin-cflags-y))
 
-  export PLUGINCC GCC_PLUGINS_CFLAGS GCC_PLUGIN SANCOV_PLUGIN
+  export PLUGINCC GCC_PLUGINS_CFLAGS GCC_PLUGIN GCC_PLUGIN_SUBDIR SANCOV_PLUGIN
 
   ifneq ($(PLUGINCC),)
     # SANCOV_PLUGIN can be only in CFLAGS_KCOV because avoid duplication.
@@ -30,6 +30,7 @@ ifdef CONFIG_GCC_PLUGINS
 
   KBUILD_CFLAGS += $(GCC_PLUGINS_CFLAGS)
   GCC_PLUGIN := $(gcc-plugin-y)
+  GCC_PLUGIN_SUBDIR := $(gcc-plugin-subdir-y)
 endif
 
 # If plugins aren't supported, abort the build before hard-to-read compiler

scripts/gcc-plugins/Makefile

@@ -12,15 +12,18 @@ else
   export HOST_EXTRACXXFLAGS
 endif
 
-export GCCPLUGINS_DIR HOSTLIBS
-
 ifneq ($(CFLAGS_KCOV), $(SANCOV_PLUGIN))
   GCC_PLUGIN := $(filter-out $(SANCOV_PLUGIN), $(GCC_PLUGIN))
 endif
 
-$(HOSTLIBS)-y := $(GCC_PLUGIN)
+export HOSTLIBS
+
+$(HOSTLIBS)-y := $(foreach p,$(GCC_PLUGIN),$(if $(findstring /,$(p)),,$(p)))
 always := $($(HOSTLIBS)-y)
 
 $(foreach p,$($(HOSTLIBS)-y:%.so=%),$(eval $(p)-objs := $(p).o))
 
+subdir-y := $(GCC_PLUGIN_SUBDIR)
+subdir-  += $(GCC_PLUGIN_SUBDIR)
+
 clean-files += *.so