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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
48 lines
1.1 KiB
Makefile
48 lines
1.1 KiB
Makefile
# SPDX-License-Identifier: GPL-2.0
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# ===========================================================================
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# Post-link x86 pass
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# ===========================================================================
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#
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# 1. Separate relocations from vmlinux into vmlinux.relocs.
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# 2. Strip relocations from vmlinux.
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PHONY := __archpost
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__archpost:
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-include include/config/auto.conf
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include $(srctree)/scripts/Kbuild.include
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CMD_RELOCS = arch/x86/tools/relocs
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OUT_RELOCS = arch/x86/boot/compressed
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quiet_cmd_relocs = RELOCS $(OUT_RELOCS)/$@.relocs
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cmd_relocs = \
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mkdir -p $(OUT_RELOCS); \
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$(CMD_RELOCS) $@ > $(OUT_RELOCS)/$@.relocs; \
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$(CMD_RELOCS) --abs-relocs $@
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quiet_cmd_strip_relocs = RSTRIP $@
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cmd_strip_relocs = \
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$(OBJCOPY) --remove-section='.rel.*' --remove-section='.rel__*' \
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--remove-section='.rela.*' --remove-section='.rela__*' $@
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# `@true` prevents complaint when there is nothing to be done
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vmlinux: FORCE
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@true
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ifeq ($(CONFIG_X86_NEED_RELOCS),y)
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$(call cmd,relocs)
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$(call cmd,strip_relocs)
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endif
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%.ko: FORCE
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@true
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clean:
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@rm -f $(OUT_RELOCS)/vmlinux.relocs
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PHONY += FORCE clean
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FORCE:
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.PHONY: $(PHONY)
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