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authorPeter Collingbourne <pcc@google.com>2019-07-31 18:18:42 -0700
committerWill Deacon <will@kernel.org>2019-08-05 12:35:35 +0100
commit5cf896fb6be3effd9aea455b22213e27be8bdb1d (patch)
tree563e83ed04a2de71b65e831896bf52e0eaf31af0
parent66cbdf5d0c96f5fe570b548e764583ea9d793077 (diff)
arm64: Add support for relocating the kernel with RELR relocations
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> Signed-off-by: Will Deacon <will@kernel.org>
-rw-r--r--Makefile4
-rw-r--r--arch/Kconfig14
-rw-r--r--arch/arm64/Kconfig1
-rw-r--r--arch/arm64/kernel/head.S96
-rw-r--r--arch/arm64/kernel/vmlinux.lds.S9
-rw-r--r--init/Kconfig3
-rwxr-xr-xscripts/tools-support-relr.sh16
7 files changed, 137 insertions, 6 deletions
diff --git a/Makefile b/Makefile
index 23cdf1f41364..9e6ec0c9962c 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 a7b57dd42c26..aa6bdb3df5c1 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -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"
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 3adcec05b1f6..2681eb79c40b 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 2cdacd1c141b..cc23302e9d95 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 7fa008374907..31716afa30f6 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 bd7d650d4a99..d96127ebc44e 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 $(cc-option,-Wmaybe-uninitialized)
help
diff --git a/scripts/tools-support-relr.sh b/scripts/tools-support-relr.sh
new file mode 100755
index 000000000000..97a2c844a95e
--- /dev/null
+++ b/scripts/tools-support-relr.sh
@@ -0,0 +1,16 @@
+#!/bin/sh -eu
+# SPDX-License-Identifier: GPL-2.0
+
+tmp_file=$(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

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