If the toolchain supports RELR relocation packing, build the virtually relocatable kernels as Position Independent (PIE) Executables. This results in more efficient relocation processing for the virtual displacement of the kernel applied at boot, using RELR relocations that take up only a fraction of the space occupied by ordinary RELA relocations.
More importantly, it instructs the linker to generate a binary that is really meant to be relocated at boot, using data structures that are intended for this purpose. Doing so is important for a couple of reasons: - Relying on --emit-relocs is problematic, because it produces the static relocations that are consumed by the linker as input, and these are not meant for describing a runtime relocatable image. For example, the linker may apply relaxations that result in the code and the static relocation going out of sync (and ld.bfd and ld.lld already handle this in a different way). - The 'relocs' tool relies on manually kept allow/deny lists of symbol names. These are needed because ELF absolute/relative symbol designations are often inaccurate. - x86 deviates from other architectures in the kernel when it comes to its implementation of boot-time relocation, making it difficult to implement further enhancements (e.g., fgkaslr, EFI zboot) in a portable manner. Note that this means that all codegen on x86_64 should be position independent, to be compatible with PIE linking, but only if KASLR is enabled. On i386, no changes to the codegen are needed, as the ordinary position dependent relocation model is supported by the linker when operating in PIE mode. Signed-off-by: Ard Biesheuvel <[email protected]> --- arch/x86/Kconfig | 3 ++- arch/x86/Makefile | 5 +++++ arch/x86/kernel/vmlinux.lds.S | 18 ++++++++++++++++++ 3 files changed, 25 insertions(+), 1 deletion(-) diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig index b3a64cfe04cf..2aa50aa8dc68 100644 --- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -103,6 +103,7 @@ config X86 select ARCH_HAS_NONLEAF_PMD_YOUNG if PGTABLE_LEVELS > 2 select ARCH_HAS_UACCESS_FLUSHCACHE if X86_64 select ARCH_HAS_COPY_MC if X86_64 + select ARCH_HAS_RELR select ARCH_HAS_SET_MEMORY select ARCH_HAS_SET_DIRECT_MAP select ARCH_HAS_STRICT_KERNEL_RWX @@ -2129,7 +2130,7 @@ config RANDOMIZE_BASE # Relocation on x86 needs some additional build support config X86_NEED_RELOCS def_bool y - depends on RELOCATABLE + depends on RELOCATABLE && !TOOLS_SUPPORT_RELR select ARCH_VMLINUX_NEEDS_RELOCS config PHYSICAL_ALIGN diff --git a/arch/x86/Makefile b/arch/x86/Makefile index b211d6c950aa..7eac705c4ff4 100644 --- a/arch/x86/Makefile +++ b/arch/x86/Makefile @@ -258,6 +258,11 @@ endif KBUILD_LDFLAGS += -m elf_$(UTS_MACHINE) +ldflags-pie-$(CONFIG_LD_IS_LLD) := --apply-dynamic-relocs +ldflags-pie-$(CONFIG_LD_IS_BFD) := -z call-nop=suffix-nop +ldflags-$(CONFIG_RELOCATABLE_PIE) := --pie -z notext $(ldflags-pie-y) +LDFLAGS_vmlinux += $(ldflags-y) + # # The 64-bit kernel must be aligned to 2MB. Pass -z max-page-size=0x200000 to # the linker to force 2MB page size regardless of the default page size used diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S index 6772fe9a9957..cfaf6ab80684 100644 --- a/arch/x86/kernel/vmlinux.lds.S +++ b/arch/x86/kernel/vmlinux.lds.S @@ -127,6 +127,9 @@ PHDRS { text PT_LOAD FLAGS(5); /* R_E */ data PT_LOAD FLAGS(6); /* RW_ */ note PT_NOTE FLAGS(0); /* ___ */ +#ifdef CONFIG_RELOCATABLE_PIE + dynamic PT_DYNAMIC; +#endif } SECTIONS @@ -201,6 +204,21 @@ SECTIONS DATA_SEGMENT_START INIT_DATA_SECTION(16) :data +#ifdef CONFIG_RELOCATABLE_PIE + /DISCARD/ : { + *(.interp .dynbss .eh_frame .sframe .relr.auth.dyn) + } + + .dynamic : { *(.dynamic) } :dynamic :data + .dynstr : { *(.dynstr) } :data + .dynsym : { *(.dynsym) } + .gnu.hash : { *(.gnu.hash) } + .hash : { *(.hash) } + .init.rela : { *(.rela.*) *(.rela_*) } + .init.rel : { *(.rel.*) *(.rel_*) } + .init.relr : { *(.relr.*) } +#endif + .x86_cpu_dev.init : AT(ADDR(.x86_cpu_dev.init) - LOAD_OFFSET) { __x86_cpu_dev_start = .; *(.x86_cpu_dev.init) -- 2.47.3
