gcc-plugins: Undefine LATENT_ENTROPY_PLUGIN when plugin disabled for a file

Commit 36d4b36 ("lib/nodemask: inline next_node_in() and
node_random()") refactored some code by moving node_random() from
lib/nodemask.c to include/linux/nodemask.h, thus requiring nodemask.h to
include random.h, which conditionally defines add_latent_entropy()
depending on whether the macro LATENT_ENTROPY_PLUGIN is defined.

This broke the build on powerpc, where nodemask.h is indirectly included
in arch/powerpc/kernel/prom_init.c, part of the early boot machinery that
is excluded from the latent entropy plugin using
DISABLE_LATENT_ENTROPY_PLUGIN. It turns out that while we add a gcc flag
to disable the actual plugin, we don't undefine LATENT_ENTROPY_PLUGIN.

This leads to the following:

    CC      arch/powerpc/kernel/prom_init.o
  In file included from ./include/linux/nodemask.h:97,
                   from ./include/linux/mmzone.h:17,
                   from ./include/linux/gfp.h:7,
                   from ./include/linux/xarray.h:15,
                   from ./include/linux/radix-tree.h:21,
                   from ./include/linux/idr.h:15,
                   from ./include/linux/kernfs.h:12,
                   from ./include/linux/sysfs.h:16,
                   from ./include/linux/kobject.h:20,
                   from ./include/linux/pci.h:35,
                   from arch/powerpc/kernel/prom_init.c:24:
  ./include/linux/random.h: In function 'add_latent_entropy':
  ./include/linux/random.h:25:46: error: 'latent_entropy' undeclared (first use in this function); did you mean 'add_latent_entropy'?
     25 |         add_device_randomness((const void *)&latent_entropy, sizeof(latent_entropy));
        |                                              ^~~~~~~~~~~~~~
        |                                              add_latent_entropy
  ./include/linux/random.h:25:46: note: each undeclared identifier is reported only once for each function it appears in
  make[2]: *** [scripts/Makefile.build:249: arch/powerpc/kernel/prom_init.o] Fehler 1
  make[1]: *** [scripts/Makefile.build:465: arch/powerpc/kernel] Fehler 2
  make: *** [Makefile:1855: arch/powerpc] Error 2

Change the DISABLE_LATENT_ENTROPY_PLUGIN flags to undefine
LATENT_ENTROPY_PLUGIN for files where the plugin is disabled.

Cc: Yury Norov <[email protected]>
Fixes: 38addce ("gcc-plugins: Add latent_entropy plugin")
Link: https://bugzilla.kernel.org/show_bug.cgi?id=216367
Link: https://lore.kernel.org/linuxppc-dev/[email protected]/
Reported-by: Erhard Furtner <[email protected]>
Signed-off-by: Andrew Donnellan <[email protected]>
Reviewed-by: Yury Norov <[email protected]>
Signed-off-by: Kees Cook <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: ajdlinux

scripts/Makefile.gcc-plugins

@@ -4,7 +4,7 @@ gcc-plugin-$(CONFIG_GCC_PLUGIN_LATENT_ENTROPY)	+= latent_entropy_plugin.so
 gcc-plugin-cflags-$(CONFIG_GCC_PLUGIN_LATENT_ENTROPY)		\
 		+= -DLATENT_ENTROPY_PLUGIN
 ifdef CONFIG_GCC_PLUGIN_LATENT_ENTROPY
-    DISABLE_LATENT_ENTROPY_PLUGIN += -fplugin-arg-latent_entropy_plugin-disable
+    DISABLE_LATENT_ENTROPY_PLUGIN += -fplugin-arg-latent_entropy_plugin-disable -ULATENT_ENTROPY_PLUGIN
 endif
 export DISABLE_LATENT_ENTROPY_PLUGIN