Re: [tip: objtool/core] jump_label, x86: Allow short NOPs

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Tue, May 18, 2021 at 10:24:43PM +0200, Peter Zijlstra wrote:
> OK, willy followed up on IRC, and it turns out there's a kbuild
> dependency missing; then objtool changes we don't rebuild:
> 
>   arch/x86/entry/vdso/vma.o
> 
> even though we should, this led to an unpatched 2 byte jump-label and
> things went sideways. I'm not sure I understand the whole build
> machinery well enough to know where to begin chasing this.
> 
> Now, this file is mighty magical, due to:
> 
> arch/x86/entry/vdso/Makefile:OBJECT_FILES_NON_STANDARD  := y
> arch/x86/entry/vdso/Makefile:OBJECT_FILES_NON_STANDARD_vma.o    := n
> 
> Maybe that's related.

I'm not exactly thrilled that objtool now has the power to easily brick
a system :-/  Is it really worth it?

Anyway, here's one way to fix it.  Maybe Masahiro has a better idea.

From f88b208677953bc445db08ac46b6e4259217bb8a Mon Sep 17 00:00:00 2001
Message-Id: <f88b208677953bc445db08ac46b6e4259217bb8a.1621384807.git.jpoimboe@redhat.com>
From: Josh Poimboeuf <jpoimboe@redhat.com>
Date: Tue, 18 May 2021 18:59:15 -0500
Subject: [PATCH] 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@infradead.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
 scripts/Makefile.build | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 949f723efe53..34d257653fb4 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))
-- 
2.31.1