[tip: objtool/core] kbuild: Fix objtool dependency for 'OBJECT_FILES_NON_STANDARD_<obj> := n'

[tip-bot2 for Josh Poimboeuf]

The following commit has been merged into the objtool/core branch of tip:

Commit-ID:     8852c552402979508fdc395ae07aa8761aa46045
Gitweb:        https://git.kernel.org/tip/8852c552402979508fdc395ae07aa8761aa46045
Author:        Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
AuthorDate:    Tue, 18 May 2021 18:59:15 -05:00
Committer:     Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
CommitterDate: Wed, 19 May 2021 15:07:20 -05:00

kbuild: Fix objtool dependency for 'OBJECT_FILES_NON_STANDARD_<obj> := n'

"OBJECT_FILES_NON_STANDARD_vma.o := n" has a dependency bug.  When
objtool source is updated, the affected object doesn't get re-analyzed
by objtool.

Peter's new variable-sized jump label feature relies on objtool
rewriting the object file.  Otherwise the system can fail to boot.  That
effectively upgrades this minor dependency issue to a major bug.

The problem is that variables in prerequisites are expanded early,
during the read-in phase.  The '$(objtool_dep)' variable indirectly uses
'$@', which isn't yet available when the target prerequisites are
evaluated.

Use '.SECONDEXPANSION:' which causes '$(objtool_dep)' to be expanded in
a later phase, after the target-specific '$@' variable has been defined.

Fixes: b9ab5ebb14ec ("objtool: Add CONFIG_STACK_VALIDATION option")
Fixes: ab3257042c26 ("jump_label, x86: Allow short NOPs")
Reported-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
 scripts/Makefile.build | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 949f723..34d2576 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -268,7 +268,8 @@ define rule_as_o_S
 endef
 
 # Built-in and composite module parts
-$(obj)/%.o: $(src)/%.c $(recordmcount_source) $(objtool_dep) FORCE
+.SECONDEXPANSION:
+$(obj)/%.o: $(src)/%.c $(recordmcount_source) $$(objtool_dep) FORCE
 	$(call if_changed_rule,cc_o_c)
 	$(call cmd,force_checksrc)
 
@@ -349,7 +350,7 @@ cmd_modversions_S =								\
 	fi
 endif
 
-$(obj)/%.o: $(src)/%.S $(objtool_dep) FORCE
+$(obj)/%.o: $(src)/%.S $$(objtool_dep) FORCE
 	$(call if_changed_rule,as_o_S)
 
 targets += $(filter-out $(subdir-builtin), $(real-obj-y))