[PATCH v2] x86: Avoid relocation information in final vmlinux

[Petr Pavlu]

The Linux build process on x86 roughly consists of compiling all input
files, statically linking them into a vmlinux ELF file, and then taking
and turning this file into an actual bzImage bootable file.

vmlinux has in this process two main purposes:
1) It is an intermediate build target on the way to produce the final
   bootable image.
2) It is a file that is expected to be used by debuggers and standard
   ELF tooling to work with the built kernel.

For the second purpose, a vmlinux file is typically collected by various
package build recipes, such as distribution spec files, including the
kernel's own binrpm-pkg target.

When building a kernel supporting KASLR with CONFIG_X86_NEED_RELOCS,
vmlinux contains also relocation information produced by using the
--emit-relocs linker option. This is utilized by subsequent build steps
to create vmlinux.relocs and produce a relocatable image. However, the
information is not needed by debuggers and other standard ELF tooling.

The issue is then that the collected vmlinux file and hence distribution
packages end up unnecessarily large because of this extra data. The
following is a size comparison of vmlinux v6.0-rc5 with and without the
relocation information:
| Configuration      | With relocs | Stripped relocs |
| x86_64_defconfig   |       70 MB |           43 MB |
| +CONFIG_DEBUG_INFO |      818 MB |          367 MB |

The patch optimizes a resulting vmlinux by adding a postlink step that
splits the relocation information into vmlinux.relocs and then strips it
from the vmlinux binary.

Signed-off-by: Petr Pavlu <petr.pavlu@xxxxxxxx>
---

Notes:
    Changes since v1 [1]:
    - Fix the command to remove relocations to work with llvm-objcopy too.
    
    [1] https://lore.kernel.org/lkml/20220913132911.6850-1-petr.pavlu@xxxxxxxx/

 .gitignore                        |  1 +
 arch/x86/Makefile.postlink        | 41 +++++++++++++++++++++++++++++++
 arch/x86/boot/compressed/Makefile | 10 +++-----
 3 files changed, 46 insertions(+), 6 deletions(-)
 create mode 100644 arch/x86/Makefile.postlink

diff --git a/.gitignore b/.gitignore
index 265959544978..cd4ef88584ea 100644
--- a/.gitignore
+++ b/.gitignore
@@ -37,6 +37,7 @@
 *.o
 *.o.*
 *.patch
+*.relocs
 *.s
 *.so
 *.so.dbg
diff --git a/arch/x86/Makefile.postlink b/arch/x86/Makefile.postlink
new file mode 100644
index 000000000000..b38ffa4defb3
--- /dev/null
+++ b/arch/x86/Makefile.postlink
@@ -0,0 +1,41 @@
+# SPDX-License-Identifier: GPL-2.0
+# ===========================================================================
+# Post-link x86 pass
+# ===========================================================================
+#
+# 1. Separate relocations from vmlinux into vmlinux.relocs.
+# 2. Strip relocations from vmlinux.
+
+PHONY := __archpost
+__archpost:
+
+-include include/config/auto.conf
+include scripts/Kbuild.include
+
+CMD_RELOCS = arch/x86/tools/relocs
+quiet_cmd_relocs = RELOCS  $@.relocs
+      cmd_relocs = $(CMD_RELOCS) $@ > $@.relocs;$(CMD_RELOCS) --abs-relocs $@
+
+quiet_cmd_strip_relocs = RSTRIP  $@
+      cmd_strip_relocs = $(OBJCOPY) --remove-section='.rel.*' --remove-section='.rel__*' --remove-section='.rela.*' --remove-section='.rela__*' $@
+
+# `@true` prevents complaint when there is nothing to be done
+
+vmlinux: FORCE
+	@true
+ifeq ($(CONFIG_X86_NEED_RELOCS),y)
+	$(call cmd,relocs)
+	$(call cmd,strip_relocs)
+endif
+
+%.ko: FORCE
+	@true
+
+clean:
+	@rm -f vmlinux.relocs
+
+PHONY += FORCE clean
+
+FORCE:
+
+.PHONY: $(PHONY)
diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
index 35ce1a64068b..eba7709d75ae 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -120,14 +120,12 @@ $(obj)/vmlinux.bin: vmlinux FORCE
 
 targets += $(patsubst $(obj)/%,%,$(vmlinux-objs-y)) vmlinux.bin.all vmlinux.relocs
 
-CMD_RELOCS = arch/x86/tools/relocs
-quiet_cmd_relocs = RELOCS  $@
-      cmd_relocs = $(CMD_RELOCS) $< > $@;$(CMD_RELOCS) --abs-relocs $<
-$(obj)/vmlinux.relocs: vmlinux FORCE
-	$(call if_changed,relocs)
+# vmlinux.relocs is created by the vmlinux postlink step.
+vmlinux.relocs: vmlinux
+	@true
 
 vmlinux.bin.all-y := $(obj)/vmlinux.bin
-vmlinux.bin.all-$(CONFIG_X86_NEED_RELOCS) += $(obj)/vmlinux.relocs
+vmlinux.bin.all-$(CONFIG_X86_NEED_RELOCS) += vmlinux.relocs
 
 $(obj)/vmlinux.bin.gz: $(vmlinux.bin.all-y) FORCE
 	$(call if_changed,gzip)
-- 
2.35.3