linux-hardening - [RFC PATCH 02/11] kbuild: document some prerequisites

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Message-Id: <20240819160309.2218114-3-vegard.nossum@oracle.com>
Date: Mon, 19 Aug 2024 18:02:59 +0200
From: Vegard Nossum <vegard.nossum@...cle.com>
To: Masahiro Yamada <masahiroy@...nel.org>, linux-kbuild@...r.kernel.org
Cc: Nathan Chancellor <nathan@...nel.org>, Nicolas Schier <nicolas@...sle.eu>,
        Michael Ellerman <mpe@...erman.id.au>,
        Morten Linderud <morten@...derud.pw>,
        Haelwenn Monnier <contact@...odan.eu>, Jann Horn <jannh@...gle.com>,
        Kees Cook <kees@...nel.org>,
        James Bottomley <James.Bottomley@...senPartnership.com>,
        Theodore Ts'o <tytso@....edu>, linux-hardening@...r.kernel.org,
        Vegard Nossum <vegard.nossum@...cle.com>
Subject: [RFC PATCH 02/11] kbuild: document some prerequisites

When running 'make --dry-run', make will invoke itself recursively for
recipes using $(MAKE), but otherwise not execute the other commands in
a recipe.

However, if a prerequisite is missing and 'make' does not how to create it
(which will be the case when running 'make -n'), it will complain with an
error message like this:

  $ make -n
  ...
  make -f ./scripts/Makefile.modpost
  make[1]: *** No rule to make target 'modules.order', needed by 'modules-only.symvers'.  Stop.
  make: *** [Makefile:1868: modpost] Error 2

In this case, the top-level makefile has reached a recipe that ran 'make'
recursively on scripts/Makefile.modpost, which itself has a rule with
modules.order as a prerequisite. Since the file doesn't exist, and make
doesn't know how to create it, it errors out.

We can document such prerequisites (which are expected to be created by
the parent Makefile) by adding them to the PHONY list of each respective
Makefile.

Signed-off-by: Vegard Nossum <vegard.nossum@...cle.com>
---
 arch/x86/boot/compressed/Makefile | 6 ++++++
 scripts/Makefile.modfinal         | 5 +++++
 scripts/Makefile.modpost          | 4 ++++
 scripts/Makefile.vmlinux          | 7 +++++++
 scripts/Makefile.vmlinux_o        | 3 +++
 5 files changed, 25 insertions(+)

diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
index f2051644de943..ccef6f0e04bc7 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -80,6 +80,9 @@ quiet_cmd_voffset = VOFFSET $@
 
 targets += ../voffset.h
 
+# We don't know how to build this
+PHONY += vmlinux
+
 $(obj)/../voffset.h: vmlinux FORCE
 	$(call if_changed,voffset)
 
@@ -107,6 +110,9 @@ vmlinux-objs-$(CONFIG_EFI) += $(obj)/efi.o
 vmlinux-objs-$(CONFIG_EFI_MIXED) += $(obj)/efi_mixed.o
 vmlinux-libs-$(CONFIG_EFI_STUB) += $(objtree)/drivers/firmware/efi/libstub/lib.a
 
+# We don't know how to build this
+PHONY += $(objtree)/drivers/firmware/efi/libstub/lib.a
+
 $(obj)/vmlinux: $(vmlinux-objs-y) $(vmlinux-libs-y) FORCE
 	$(call if_changed,ld)
 
diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
index 1fa98b5e952b4..696888f0a0bde 100644
--- a/scripts/Makefile.modfinal
+++ b/scripts/Makefile.modfinal
@@ -15,6 +15,11 @@ include $(srctree)/scripts/Makefile.lib
 # find all modules listed in modules.order
 modules := $(call read-file, $(MODORDER))
 
+# We don't know how to build these
+PHONY += $(modules)
+PHONY += $(modules:%.o=%.mod.c)
+PHONY += scripts/module.lds
+
 __modfinal: $(modules:%.o=%.ko)
 	@:
 
diff --git a/scripts/Makefile.modpost b/scripts/Makefile.modpost
index 44936ebad161e..65f2bdc702369 100644
--- a/scripts/Makefile.modpost
+++ b/scripts/Makefile.modpost
@@ -140,6 +140,10 @@ quiet_cmd_modpost = MODPOST $@
 		echo >&2 "         if you want to proceed at your own risk.";) \
 	$(MODPOST) $(modpost-args)
 
+# We don't know how to build these
+PHONY += vmlinux.o
+PHONY += $(MODORDER)
+
 targets += $(output-symdump)
 $(output-symdump): $(modpost-deps) FORCE
 	$(call if_changed,modpost)
diff --git a/scripts/Makefile.vmlinux b/scripts/Makefile.vmlinux
index 49946cb968440..10d80e07f945c 100644
--- a/scripts/Makefile.vmlinux
+++ b/scripts/Makefile.vmlinux
@@ -18,6 +18,9 @@ quiet_cmd_cc_o_c = CC      $@
 	$(call if_changed_dep,cc_o_c)
 
 ifdef CONFIG_MODULES
+# We don't know how to build this
+PHONY += .vmlinux.export.c
+
 targets += .vmlinux.export.o
 vmlinux: .vmlinux.export.o
 endif
@@ -29,6 +32,10 @@ cmd_link_vmlinux =							\
 	$< "$(LD)" "$(KBUILD_LDFLAGS)" "$(LDFLAGS_vmlinux)";		\
 	$(if $(ARCH_POSTLINK), $(MAKE) -f $(ARCH_POSTLINK) $@, true)
 
+# We don't know how to build these
+PHONY += vmlinux.o
+PHONY += $(KBUILD_LDS)
+
 targets += vmlinux
 vmlinux: scripts/link-vmlinux.sh vmlinux.o $(KBUILD_LDS) FORCE
 	+$(call if_changed_dep,link_vmlinux)
diff --git a/scripts/Makefile.vmlinux_o b/scripts/Makefile.vmlinux_o
index 6de297916ce68..7af841eb1ce5b 100644
--- a/scripts/Makefile.vmlinux_o
+++ b/scripts/Makefile.vmlinux_o
@@ -58,6 +58,9 @@ define rule_ld_vmlinux.o
 	$(call cmd,gen_objtooldep)
 endef
 
+# We don't know how to build this
+PHONY += vmlinux.a
+
 vmlinux.o: $(initcalls-lds) vmlinux.a $(KBUILD_VMLINUX_LIBS) FORCE
 	$(call if_changed_rule,ld_vmlinux.o)
 
-- 
2.34.1