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llvm-project/lld/ELF/Arch/AArch64.cpp

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//===- AArch64.cpp --------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#include "Symbols.h"
#include "SyntheticSections.h"
#include "Target.h"
#include "Thunks.h"
#include "lld/Common/ErrorHandler.h"
#include "llvm/Object/ELF.h"
#include "llvm/Support/Endian.h"
using namespace llvm;
using namespace llvm::support::endian;
using namespace llvm::ELF;
using namespace lld;
using namespace lld::elf;
// Page(Expr) is the page address of the expression Expr, defined
// as (Expr & ~0xFFF). (This applies even if the machine page size
// supported by the platform has a different value.)
uint64_t elf::getAArch64Page(uint64_t expr) {
return expr & ~static_cast<uint64_t>(0xFFF);
}
namespace {
class AArch64 : public TargetInfo {
public:
AArch64();
RelExpr getRelExpr(RelType type, const Symbol &s,
const uint8_t *loc) const override;
RelType getDynRel(RelType type) const override;
void writeGotPlt(uint8_t *buf, const Symbol &s) const override;
void writePltHeader(uint8_t *buf) const override;
void writePlt(uint8_t *buf, uint64_t gotPltEntryAddr, uint64_t pltEntryAddr,
int32_t index, unsigned relOff) const override;
bool needsThunk(RelExpr expr, RelType type, const InputFile *file,
uint64_t branchAddr, const Symbol &s) const override;
uint32_t getThunkSectionSpacing() const override;
bool inBranchRange(RelType type, uint64_t src, uint64_t dst) const override;
bool usesOnlyLowPageBits(RelType type) const override;
void relocateOne(uint8_t *loc, RelType type, uint64_t val) const override;
RelExpr adjustRelaxExpr(RelType type, const uint8_t *data,
RelExpr expr) const override;
void relaxTlsGdToLe(uint8_t *loc, RelType type, uint64_t val) const override;
void relaxTlsGdToIe(uint8_t *loc, RelType type, uint64_t val) const override;
void relaxTlsIeToLe(uint8_t *loc, RelType type, uint64_t val) const override;
};
} // namespace
AArch64::AArch64() {
copyRel = R_AARCH64_COPY;
relativeRel = R_AARCH64_RELATIVE;
iRelativeRel = R_AARCH64_IRELATIVE;
gotRel = R_AARCH64_GLOB_DAT;
noneRel = R_AARCH64_NONE;
pltRel = R_AARCH64_JUMP_SLOT;
symbolicRel = R_AARCH64_ABS64;
tlsDescRel = R_AARCH64_TLSDESC;
tlsGotRel = R_AARCH64_TLS_TPREL64;
pltEntrySize = 16;
pltHeaderSize = 32;
defaultMaxPageSize = 65536;
// Align to the 2 MiB page size (known as a superpage or huge page).
// FreeBSD automatically promotes 2 MiB-aligned allocations.
defaultImageBase = 0x200000;
needsThunks = true;
}
RelExpr AArch64::getRelExpr(RelType type, const Symbol &s,
const uint8_t *loc) const {
switch (type) {
case R_AARCH64_ABS16:
case R_AARCH64_ABS32:
case R_AARCH64_ABS64:
case R_AARCH64_ADD_ABS_LO12_NC:
case R_AARCH64_LDST128_ABS_LO12_NC:
case R_AARCH64_LDST16_ABS_LO12_NC:
case R_AARCH64_LDST32_ABS_LO12_NC:
case R_AARCH64_LDST64_ABS_LO12_NC:
case R_AARCH64_LDST8_ABS_LO12_NC:
case R_AARCH64_MOVW_SABS_G0:
case R_AARCH64_MOVW_SABS_G1:
case R_AARCH64_MOVW_SABS_G2:
case R_AARCH64_MOVW_UABS_G0:
case R_AARCH64_MOVW_UABS_G0_NC:
case R_AARCH64_MOVW_UABS_G1:
case R_AARCH64_MOVW_UABS_G1_NC:
case R_AARCH64_MOVW_UABS_G2:
case R_AARCH64_MOVW_UABS_G2_NC:
case R_AARCH64_MOVW_UABS_G3:
return R_ABS;
case R_AARCH64_TLSDESC_ADR_PAGE21:
return R_AARCH64_TLSDESC_PAGE;
case R_AARCH64_TLSDESC_LD64_LO12:
case R_AARCH64_TLSDESC_ADD_LO12:
return R_TLSDESC;
case R_AARCH64_TLSDESC_CALL:
return R_TLSDESC_CALL;
case R_AARCH64_TLSLE_ADD_TPREL_HI12:
case R_AARCH64_TLSLE_ADD_TPREL_LO12_NC:
case R_AARCH64_TLSLE_LDST8_TPREL_LO12_NC:
case R_AARCH64_TLSLE_LDST16_TPREL_LO12_NC:
case R_AARCH64_TLSLE_LDST32_TPREL_LO12_NC:
case R_AARCH64_TLSLE_LDST64_TPREL_LO12_NC:
case R_AARCH64_TLSLE_LDST128_TPREL_LO12_NC:
case R_AARCH64_TLSLE_MOVW_TPREL_G0:
case R_AARCH64_TLSLE_MOVW_TPREL_G0_NC:
case R_AARCH64_TLSLE_MOVW_TPREL_G1:
case R_AARCH64_TLSLE_MOVW_TPREL_G1_NC:
case R_AARCH64_TLSLE_MOVW_TPREL_G2:
return R_TLS;
case R_AARCH64_CALL26:
case R_AARCH64_CONDBR19:
case R_AARCH64_JUMP26:
case R_AARCH64_TSTBR14:
return R_PLT_PC;
case R_AARCH64_PREL16:
case R_AARCH64_PREL32:
case R_AARCH64_PREL64:
case R_AARCH64_ADR_PREL_LO21:
case R_AARCH64_LD_PREL_LO19:
case R_AARCH64_MOVW_PREL_G0:
case R_AARCH64_MOVW_PREL_G0_NC:
case R_AARCH64_MOVW_PREL_G1:
case R_AARCH64_MOVW_PREL_G1_NC:
case R_AARCH64_MOVW_PREL_G2:
case R_AARCH64_MOVW_PREL_G2_NC:
case R_AARCH64_MOVW_PREL_G3:
return R_PC;
case R_AARCH64_ADR_PREL_PG_HI21:
case R_AARCH64_ADR_PREL_PG_HI21_NC:
return R_AARCH64_PAGE_PC;
case R_AARCH64_LD64_GOT_LO12_NC:
case R_AARCH64_TLSIE_LD64_GOTTPREL_LO12_NC:
return R_GOT;
case R_AARCH64_ADR_GOT_PAGE:
case R_AARCH64_TLSIE_ADR_GOTTPREL_PAGE21:
return R_AARCH64_GOT_PAGE_PC;
case R_AARCH64_NONE:
return R_NONE;
default:
error(getErrorLocation(loc) + "unknown relocation (" + Twine(type) +
") against symbol " + toString(s));
return R_NONE;
}
}
RelExpr AArch64::adjustRelaxExpr(RelType type, const uint8_t *data,
RelExpr expr) const {
if (expr == R_RELAX_TLS_GD_TO_IE) {
if (type == R_AARCH64_TLSDESC_ADR_PAGE21)
return R_AARCH64_RELAX_TLS_GD_TO_IE_PAGE_PC;
return R_RELAX_TLS_GD_TO_IE_ABS;
}
return expr;
}
bool AArch64::usesOnlyLowPageBits(RelType type) const {
switch (type) {
default:
return false;
case R_AARCH64_ADD_ABS_LO12_NC:
case R_AARCH64_LD64_GOT_LO12_NC:
case R_AARCH64_LDST128_ABS_LO12_NC:
case R_AARCH64_LDST16_ABS_LO12_NC:
case R_AARCH64_LDST32_ABS_LO12_NC:
case R_AARCH64_LDST64_ABS_LO12_NC:
case R_AARCH64_LDST8_ABS_LO12_NC:
case R_AARCH64_TLSDESC_ADD_LO12:
case R_AARCH64_TLSDESC_LD64_LO12:
case R_AARCH64_TLSIE_LD64_GOTTPREL_LO12_NC:
return true;
}
}
RelType AArch64::getDynRel(RelType type) const {
if (type == R_AARCH64_ABS64)
return type;
return R_AARCH64_NONE;
}
void AArch64::writeGotPlt(uint8_t *buf, const Symbol &) const {
write64le(buf, in.plt->getVA());
}
void AArch64::writePltHeader(uint8_t *buf) const {
const uint8_t pltData[] = {
0xf0, 0x7b, 0xbf, 0xa9, // stp x16, x30, [sp,#-16]!
0x10, 0x00, 0x00, 0x90, // adrp x16, Page(&(.plt.got[2]))
0x11, 0x02, 0x40, 0xf9, // ldr x17, [x16, Offset(&(.plt.got[2]))]
0x10, 0x02, 0x00, 0x91, // add x16, x16, Offset(&(.plt.got[2]))
0x20, 0x02, 0x1f, 0xd6, // br x17
0x1f, 0x20, 0x03, 0xd5, // nop
0x1f, 0x20, 0x03, 0xd5, // nop
0x1f, 0x20, 0x03, 0xd5 // nop
};
memcpy(buf, pltData, sizeof(pltData));
uint64_t got = in.gotPlt->getVA();
uint64_t plt = in.plt->getVA();
relocateOne(buf + 4, R_AARCH64_ADR_PREL_PG_HI21,
getAArch64Page(got + 16) - getAArch64Page(plt + 4));
relocateOne(buf + 8, R_AARCH64_LDST64_ABS_LO12_NC, got + 16);
relocateOne(buf + 12, R_AARCH64_ADD_ABS_LO12_NC, got + 16);
}
void AArch64::writePlt(uint8_t *buf, uint64_t gotPltEntryAddr,
uint64_t pltEntryAddr, int32_t index,
unsigned relOff) const {
const uint8_t inst[] = {
0x10, 0x00, 0x00, 0x90, // adrp x16, Page(&(.plt.got[n]))
0x11, 0x02, 0x40, 0xf9, // ldr x17, [x16, Offset(&(.plt.got[n]))]
0x10, 0x02, 0x00, 0x91, // add x16, x16, Offset(&(.plt.got[n]))
0x20, 0x02, 0x1f, 0xd6 // br x17
};
memcpy(buf, inst, sizeof(inst));
relocateOne(buf, R_AARCH64_ADR_PREL_PG_HI21,
getAArch64Page(gotPltEntryAddr) - getAArch64Page(pltEntryAddr));
relocateOne(buf + 4, R_AARCH64_LDST64_ABS_LO12_NC, gotPltEntryAddr);
relocateOne(buf + 8, R_AARCH64_ADD_ABS_LO12_NC, gotPltEntryAddr);
}
bool AArch64::needsThunk(RelExpr expr, RelType type, const InputFile *file,
uint64_t branchAddr, const Symbol &s) const {
// ELF for the ARM 64-bit architecture, section Call and Jump relocations
// only permits range extension thunks for R_AARCH64_CALL26 and
// R_AARCH64_JUMP26 relocation types.
if (type != R_AARCH64_CALL26 && type != R_AARCH64_JUMP26)
return false;
uint64_t dst = (expr == R_PLT_PC) ? s.getPltVA() : s.getVA();
return !inBranchRange(type, branchAddr, dst);
}
uint32_t AArch64::getThunkSectionSpacing() const {
// See comment in Arch/ARM.cpp for a more detailed explanation of
// getThunkSectionSpacing(). For AArch64 the only branches we are permitted to
// Thunk have a range of +/- 128 MiB
return (128 * 1024 * 1024) - 0x30000;
}
bool AArch64::inBranchRange(RelType type, uint64_t src, uint64_t dst) const {
if (type != R_AARCH64_CALL26 && type != R_AARCH64_JUMP26)
return true;
// The AArch64 call and unconditional branch instructions have a range of
// +/- 128 MiB.
uint64_t range = 128 * 1024 * 1024;
if (dst > src) {
// Immediate of branch is signed.
range -= 4;
return dst - src <= range;
}
return src - dst <= range;
}
static void write32AArch64Addr(uint8_t *l, uint64_t imm) {
uint32_t immLo = (imm & 0x3) << 29;
uint32_t immHi = (imm & 0x1FFFFC) << 3;
uint64_t mask = (0x3 << 29) | (0x1FFFFC << 3);
write32le(l, (read32le(l) & ~mask) | immLo | immHi);
}
// Return the bits [Start, End] from Val shifted Start bits.
// For instance, getBits(0xF0, 4, 8) returns 0xF.
static uint64_t getBits(uint64_t val, int start, int end) {
uint64_t mask = ((uint64_t)1 << (end + 1 - start)) - 1;
return (val >> start) & mask;
}
static void or32le(uint8_t *p, int32_t v) { write32le(p, read32le(p) | v); }
// Update the immediate field in a AARCH64 ldr, str, and add instruction.
static void or32AArch64Imm(uint8_t *l, uint64_t imm) {
or32le(l, (imm & 0xFFF) << 10);
}
// Update the immediate field in an AArch64 movk, movn or movz instruction
// for a signed relocation, and update the opcode of a movn or movz instruction
// to match the sign of the operand.
static void writeSMovWImm(uint8_t *loc, uint32_t imm) {
uint32_t inst = read32le(loc);
// Opcode field is bits 30, 29, with 10 = movz, 00 = movn and 11 = movk.
if (!(inst & (1 << 29))) {
// movn or movz.
if (imm & 0x10000) {
// Change opcode to movn, which takes an inverted operand.
imm ^= 0xFFFF;
inst &= ~(1 << 30);
} else {
// Change opcode to movz.
inst |= 1 << 30;
}
}
write32le(loc, inst | ((imm & 0xFFFF) << 5));
}
void AArch64::relocateOne(uint8_t *loc, RelType type, uint64_t val) const {
switch (type) {
case R_AARCH64_ABS16:
case R_AARCH64_PREL16:
checkIntUInt(loc, val, 16, type);
write16le(loc, val);
break;
case R_AARCH64_ABS32:
case R_AARCH64_PREL32:
checkIntUInt(loc, val, 32, type);
write32le(loc, val);
break;
case R_AARCH64_ABS64:
case R_AARCH64_PREL64:
write64le(loc, val);
break;
case R_AARCH64_ADD_ABS_LO12_NC:
or32AArch64Imm(loc, val);
break;
case R_AARCH64_ADR_GOT_PAGE:
case R_AARCH64_ADR_PREL_PG_HI21:
case R_AARCH64_TLSIE_ADR_GOTTPREL_PAGE21:
case R_AARCH64_TLSDESC_ADR_PAGE21:
checkInt(loc, val, 33, type);
LLVM_FALLTHROUGH;
case R_AARCH64_ADR_PREL_PG_HI21_NC:
write32AArch64Addr(loc, val >> 12);
break;
case R_AARCH64_ADR_PREL_LO21:
checkInt(loc, val, 21, type);
write32AArch64Addr(loc, val);
break;
case R_AARCH64_JUMP26:
// Normally we would just write the bits of the immediate field, however
// when patching instructions for the cpu errata fix -fix-cortex-a53-843419
// we want to replace a non-branch instruction with a branch immediate
// instruction. By writing all the bits of the instruction including the
// opcode and the immediate (0 001 | 01 imm26) we can do this
// transformation by placing a R_AARCH64_JUMP26 relocation at the offset of
// the instruction we want to patch.
write32le(loc, 0x14000000);
LLVM_FALLTHROUGH;
case R_AARCH64_CALL26:
checkInt(loc, val, 28, type);
or32le(loc, (val & 0x0FFFFFFC) >> 2);
break;
case R_AARCH64_CONDBR19:
case R_AARCH64_LD_PREL_LO19:
checkAlignment(loc, val, 4, type);
checkInt(loc, val, 21, type);
or32le(loc, (val & 0x1FFFFC) << 3);
break;
case R_AARCH64_LDST8_ABS_LO12_NC:
case R_AARCH64_TLSLE_LDST8_TPREL_LO12_NC:
or32AArch64Imm(loc, getBits(val, 0, 11));
break;
case R_AARCH64_LDST16_ABS_LO12_NC:
case R_AARCH64_TLSLE_LDST16_TPREL_LO12_NC:
checkAlignment(loc, val, 2, type);
or32AArch64Imm(loc, getBits(val, 1, 11));
break;
case R_AARCH64_LDST32_ABS_LO12_NC:
case R_AARCH64_TLSLE_LDST32_TPREL_LO12_NC:
checkAlignment(loc, val, 4, type);
or32AArch64Imm(loc, getBits(val, 2, 11));
break;
case R_AARCH64_LDST64_ABS_LO12_NC:
case R_AARCH64_LD64_GOT_LO12_NC:
case R_AARCH64_TLSIE_LD64_GOTTPREL_LO12_NC:
case R_AARCH64_TLSLE_LDST64_TPREL_LO12_NC:
case R_AARCH64_TLSDESC_LD64_LO12:
checkAlignment(loc, val, 8, type);
or32AArch64Imm(loc, getBits(val, 3, 11));
break;
case R_AARCH64_LDST128_ABS_LO12_NC:
case R_AARCH64_TLSLE_LDST128_TPREL_LO12_NC:
checkAlignment(loc, val, 16, type);
or32AArch64Imm(loc, getBits(val, 4, 11));
break;
case R_AARCH64_MOVW_UABS_G0:
checkUInt(loc, val, 16, type);
LLVM_FALLTHROUGH;
case R_AARCH64_MOVW_UABS_G0_NC:
or32le(loc, (val & 0xFFFF) << 5);
break;
case R_AARCH64_MOVW_UABS_G1:
checkUInt(loc, val, 32, type);
LLVM_FALLTHROUGH;
case R_AARCH64_MOVW_UABS_G1_NC:
or32le(loc, (val & 0xFFFF0000) >> 11);
break;
case R_AARCH64_MOVW_UABS_G2:
checkUInt(loc, val, 48, type);
LLVM_FALLTHROUGH;
case R_AARCH64_MOVW_UABS_G2_NC:
or32le(loc, (val & 0xFFFF00000000) >> 27);
break;
case R_AARCH64_MOVW_UABS_G3:
or32le(loc, (val & 0xFFFF000000000000) >> 43);
break;
case R_AARCH64_MOVW_PREL_G0:
case R_AARCH64_MOVW_SABS_G0:
case R_AARCH64_TLSLE_MOVW_TPREL_G0:
checkInt(loc, val, 17, type);
LLVM_FALLTHROUGH;
case R_AARCH64_MOVW_PREL_G0_NC:
case R_AARCH64_TLSLE_MOVW_TPREL_G0_NC:
writeSMovWImm(loc, val);
break;
case R_AARCH64_MOVW_PREL_G1:
case R_AARCH64_MOVW_SABS_G1:
case R_AARCH64_TLSLE_MOVW_TPREL_G1:
checkInt(loc, val, 33, type);
LLVM_FALLTHROUGH;
case R_AARCH64_MOVW_PREL_G1_NC:
case R_AARCH64_TLSLE_MOVW_TPREL_G1_NC:
writeSMovWImm(loc, val >> 16);
break;
case R_AARCH64_MOVW_PREL_G2:
case R_AARCH64_MOVW_SABS_G2:
case R_AARCH64_TLSLE_MOVW_TPREL_G2:
checkInt(loc, val, 49, type);
LLVM_FALLTHROUGH;
case R_AARCH64_MOVW_PREL_G2_NC:
writeSMovWImm(loc, val >> 32);
break;
case R_AARCH64_MOVW_PREL_G3:
writeSMovWImm(loc, val >> 48);
break;
case R_AARCH64_TSTBR14:
checkInt(loc, val, 16, type);
or32le(loc, (val & 0xFFFC) << 3);
break;
case R_AARCH64_TLSLE_ADD_TPREL_HI12:
checkUInt(loc, val, 24, type);
or32AArch64Imm(loc, val >> 12);
break;
case R_AARCH64_TLSLE_ADD_TPREL_LO12_NC:
case R_AARCH64_TLSDESC_ADD_LO12:
or32AArch64Imm(loc, val);
break;
default:
llvm_unreachable("unknown relocation");
}
}
void AArch64::relaxTlsGdToLe(uint8_t *loc, RelType type, uint64_t val) const {
// TLSDESC Global-Dynamic relocation are in the form:
// adrp x0, :tlsdesc:v [R_AARCH64_TLSDESC_ADR_PAGE21]
// ldr x1, [x0, #:tlsdesc_lo12:v [R_AARCH64_TLSDESC_LD64_LO12]
// add x0, x0, :tlsdesc_los:v [R_AARCH64_TLSDESC_ADD_LO12]
// .tlsdesccall [R_AARCH64_TLSDESC_CALL]
// blr x1
// And it can optimized to:
// movz x0, #0x0, lsl #16
// movk x0, #0x10
// nop
// nop
checkUInt(loc, val, 32, type);
switch (type) {
case R_AARCH64_TLSDESC_ADD_LO12:
case R_AARCH64_TLSDESC_CALL:
write32le(loc, 0xd503201f); // nop
return;
case R_AARCH64_TLSDESC_ADR_PAGE21:
write32le(loc, 0xd2a00000 | (((val >> 16) & 0xffff) << 5)); // movz
return;
case R_AARCH64_TLSDESC_LD64_LO12:
write32le(loc, 0xf2800000 | ((val & 0xffff) << 5)); // movk
return;
default:
llvm_unreachable("unsupported relocation for TLS GD to LE relaxation");
}
}
void AArch64::relaxTlsGdToIe(uint8_t *loc, RelType type, uint64_t val) const {
// TLSDESC Global-Dynamic relocation are in the form:
// adrp x0, :tlsdesc:v [R_AARCH64_TLSDESC_ADR_PAGE21]
// ldr x1, [x0, #:tlsdesc_lo12:v [R_AARCH64_TLSDESC_LD64_LO12]
// add x0, x0, :tlsdesc_los:v [R_AARCH64_TLSDESC_ADD_LO12]
// .tlsdesccall [R_AARCH64_TLSDESC_CALL]
// blr x1
// And it can optimized to:
// adrp x0, :gottprel:v
// ldr x0, [x0, :gottprel_lo12:v]
// nop
// nop
switch (type) {
case R_AARCH64_TLSDESC_ADD_LO12:
case R_AARCH64_TLSDESC_CALL:
write32le(loc, 0xd503201f); // nop
break;
case R_AARCH64_TLSDESC_ADR_PAGE21:
write32le(loc, 0x90000000); // adrp
relocateOne(loc, R_AARCH64_TLSIE_ADR_GOTTPREL_PAGE21, val);
break;
case R_AARCH64_TLSDESC_LD64_LO12:
write32le(loc, 0xf9400000); // ldr
relocateOne(loc, R_AARCH64_TLSIE_LD64_GOTTPREL_LO12_NC, val);
break;
default:
llvm_unreachable("unsupported relocation for TLS GD to LE relaxation");
}
}
void AArch64::relaxTlsIeToLe(uint8_t *loc, RelType type, uint64_t val) const {
checkUInt(loc, val, 32, type);
if (type == R_AARCH64_TLSIE_ADR_GOTTPREL_PAGE21) {
// Generate MOVZ.
uint32_t regNo = read32le(loc) & 0x1f;
write32le(loc, (0xd2a00000 | regNo) | (((val >> 16) & 0xffff) << 5));
return;
}
if (type == R_AARCH64_TLSIE_LD64_GOTTPREL_LO12_NC) {
// Generate MOVK.
uint32_t regNo = read32le(loc) & 0x1f;
write32le(loc, (0xf2800000 | regNo) | ((val & 0xffff) << 5));
return;
}
llvm_unreachable("invalid relocation for TLS IE to LE relaxation");
}
// AArch64 may use security features in variant PLT sequences. These are:
// Pointer Authentication (PAC), introduced in armv8.3-a and Branch Target
// Indicator (BTI) introduced in armv8.5-a. The additional instructions used
// in the variant Plt sequences are encoded in the Hint space so they can be
// deployed on older architectures, which treat the instructions as a nop.
// PAC and BTI can be combined leading to the following combinations:
// writePltHeader
// writePltHeaderBti (no PAC Header needed)
// writePlt
// writePltBti (BTI only)
// writePltPac (PAC only)
// writePltBtiPac (BTI and PAC)
//
// When PAC is enabled the dynamic loader encrypts the address that it places
// in the .got.plt using the pacia1716 instruction which encrypts the value in
// x17 using the modifier in x16. The static linker places autia1716 before the
// indirect branch to x17 to authenticate the address in x17 with the modifier
// in x16. This makes it more difficult for an attacker to modify the value in
// the .got.plt.
//
// When BTI is enabled all indirect branches must land on a bti instruction.
// The static linker must place a bti instruction at the start of any PLT entry
// that may be the target of an indirect branch. As the PLT entries call the
// lazy resolver indirectly this must have a bti instruction at start. In
// general a bti instruction is not needed for a PLT entry as indirect calls
// are resolved to the function address and not the PLT entry for the function.
// There are a small number of cases where the PLT address can escape, such as
// taking the address of a function or ifunc via a non got-generating
// relocation, and a shared library refers to that symbol.
//
// We use the bti c variant of the instruction which permits indirect branches
// (br) via x16/x17 and indirect function calls (blr) via any register. The ABI
// guarantees that all indirect branches from code requiring BTI protection
// will go via x16/x17
namespace {
class AArch64BtiPac final : public AArch64 {
public:
AArch64BtiPac();
void writePltHeader(uint8_t *buf) const override;
void writePlt(uint8_t *buf, uint64_t gotPltEntryAddr, uint64_t pltEntryAddr,
int32_t index, unsigned relOff) const override;
private:
bool btiHeader; // bti instruction needed in PLT Header
bool btiEntry; // bti instruction needed in PLT Entry
bool pacEntry; // autia1716 instruction needed in PLT Entry
};
} // namespace
AArch64BtiPac::AArch64BtiPac() {
btiHeader = (config->andFeatures & GNU_PROPERTY_AARCH64_FEATURE_1_BTI);
// A BTI (Branch Target Indicator) Plt Entry is only required if the
// address of the PLT entry can be taken by the program, which permits an
// indirect jump to the PLT entry. This can happen when the address
// of the PLT entry for a function is canonicalised due to the address of
// the function in an executable being taken by a shared library.
// FIXME: There is a potential optimization to omit the BTI if we detect
// that the address of the PLT entry isn't taken.
btiEntry = btiHeader && !config->shared;
pacEntry = (config->andFeatures & GNU_PROPERTY_AARCH64_FEATURE_1_PAC);
if (btiEntry || pacEntry)
pltEntrySize = 24;
}
void AArch64BtiPac::writePltHeader(uint8_t *buf) const {
const uint8_t btiData[] = { 0x5f, 0x24, 0x03, 0xd5 }; // bti c
const uint8_t pltData[] = {
0xf0, 0x7b, 0xbf, 0xa9, // stp x16, x30, [sp,#-16]!
0x10, 0x00, 0x00, 0x90, // adrp x16, Page(&(.plt.got[2]))
0x11, 0x02, 0x40, 0xf9, // ldr x17, [x16, Offset(&(.plt.got[2]))]
0x10, 0x02, 0x00, 0x91, // add x16, x16, Offset(&(.plt.got[2]))
0x20, 0x02, 0x1f, 0xd6, // br x17
0x1f, 0x20, 0x03, 0xd5, // nop
0x1f, 0x20, 0x03, 0xd5 // nop
};
const uint8_t nopData[] = { 0x1f, 0x20, 0x03, 0xd5 }; // nop
uint64_t got = in.gotPlt->getVA();
uint64_t plt = in.plt->getVA();
if (btiHeader) {
// PltHeader is called indirectly by plt[N]. Prefix pltData with a BTI C
// instruction.
memcpy(buf, btiData, sizeof(btiData));
buf += sizeof(btiData);
plt += sizeof(btiData);
}
memcpy(buf, pltData, sizeof(pltData));
relocateOne(buf + 4, R_AARCH64_ADR_PREL_PG_HI21,
getAArch64Page(got + 16) - getAArch64Page(plt + 8));
relocateOne(buf + 8, R_AARCH64_LDST64_ABS_LO12_NC, got + 16);
relocateOne(buf + 12, R_AARCH64_ADD_ABS_LO12_NC, got + 16);
if (!btiHeader)
// We didn't add the BTI c instruction so round out size with NOP.
memcpy(buf + sizeof(pltData), nopData, sizeof(nopData));
}
void AArch64BtiPac::writePlt(uint8_t *buf, uint64_t gotPltEntryAddr,
uint64_t pltEntryAddr, int32_t index,
unsigned relOff) const {
// The PLT entry is of the form:
// [btiData] addrInst (pacBr | stdBr) [nopData]
const uint8_t btiData[] = { 0x5f, 0x24, 0x03, 0xd5 }; // bti c
const uint8_t addrInst[] = {
0x10, 0x00, 0x00, 0x90, // adrp x16, Page(&(.plt.got[n]))
0x11, 0x02, 0x40, 0xf9, // ldr x17, [x16, Offset(&(.plt.got[n]))]
0x10, 0x02, 0x00, 0x91 // add x16, x16, Offset(&(.plt.got[n]))
};
const uint8_t pacBr[] = {
0x9f, 0x21, 0x03, 0xd5, // autia1716
0x20, 0x02, 0x1f, 0xd6 // br x17
};
const uint8_t stdBr[] = {
0x20, 0x02, 0x1f, 0xd6, // br x17
0x1f, 0x20, 0x03, 0xd5 // nop
};
const uint8_t nopData[] = { 0x1f, 0x20, 0x03, 0xd5 }; // nop
if (btiEntry) {
memcpy(buf, btiData, sizeof(btiData));
buf += sizeof(btiData);
pltEntryAddr += sizeof(btiData);
}
memcpy(buf, addrInst, sizeof(addrInst));
relocateOne(buf, R_AARCH64_ADR_PREL_PG_HI21,
getAArch64Page(gotPltEntryAddr) -
getAArch64Page(pltEntryAddr));
relocateOne(buf + 4, R_AARCH64_LDST64_ABS_LO12_NC, gotPltEntryAddr);
relocateOne(buf + 8, R_AARCH64_ADD_ABS_LO12_NC, gotPltEntryAddr);
if (pacEntry)
memcpy(buf + sizeof(addrInst), pacBr, sizeof(pacBr));
else
memcpy(buf + sizeof(addrInst), stdBr, sizeof(stdBr));
if (!btiEntry)
// We didn't add the BTI c instruction so round out size with NOP.
memcpy(buf + sizeof(addrInst) + sizeof(stdBr), nopData, sizeof(nopData));
}
static TargetInfo *getTargetInfo() {
if (config->andFeatures & (GNU_PROPERTY_AARCH64_FEATURE_1_BTI |
GNU_PROPERTY_AARCH64_FEATURE_1_PAC)) {
static AArch64BtiPac t;
return &t;
}
static AArch64 t;
return &t;
}
TargetInfo *elf::getAArch64TargetInfo() { return getTargetInfo(); }
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