kbuild: Abort build if SUBDIRS used

Forwarded: not-needed
Bug-Debian: https://bugs.debian.org/987575

DKMS and module-assistant both build OOT modules as root.  If they
build an old OOT module that still use SUBDIRS this causes Kbuild
to try building a full kernel, which obviously fails but not before
deleting files from the installed headers package.

To avoid such mishaps, detect this situation and abort the build.

The error message is based on that used in commit 0126be38d988
"kbuild: announce removal of SUBDIRS if used".

Gbp-Pq: Topic debian
Gbp-Pq: Name kbuild-abort-build-if-subdirs-used.patch

diff --git a/Makefile b/Makefile
index 0c788fc3c44ff7009eb3e719c14d21bb0ec8e2a9..0974379c695f857bc2bb1b862f6df0350740811c 100644 (file)
--- a/Makefile
+++ b/Makefile
@@ -218,6 +218,18 @@ ifeq ("$(origin M)", "command line")
   KBUILD_EXTMOD := $(M)
 endif
 
+# Old syntax make ... SUBDIRS=$PWD should be rejected to avoid mishaps
+# (see Debian bugs #982334, #987575)
+ifndef KBUILD_EXTMOD
+  ifdef SUBDIRS
+    $(warning =============== ERROR ==============)
+    $(warning 'SUBDIRS' was removed in Linux 5.3)
+    $(warning Use 'M=' or 'KBUILD_EXTMOD=' instead)
+    $(warning ====================================)
+    $(error .)
+  endif
+endif
+
 $(if $(word 2, $(KBUILD_EXTMOD)), \
        $(error building multiple external modules is not supported))