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 cac7f55565cbfc861135ce1cc457bd1d6f5544fc..0f41544bab6bb2694e2215ada969d3efa2edd79b 100644 (file)
--- a/Makefile
+++ b/Makefile
@@ -134,6 +134,18 @@ 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))