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arm64: Add support for relocating the kernel with RELR relocations

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RELR is a relocation packing format for relative relocations.
The format is described in a generic-abi proposal:
https://groups.google.com/d/topic/generic-abi/bX460iggiKg/discussion

The LLD linker can be instructed to pack relocations in the RELR
format by passing the flag --pack-dyn-relocs=relr.

This patch adds a new config option, CONFIG_RELR. Enabling this option
instructs the linker to pack vmlinux's relative relocations in the RELR
format, and causes the kernel to apply the relocations at startup along
with the RELA relocations. RELA relocations still need to be applied
because the linker will emit RELA relative relocations if they are
unrepresentable in the RELR format (i.e. address not a multiple of 2).

Enabling CONFIG_RELR reduces the size of a defconfig kernel image
with CONFIG_RANDOMIZE_BASE by 3.5MB/16% uncompressed, or 550KB/5%
compressed (lz4).

Signed-off-by: Peter Collingbourne <[email protected]>
Tested-by: Nick Desaulniers <[email protected]>
Reviewed-by: Nick Desaulniers <[email protected]>
Signed-off-by: Will Deacon <[email protected]>
This commit is contained in:
Peter Collingbourne 2019-07-31 18:18:42 -07:00 • committed by Will Deacon
commit 5cf896fb6b
7 changed files with 137 additions and 6 deletions
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14
arch/Kconfig
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@ -925,6 +925,20 @@ config LOCK_EVENT_COUNTS
the chance of application behavior change because of timing
differences. The counts are reported via debugfs.
# Select if the architecture has support for applying RELR relocations.
config ARCH_HAS_RELR
bool
config RELR
bool "Use RELR relocation packing"
depends on ARCH_HAS_RELR && TOOLS_SUPPORT_RELR
default y
help
Store the kernel's dynamic relocations in the RELR relocation packing
format. Requires a compatible linker (LLD supports this feature), as
well as compatible NM and OBJCOPY utilities (llvm-nm and llvm-objcopy
are compatible).
source "kernel/gcov/Kconfig"
source "scripts/gcc-plugins/Kconfig"