kbuild: Fix corner caches with .cache.mk

This two-patches fixes two corner cases with .cache.mk that have been reported recently. Neither problem was catastrophic, but certainly several people ran into the problem solved by the first patch (can't build after gcc upgrade) and wasted time debugging, so it's really a good idea to fix.

For both patches, please make sure to give them an extra thorough review. I _think_ I understand enough about $(MAKECMDGOALS), but I'd never explored that feature of make before writing these patches so the patches certainly need someone more experienced to give them a careful look.

I've only got one more day of work before I'm on Christmas vacation for 2 weeks, so if there are problems with these patches please give me a bit of time to fix. ...or, if someone is in a hurry, I wouldn't object to someone else hijacking them and posting fixes.

Douglas Anderson (2):
kbuild: Require a 'make clean' if we detect gcc changed underneath us
kbuild: Don't mess with the .cache.mk when installing

 scripts/Kbuild.include | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

2.15.1.620.gb9897f4670-goog

Several people reported that the commit 3298b690b21c ("kbuild: Add a cache for generated variables") caused them problems when they updated gcc versions. Specifically the reports all looked something similar to this:

Masahiro Yamada determined that the problem was with:

  NOSTDINC_FLAGS += -nostdinc -isystem $(call shell-cached,$(CC)
  -print-file-name=include)

Specifically that the stale result of -print-file-name is stored in the cache file. It was determined that a "make clean" fixed the problems in all cases.

In this particular case we could certainly try to clean just the cache when we detect a gcc update, but it seems like overall it's a bad idea to do an incremental build when gcc changes. We should warn the user and tell them that they need a 'make clean'.

---

 scripts/Kbuild.include | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

index 065324a8046f..56dee20fb2b8 100644
@@ -222,6 +222,10 @@ cc-version = $(call shell-cached,$(CONFIG_SHELL) $(srctree)/scripts/gcc-version.
 cc-fullversion = $(call shell-cached,$(CONFIG_SHELL) \
 	$(srctree)/scripts/gcc-version.sh -p $(CC))
 
+# cc-fullversion-uncached
+cc-fullversion-uncached := $(shell $(CONFIG_SHELL) \
+	$(srctree)/scripts/gcc-version.sh -p $(CC))
+
 # cc-ifversion
 # Usage:  EXTRA_CFLAGS += $(call cc-ifversion, -lt, 0402, -O1)
 cc-ifversion = $(shell [ $(cc-version) $(1) $(2) ] && echo $(3) || echo $(4))

@@ -475,3 +479,14 @@ endif
 endef
 #
 ###############################################################################
+
+# Require a 'make clean' if the compiler changed; not only does the .cache.mk
+# need to be thrown out but we should also start with fresh object files.
+#
+# NOTE: it's important that we don't error out when the goal is actually to
+# try to make clean, distclean or mrproper.
+ifeq ($(filter %clean,$(MAKECMDGOALS))$(filter mrproper,$(MAKECMDGOALS)),)
+  ifneq ($(cc-fullversion-uncached),$(cc-fullversion))
+    $(error Detected new CC version ($(cc-fullversion-uncached) vs $(cc-fullversion)).  Please 'make clean')
+  endif
+endif

2.15.1.620.gb9897f4670-goog

As pointed out by Masahiro Yamada people often run "sudo make install" or "sudo make modules_install". In theory, that could cause a cache file (or the directory containing it) to be created by root. After doing this then subsequent invocations to make would yell with a whole bunch of warnings like:

/bin/sh: ./.cache.mk: Permission denied

These yells would be mostly harmless (we'd just go on running without the cache), but they're ugly.

The above situation would be fairly unlikely because it should only happen if someone does "sudo make install" before doing a normal "make", which is an invalid thing to do. If you did a normal "make" before the "sudo make install" then all the cache files and directories should have already been created by a normal user and, even if the superuser needed to add to the existing files, we wouldn't end up with a permission problem.

The above situation would also not have been catastrophic because presumably if the user was able to run "sudo make install" then they should also be able to run "sudo make clean" to clean things up.

However, even though the problem described is relatively minor, it probably makes sense to add a heuristic to avoid creating cache files when one of the make goals looks like an install. This heuristic doesn't add a ton of overhead and it might save someone time tracking down strange "Permission denied" messages. We'll consider this heuristic good enough because the problem really shouldn't be that serious.

---

 scripts/Kbuild.include | 8 ++++++++
 1 file changed, 8 insertions(+)

index 56dee20fb2b8..e18235cd9570 100644
@@ -124,6 +124,12 @@ __sanitize-opt = $(subst $$,_,$(subst $(right_paren),_,$(subst $(left_paren),_,$
 # previous invocation of make!) we'll return the value.  If not, we'll
 # compute it and store the result for future runs.
 #
+# NOTE: we won't ever add to the cache if one of the make goals looks like
+# it is installing.  Technically there's no reason we can't cache things
+# related to install but it's likely that 'make install' is called as root.
+# If we create the cache file as root we might have a hard time adding to it
+# (or deleting it in make clean).
+#
 # This is a bit of voodoo, but basic explanation is that if the variable
 # was undefined then we'll evaluate the shell command and store the result
 # into the variable.  We'll then store that value in the cache and finally

@@ -137,12 +143,14 @@ __sanitize-opt = $(subst $$,_,$(subst $(right_paren),_,$(subst $(left_paren),_,$
 define __run-and-store
 ifeq ($(origin $(1)),undefined)
   $$(eval $(1) := $$(shell $$(2)))
+ifeq ($(filter %install,$(MAKECMDGOALS)),)
 ifeq ($(create-cache-dir),1)
   $$(shell mkdir -p $(dir $(make-cache)))
   $$(eval create-cache-dir :=)
 endif
   $$(shell echo '$(1) := $$($(1))' >> $(make-cache))
 endif
+endif
 endef
 __shell-cached = $(eval $(call __run-and-store,$(1)))$($(1))
 shell-cached = $(call __shell-cached,__cached_$(call __sanitize-opt,$(1)),$(1))

2.15.1.620.gb9897f4670-goog