| Message ID | 20190801011842.199786-1-pcc@google.com (mailing list archive) |
|---|---|
| State | Mainlined |
| Commit | 5cf896fb6be3effd9aea455b22213e27be8bdb1d |
| Headers | show |
| Series | [v3] arm64: Add support for relocating the kernel with RELR relocations | expand |
On Wed, Jul 31, 2019 at 06:18:42PM -0700, Peter Collingbourne wrote: > 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 <pcc@google.com> > Tested-by: Nick Desaulniers <ndesaulniers@google.com> > Reviewed-by: Nick Desaulniers <ndesaulniers@google.com> > --- > > Notes: > Changes in v3: > - Move Kconfig/Makefile logic to arch-independent location > - Tweak CONFIG_RELR documentation to remove "currently" Excellent, thanks. Queued for 5.4. One more question: is there any benefit to supporting this for loadable modules as well? Will
On Thu, Aug 1, 2019 at 5:05 AM Will Deacon <will@kernel.org> wrote: > > On Wed, Jul 31, 2019 at 06:18:42PM -0700, Peter Collingbourne wrote: > > 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 <pcc@google.com> > > Tested-by: Nick Desaulniers <ndesaulniers@google.com> > > Reviewed-by: Nick Desaulniers <ndesaulniers@google.com> > > --- > > > > Notes: > > Changes in v3: > > - Move Kconfig/Makefile logic to arch-independent location > > - Tweak CONFIG_RELR documentation to remove "currently" > > Excellent, thanks. Queued for 5.4. Thanks. > One more question: is there any benefit to supporting this for loadable > modules as well? It looks like there would be, but it would be much smaller than the kernel itself, as well as being smaller than the benefit of building modules as shared objects instead of object files, which as I mentioned to Nick is a prerequisite for the linker to be able to emit RELR relocations. The kernel appears to proportionally contain many more RELATIVE relocations than the modules, probably because of __ksymtab which is almost entirely RELATIVE relocations. Shared objects should help significantly with binary size because the linker would then be able to statically resolve relocations between sections, avoiding the need to store them explicitly in the module, but they have the downside of requiring PIC which could hurt performance. As a quick experiment I did: diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile index d3776c945a173..015898faba590 100644 --- a/arch/arm64/Makefile +++ b/arch/arm64/Makefile @@ -65,7 +65,7 @@ endif KBUILD_CFLAGS += -mgeneral-regs-only $(lseinstr) $(brokengasinst) $(compat_vdso) KBUILD_CFLAGS += -fno-asynchronous-unwind-tables -KBUILD_CFLAGS += $(call cc-disable-warning, psabi) +KBUILD_CFLAGS += $(call cc-disable-warning, psabi) -fPIC KBUILD_AFLAGS += $(lseinstr) $(brokengasinst) $(compat_vdso) KBUILD_CFLAGS += $(call cc-option,-mabi=lp64) and built a defconfig kernel. I then did: $ for i in **/*.ko ; do ld.lld -shared -o ${i}.so $i -Bsymbolic -N 2>/dev/null ; done $ for i in **/*.ko ; do ld.lld -shared -o ${i}.relr.so $i -Bsymbolic -N --pack-dyn-relocs=relr 2>/dev/null ; done That gave me 494 out of 525 modules that can currently be linked as shared objects, and 491 that can be linked with RELR (looks like there's an LLD bug preventing linking the other 3). The binary sizes are: $ cat `ls **/*.ko.relr.so | sed -e 's/\.ko\.relr\.so$/.ko/g'` | wc -c 279730136 $ cat `ls **/*.ko.relr.so | sed -e 's/\.ko\.relr\.so$/.ko.so/g'` | wc -c 134944384 $ cat **/*.ko.relr.so | wc -c 134617216 So that's roughly >50% size improvement from shared objects and 0.3% from RELR. Peter
diff --git a/Makefile b/Makefile index 5ee6f68898693..23ed9dbef12a1 100644 --- a/Makefile +++ b/Makefile @@ -912,6 +912,10 @@ ifeq ($(CONFIG_STRIP_ASM_SYMS),y) LDFLAGS_vmlinux += $(call ld-option, -X,) endif +ifeq ($(CONFIG_RELR),y) +LDFLAGS_vmlinux += --pack-dyn-relocs=relr +endif + # insure the checker run with the right endianness CHECKFLAGS += $(if $(CONFIG_CPU_BIG_ENDIAN),-mbig-endian,-mlittle-endian) diff --git a/arch/Kconfig b/arch/Kconfig index d4c1f0551dfe0..719b27275f86a 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -936,6 +936,20 @@ 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" diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig index 399f595ef852e..ac1bc9cc22a26 100644 --- a/arch/arm64/Kconfig +++ b/arch/arm64/Kconfig @@ -1467,6 +1467,7 @@ endif config RELOCATABLE bool + select ARCH_HAS_RELR help This builds the kernel as a Position Independent Executable (PIE), which retains all relocation metadata required to relocate the diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S index 2cdacd1c141b9..cc23302e9d95e 100644 --- a/arch/arm64/kernel/head.S +++ b/arch/arm64/kernel/head.S @@ -102,6 +102,8 @@ pe_header: * x23 stext() .. start_kernel() physical misalignment/KASLR offset * x28 __create_page_tables() callee preserved temp register * x19/x20 __primary_switch() callee preserved temp registers + * x24 __primary_switch() .. relocate_kernel() + * current RELR displacement */ ENTRY(stext) bl preserve_boot_args @@ -834,14 +836,93 @@ __relocate_kernel: 0: cmp x9, x10 b.hs 1f - ldp x11, x12, [x9], #24 - ldr x13, [x9, #-8] - cmp w12, #R_AARCH64_RELATIVE + ldp x12, x13, [x9], #24 + ldr x14, [x9, #-8] + cmp w13, #R_AARCH64_RELATIVE b.ne 0b - add x13, x13, x23 // relocate - str x13, [x11, x23] + add x14, x14, x23 // relocate + str x14, [x12, x23] b 0b -1: ret + +1: +#ifdef CONFIG_RELR + /* + * Apply RELR relocations. + * + * RELR is a compressed format for storing relative relocations. The + * encoded sequence of entries looks like: + * [ AAAAAAAA BBBBBBB1 BBBBBBB1 ... AAAAAAAA BBBBBB1 ... ] + * + * i.e. start with an address, followed by any number of bitmaps. The + * address entry encodes 1 relocation. The subsequent bitmap entries + * encode up to 63 relocations each, at subsequent offsets following + * the last address entry. + * + * The bitmap entries must have 1 in the least significant bit. The + * assumption here is that an address cannot have 1 in lsb. Odd + * addresses are not supported. Any odd addresses are stored in the RELA + * section, which is handled above. + * + * Excluding the least significant bit in the bitmap, each non-zero + * bit in the bitmap represents a relocation to be applied to + * a corresponding machine word that follows the base address + * word. The second least significant bit represents the machine + * word immediately following the initial address, and each bit + * that follows represents the next word, in linear order. As such, + * a single bitmap can encode up to 63 relocations in a 64-bit object. + * + * In this implementation we store the address of the next RELR table + * entry in x9, the address being relocated by the current address or + * bitmap entry in x13 and the address being relocated by the current + * bit in x14. + * + * Because addends are stored in place in the binary, RELR relocations + * cannot be applied idempotently. We use x24 to keep track of the + * currently applied displacement so that we can correctly relocate if + * __relocate_kernel is called twice with non-zero displacements (i.e. + * if there is both a physical misalignment and a KASLR displacement). + */ + ldr w9, =__relr_offset // offset to reloc table + ldr w10, =__relr_size // size of reloc table + add x9, x9, x11 // __va(.relr) + add x10, x9, x10 // __va(.relr) + sizeof(.relr) + + sub x15, x23, x24 // delta from previous offset + cbz x15, 7f // nothing to do if unchanged + mov x24, x23 // save new offset + +2: cmp x9, x10 + b.hs 7f + ldr x11, [x9], #8 + tbnz x11, #0, 3f // branch to handle bitmaps + add x13, x11, x23 + ldr x12, [x13] // relocate address entry + add x12, x12, x15 + str x12, [x13], #8 // adjust to start of bitmap + b 2b + +3: mov x14, x13 +4: lsr x11, x11, #1 + cbz x11, 6f + tbz x11, #0, 5f // skip bit if not set + ldr x12, [x14] // relocate bit + add x12, x12, x15 + str x12, [x14] + +5: add x14, x14, #8 // move to next bit's address + b 4b + +6: /* + * Move to the next bitmap's address. 8 is the word size, and 63 is the + * number of significant bits in a bitmap entry. + */ + add x13, x13, #(8 * 63) + b 2b + +7: +#endif + ret + ENDPROC(__relocate_kernel) #endif @@ -854,6 +935,9 @@ __primary_switch: adrp x1, init_pg_dir bl __enable_mmu #ifdef CONFIG_RELOCATABLE +#ifdef CONFIG_RELR + mov x24, #0 // no RELR displacement yet +#endif bl __relocate_kernel #ifdef CONFIG_RANDOMIZE_BASE ldr x8, =__primary_switched diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S index 7fa0083749078..31716afa30f65 100644 --- a/arch/arm64/kernel/vmlinux.lds.S +++ b/arch/arm64/kernel/vmlinux.lds.S @@ -200,6 +200,15 @@ SECTIONS __rela_offset = ABSOLUTE(ADDR(.rela.dyn) - KIMAGE_VADDR); __rela_size = SIZEOF(.rela.dyn); +#ifdef CONFIG_RELR + .relr.dyn : ALIGN(8) { + *(.relr.dyn) + } + + __relr_offset = ABSOLUTE(ADDR(.relr.dyn) - KIMAGE_VADDR); + __relr_size = SIZEOF(.relr.dyn); +#endif + . = ALIGN(SEGMENT_ALIGN); __initdata_end = .; __init_end = .; diff --git a/init/Kconfig b/init/Kconfig index bd7d650d4a996..d96127ebc44e0 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -30,6 +30,9 @@ config CC_CAN_LINK config CC_HAS_ASM_GOTO def_bool $(success,$(srctree)/scripts/gcc-goto.sh $(CC)) +config TOOLS_SUPPORT_RELR + def_bool $(success,env "CC=$(CC)" "LD=$(LD)" "NM=$(NM)" "OBJCOPY=$(OBJCOPY)" $(srctree)/scripts/tools-support-relr.sh) + config CC_HAS_WARN_MAYBE_UNINITIALIZED def_bool $(cc-option,-Wmaybe-uninitialized) help diff --git a/scripts/tools-support-relr.sh b/scripts/tools-support-relr.sh new file mode 100755 index 0000000000000..97a2c844a95e4 --- /dev/null +++ b/scripts/tools-support-relr.sh @@ -0,0 +1,16 @@ +#!/bin/sh -eu +# SPDX-License-Identifier: GPL-2.0 + +tmp_file=$(mktemp) +trap "rm -f $tmp_file.o $tmp_file $tmp_file.bin" EXIT + +cat << "END" | "$CC" -c -x c - -o $tmp_file.o >/dev/null 2>&1 +void *p = &p; +END +"$LD" $tmp_file.o -shared -Bsymbolic --pack-dyn-relocs=relr -o $tmp_file + +# Despite printing an error message, GNU nm still exits with exit code 0 if it +# sees a relr section. So we need to check that nothing is printed to stderr. +test -z "$("$NM" $tmp_file 2>&1 >/dev/null)" + +"$OBJCOPY" -O binary $tmp_file $tmp_file.bin