[PATCH v5] 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 tar-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 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 |

Optimize 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>
---

Changes since v4 [1]:
- Update the example target which is mentioned in the patch description
  to collect vmlinux from binrpm-pkg to tar-pkg, to reflect fc8c2d8ff206
  ("kbuild: Stop including vmlinux.bz2 in the rpm's").

Changes since v3 [2]:
- Update the Kbuild.include path in arch/x86/Makefile.postlink to work
  after 67d7c3023a67 ("kbuild: remove --include-dir MAKEFLAG from top
  Makefile").

Changes since v2 [3]:
- Ignore only the moved vmlinux.relocs, add it to .gitignore and
  Documentation/dontdiff.
- Clean up the patch description.

Changes since v1 [4]:
- Fix the command to remove relocations to work with llvm-objcopy too.

[1] https://lore.kernel.org/lkml/20230227131829.26824-1-petr.pavlu@xxxxxxxx/
[2] https://lore.kernel.org/lkml/20221211141227.7622-1-petr.pavlu@xxxxxxxx/
[3] https://lore.kernel.org/lkml/20220927084632.14531-1-petr.pavlu@xxxxxxxx/
[4] https://lore.kernel.org/lkml/20220913132911.6850-1-petr.pavlu@xxxxxxxx/

 .gitignore                          |  1 +
 Documentation/dontdiff              |  1 +
 arch/x86/Makefile.postlink          | 41 +++++++++++++++++++++++++++++
 arch/x86/boot/compressed/.gitignore |  1 -
 arch/x86/boot/compressed/Makefile   | 10 +++----
 5 files changed, 47 insertions(+), 7 deletions(-)
 create mode 100644 arch/x86/Makefile.postlink

diff --git a/.gitignore b/.gitignore
index 70ec6037fa7a..9bafd3c6bb5f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -65,6 +65,7 @@ modules.order
 /vmlinux
 /vmlinux.32
 /vmlinux.map
+/vmlinux.relocs
 /vmlinux.symvers
 /vmlinux-gdb.py
 /vmlinuz
diff --git a/Documentation/dontdiff b/Documentation/dontdiff
index 3c399f132e2d..a62ad01e6d11 100644
--- a/Documentation/dontdiff
+++ b/Documentation/dontdiff
@@ -254,6 +254,7 @@ vmlinux.aout
 vmlinux.bin.all
 vmlinux.lds
 vmlinux.map
+vmlinux.relocs
 vmlinux.symvers
 vmlinuz
 voffset.h
diff --git a/arch/x86/Makefile.postlink b/arch/x86/Makefile.postlink
new file mode 100644
index 000000000000..195af937aa4d
--- /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 $(srctree)/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/.gitignore b/arch/x86/boot/compressed/.gitignore
index 25805199a506..b2968175fc27 100644
--- a/arch/x86/boot/compressed/.gitignore
+++ b/arch/x86/boot/compressed/.gitignore
@@ -1,7 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0-only
 relocs
 vmlinux.bin.all
-vmlinux.relocs
 vmlinux.lds
 mkpiggy
 piggy.S
diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
index 6b6cfe607bdb..19d1fb601796 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -121,14 +121,12 @@ $(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