kbuild: Create intermediate vmlinux build with relocations preserved

The imperative paradigm used to build vmlinux, extract some info from it
or perform some checks on it, and subsequently modify it again goes
against the declarative paradigm that is usually employed for defining
make rules.

In particular, the Makefile.postlink files that consume their input via
an output rule result in some dodgy logic in the decompressor makefiles
for RISC-V and x86, given that the vmlinux.relocs input file needed to
generate the arch-specific relocation tables may not exist or be out of
date, but cannot be constructed using the ordinary Make dependency based
rules, because the info needs to be extracted while vmlinux is in its
ephemeral, non-stripped form.

So instead, for architectures that require the static relocations that
are emitted into vmlinux when passing --emit-relocs to the linker, and
are subsequently stripped out again, introduce an intermediate vmlinux
target called vmlinux.unstripped, and organize the reset of the build
logic accordingly:

- vmlinux.unstripped is created only once, and not updated again
- build rules under arch/*/boot can depend on vmlinux.unstripped without
  running the risk of the data disappearing or being out of date
- the final vmlinux generated by the build is not bloated with static
  relocations that are never needed again after the build completes.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

1 files changed, 21 insertions, 7 deletions

diff --git a/scripts/Makefile.vmlinux b/scripts/Makefile.vmlinux
index 487f0bf716ad..b0a6cd5b818c 100644
--- a/scripts/Makefile.vmlinux
+++ b/scripts/Makefile.vmlinux
@@ -9,6 +9,20 @@ include $(srctree)/scripts/Makefile.lib
 
 targets :=
 
+ifdef CONFIG_ARCH_VMLINUX_NEEDS_RELOCS
+vmlinux-final := vmlinux.unstripped
+
+quiet_cmd_strip_relocs = RSTRIP  $@
+      cmd_strip_relocs = $(OBJCOPY) --remove-section='.rel*' $< $@
+
+vmlinux: $(vmlinux-final) FORCE
+	$(call if_changed,strip_relocs)
+
+targets += vmlinux
+else
+vmlinux-final := vmlinux
+endif
+
 %.o: %.c FORCE
 	$(call if_changed_rule,cc_o_c)
 
@@ -47,7 +61,7 @@ targets += .builtin-dtbs-list
 
 ifdef CONFIG_GENERIC_BUILTIN_DTB
 targets += .builtin-dtbs.S .builtin-dtbs.o
-vmlinux: .builtin-dtbs.o
+$(vmlinux-final): .builtin-dtbs.o
 endif
 
 # vmlinux
@@ -55,11 +69,11 @@ endif
 
 ifdef CONFIG_MODULES
 targets += .vmlinux.export.o
-vmlinux: .vmlinux.export.o
+$(vmlinux-final): .vmlinux.export.o
 endif
 
 ifdef CONFIG_ARCH_WANTS_PRE_LINK_VMLINUX
-vmlinux: arch/$(SRCARCH)/tools/vmlinux.arch.o
+$(vmlinux-final): arch/$(SRCARCH)/tools/vmlinux.arch.o
 
 arch/$(SRCARCH)/tools/vmlinux.arch.o: vmlinux.o FORCE
 	$(Q)$(MAKE) $(build)=arch/$(SRCARCH)/tools $@
@@ -72,11 +86,11 @@ cmd_link_vmlinux =							\
 	$< "$(LD)" "$(KBUILD_LDFLAGS)" "$(LDFLAGS_vmlinux)" "$@";	\
 	$(if $(ARCH_POSTLINK), $(MAKE) -f $(ARCH_POSTLINK) $@, true)
 
-targets += vmlinux
-vmlinux: scripts/link-vmlinux.sh vmlinux.o $(KBUILD_LDS) FORCE
+targets += $(vmlinux-final)
+$(vmlinux-final): scripts/link-vmlinux.sh vmlinux.o $(KBUILD_LDS) FORCE
 	+$(call if_changed_dep,link_vmlinux)
 ifdef CONFIG_DEBUG_INFO_BTF
-vmlinux: $(RESOLVE_BTFIDS)
+$(vmlinux-final): $(RESOLVE_BTFIDS)
 endif
 
 ifdef CONFIG_BUILDTIME_TABLE_SORT
@@ -96,7 +110,7 @@ modules.builtin.ranges: $(srctree)/scripts/generate_builtin_ranges.awk \
 			modules.builtin vmlinux.map vmlinux.o.map FORCE
 	$(call if_changed,modules_builtin_ranges)
 
-vmlinux.map: vmlinux
+vmlinux.map: $(vmlinux-final)
 	@:
 
 endif