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x86/build: Avoid relocation information in final vmlinux

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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@suse.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lore.kernel.org/r/20220927084632.14531-1-petr.pavlu@suse.com
This commit is contained in:
Petr Pavlu 2023-03-27 11:54:06 +02:00 committed by Borislav Petkov (AMD)
parent 858fd168a9
commit 9d9173e9ce
2 changed files with 50 additions and 5 deletions
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47
arch/x86/Makefile.postlink Normal file
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@ -0,0 +1,47 @@
# 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
OUT_RELOCS = arch/x86/boot/compressed
quiet_cmd_relocs = RELOCS $(OUT_RELOCS)/$@.relocs
cmd_relocs = \
mkdir -p $(OUT_RELOCS); \
$(CMD_RELOCS) $@ > $(OUT_RELOCS)/$@.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 $(OUT_RELOCS)/vmlinux.relocs
PHONY += FORCE clean
FORCE:
.PHONY: $(PHONY)

8
arch/x86/boot/compressed/Makefile
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@ -121,11 +121,9 @@ $(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.
$(obj)/vmlinux.relocs: vmlinux
@true
vmlinux.bin.all-y := $(obj)/vmlinux.bin
vmlinux.bin.all-$(CONFIG_X86_NEED_RELOCS) += $(obj)/vmlinux.relocs