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[PAC][lld] Do not emit AUTH relocs against undef weak non-preemptible symbols #194636

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[PAC][lld] Do not emit AUTH relocs against undef weak non-preemptible symbols #194636

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f56b506
[PAC][lld] Do not emit AUTH relocs against undef weak non-preemptible…
kovdan01 Apr 28, 2026
565d156
Support R_AARCH64_AUTH_TLSDESC_CALL
kovdan01 May 18, 2026
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86 changes: 77 additions & 9 deletions lld/ELF/Arch/AArch64.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -102,6 +102,8 @@ class AArch64 : public TargetInfo {
void relaxTlsGdToLe(uint8_t *loc, const Relocation &rel, uint64_t val) const;
void relaxTlsGdToIe(uint8_t *loc, const Relocation &rel, uint64_t val) const;
void relaxTlsIeToLe(uint8_t *loc, const Relocation &rel, uint64_t val) const;
void relaxAuthTlsDescForNonPreemptibleUndefWeak(uint8_t *loc,
const Relocation &rel) const;
};

struct AArch64Relaxer {
Expand Down Expand Up @@ -206,6 +208,20 @@ void AArch64::scanSectionImpl(InputSectionBase &sec, Relocs<RelTy> rels) {
// Relocation types that only need a RelExpr set `expr` and break out of
// the switch to reach rs.process(). Types that need special handling
// (fast-path helpers, TLS) call a handler and use `continue`.

auto handleTlsDescAuth = [&sym, &sec, type, offset,
addend](RelExpr tlsdescExpr) {
sym.setFlags(NEEDS_TLSDESC_AUTH);
if (sym.isUndefWeak() && !sym.isPreemptible) {
// Resolves statically to null. Handle in
// relaxAuthTlsDescForNonPreemptibleUndefWeak
sec.addReloc({R_TPREL, type, offset, addend, &sym});
} else {
sym.setFlags(NEEDS_TLSDESC);
sec.addReloc({tlsdescExpr, type, offset, addend, &sym});
}
};

switch (type) {
case R_AARCH64_NONE:
continue;
Expand Down Expand Up @@ -357,13 +373,16 @@ void AArch64::scanSectionImpl(InputSectionBase &sec, Relocs<RelTy> rels) {
// only supports the descriptor based TLS (TLSDESC).
// https://github.com/ARM-software/abi-aa/blob/main/pauthabielf64/pauthabielf64.rst#general-restrictions
case R_AARCH64_AUTH_TLSDESC_ADR_PAGE21:
sym.setFlags(NEEDS_TLSDESC | NEEDS_TLSDESC_AUTH);
sec.addReloc({RE_AARCH64_TLSDESC_PAGE, type, offset, addend, &sym});
handleTlsDescAuth(RE_AARCH64_TLSDESC_PAGE);
continue;
case R_AARCH64_AUTH_TLSDESC_LD64_LO12:
case R_AARCH64_AUTH_TLSDESC_ADD_LO12:
sym.setFlags(NEEDS_TLSDESC | NEEDS_TLSDESC_AUTH);
sec.addReloc({R_TLSDESC, type, offset, addend, &sym});
handleTlsDescAuth(R_TLSDESC);
continue;
case R_AARCH64_AUTH_TLSDESC_CALL:
sym.setFlags(NEEDS_TLSDESC_AUTH);
if (sym.isUndefWeak() && !sym.isPreemptible)
sec.addReloc({R_TPREL, type, offset, addend, &sym});
continue;

default:
Expand Down Expand Up @@ -645,11 +664,22 @@ void AArch64::relocate(uint8_t *loc, const Relocation &rel,
write64(ctx, loc, val);
break;
case R_AARCH64_AUTH_ABS64:
// This is used for the addend of a .relr.auth.dyn entry,
// which is a 32-bit value; the upper 32 bits are used to
// encode the schema.
checkInt(ctx, loc, val, 32, rel);
write32(ctx, loc, val);
if (rel.sym->isUndefined() && !rel.sym->isPreemptible) {
// Undefined weak non-preemptible symbols are statically resolved to the
// addend. No dynamic relocation and corresponding signing schema encoding
// is needed.
//
// Note: at this point, binding of undefined weak non-preemptible symbols
// has already been changed from weak to local by computeBinding call, so
// just check against isUndefined().
write64(ctx, loc, val);
} else {
// This is used for the addend of a .relr.auth.dyn entry,
// which is a 32-bit value; the upper 32 bits are used to
// encode the schema.
checkInt(ctx, loc, val, 32, rel);
write32(ctx, loc, val);
}
break;
case R_AARCH64_TLS_DTPREL64:
write64(ctx, loc, val);
Expand Down Expand Up @@ -803,6 +833,35 @@ void AArch64::relocate(uint8_t *loc, const Relocation &rel,
}
}

void AArch64::relaxAuthTlsDescForNonPreemptibleUndefWeak(
uint8_t *loc, const Relocation &rel) const {
// AUTH TLSDESC relocations are in the form:
// adrp x0, :tlsdesc_auth:v [R_AARCH64_AUTH_TLSDESC_ADR_PAGE21]
// ldr x16, [x0, :tlsdesc_auth_lo12:v] [R_AARCH64_AUTH_TLSDESC_LD64_LO12]
// add x0, x0, :tlsdesc_auth_lo12:v [R_AARCH64_AUTH_TLSDESC_ADD_LO12]
// .tlsdescauthcall v [R_AARCH64_AUTH_TLSDESC_CALL]
// blraa x16, x0
// And it can optimized to:
// mov x0, #0x0
// nop
// nop
// nop

switch (rel.type) {
case R_AARCH64_AUTH_TLSDESC_ADD_LO12:
case R_AARCH64_AUTH_TLSDESC_LD64_LO12:
case R_AARCH64_AUTH_TLSDESC_CALL:
write32le(loc, 0xd503201f); // nop
return;
case R_AARCH64_AUTH_TLSDESC_ADR_PAGE21:
write32le(loc, 0xd2800000); // mov x0, #0x0
return;
default:
llvm_unreachable("unsupported relocation for non-preemptible undefined "
"weak AUTH TLSDESC relaxation");
}
}

void AArch64::relaxTlsGdToLe(uint8_t *loc, const Relocation &rel,
uint64_t val) const {
// TLSDESC Global-Dynamic relocation are in the form:
Expand Down Expand Up @@ -1081,6 +1140,15 @@ void AArch64::relocateAlloc(InputSection &sec, uint8_t *buf) const {
else
relocate(loc, rel, val);
continue;
case R_AARCH64_AUTH_TLSDESC_ADR_PAGE21:
case R_AARCH64_AUTH_TLSDESC_LD64_LO12:
case R_AARCH64_AUTH_TLSDESC_ADD_LO12:
case R_AARCH64_AUTH_TLSDESC_CALL:
if (rel.expr == R_TPREL)
relaxAuthTlsDescForNonPreemptibleUndefWeak(loc, rel);
else
relocate(loc, rel, val);
continue;
case R_AARCH64_TLSIE_ADR_GOTTPREL_PAGE21:
case R_AARCH64_TLSIE_LD64_GOTTPREL_LO12_NC:
if (rel.expr == R_TPREL)
Expand Down
19 changes: 13 additions & 6 deletions lld/ELF/Relocations.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -789,9 +789,12 @@ static void addGotAuthEntry(Ctx &ctx, Symbol &sym) {
return;
}

// Signed GOT requires dynamic relocation.
ctx.in.relaDyn->addReloc(
{R_AARCH64_AUTH_RELATIVE, ctx.in.got.get(), off, false, sym, 0, R_ABS});
// Signed GOT requires dynamic relocation unless the symbol is
// non-preemptible and undefined weak.
if (!sym.isUndefWeak()) {
ctx.in.relaDyn->addReloc(
{R_AARCH64_AUTH_RELATIVE, ctx.in.got.get(), off, false, sym, 0, R_ABS});
}
}

static void addTpOffsetGotEntry(Ctx &ctx, Symbol &sym) {
Expand Down Expand Up @@ -851,8 +854,12 @@ bool RelocScan::isStaticLinkTimeConstant(RelExpr e, RelType type,
// only the low bits are used.
if (e == R_GOT || e == R_PLT)
return ctx.target->usesOnlyLowPageBits(type) || !ctx.arg.isPic;
// R_AARCH64_AUTH_ABS64 and iRelSymbolicRel require a dynamic relocation.
if (e == RE_AARCH64_AUTH || type == ctx.target->iRelSymbolicRel)
// R_AARCH64_AUTH_ABS64 requires a dynamic relocation unless the symbol is
// non-preemptible and undefined weak.
if (e == RE_AARCH64_AUTH && (!sym.isUndefWeak() || sym.isPreemptible))
return false;
// iRelSymbolicRel requires a dynamic relocation.
if (type == ctx.target->iRelSymbolicRel)
return false;

// The behavior of an undefined weak reference is implementation defined.
Expand Down Expand Up @@ -1354,7 +1361,7 @@ void elf::postScanRelocations(Ctx &ctx) {
got->addTlsDescEntry(sym);
RelType tlsDescRel = ctx.target->tlsDescRel;
if (flags & NEEDS_TLSDESC_AUTH) {
got->addTlsDescAuthEntry();
got->addTlsDescAuthEntry(sym);
tlsDescRel = ELF::R_AARCH64_AUTH_TLSDESC;
}
ctx.in.relaDyn->addAddendOnlyRelocIfNonPreemptible(
Expand Down
20 changes: 15 additions & 5 deletions lld/ELF/SyntheticSections.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -507,7 +507,9 @@ void GotSection::addEntry(const Symbol &sym) {

void GotSection::addAuthEntry(const Symbol &sym) {
authEntries.push_back(
{(numEntries - 1) * ctx.target->gotEntrySize, sym.isFunc()});
{/*offset=*/(numEntries - 1) * ctx.target->gotEntrySize,
/*isSymbolFunc=*/sym.isFunc(),
/*isUndefWeakNonPreemptible=*/sym.isUndefWeak() && !sym.isPreemptible});
}

bool GotSection::addTlsDescEntry(const Symbol &sym) {
Expand All @@ -517,9 +519,12 @@ bool GotSection::addTlsDescEntry(const Symbol &sym) {
return true;
}

void GotSection::addTlsDescAuthEntry() {
authEntries.push_back({(numEntries - 2) * ctx.target->gotEntrySize, true});
authEntries.push_back({(numEntries - 1) * ctx.target->gotEntrySize, false});
void GotSection::addTlsDescAuthEntry(const Symbol &sym) {
authEntries.push_back({/*offset=*/(numEntries - 2) * ctx.target->gotEntrySize,
/*isSymbolFunc=*/true,
/*isUndefWeakNonPreemptible=*/false});
assert(!sym.isFunc());
addAuthEntry(sym);
}

bool GotSection::addDynTlsEntry(const Symbol &sym) {
Expand Down Expand Up @@ -578,6 +583,12 @@ void GotSection::writeTo(uint8_t *buf) {
ctx.target->writeGotHeader(buf);
ctx.target->relocateAlloc(*this, buf);
for (const AuthEntryInfo &authEntry : authEntries) {
uint8_t *dest = buf + authEntry.offset;

if (authEntry.isUndefWeakNonPreemptible) {
write64(ctx, dest, 0);
continue;
}
// https://github.com/ARM-software/abi-aa/blob/2024Q3/pauthabielf64/pauthabielf64.rst#default-signing-schema
// Signed GOT entries use the IA key for symbols of type STT_FUNC and the
// DA key for all other symbol types, with the address of the GOT entry as
Expand All @@ -587,7 +598,6 @@ void GotSection::writeTo(uint8_t *buf) {
// https://github.com/ARM-software/abi-aa/blob/2024Q3/pauthabielf64/pauthabielf64.rst#encoding-the-signing-schema
// If address diversity is set and the discriminator
// is 0 then modifier = Place
uint8_t *dest = buf + authEntry.offset;
uint64_t key = authEntry.isSymbolFunc ? /*IA=*/0b00 : /*DA=*/0b10;
uint64_t addrDiversity = 1;
write64(ctx, dest, (addrDiversity << 63) | (key << 60));
Expand Down
3 changes: 2 additions & 1 deletion lld/ELF/SyntheticSections.h
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Original file line number Diff line number Diff line change
Expand Up @@ -123,7 +123,7 @@ class GotSection final : public SyntheticSection {
void addEntry(const Symbol &sym);
void addAuthEntry(const Symbol &sym);
bool addTlsDescEntry(const Symbol &sym);
void addTlsDescAuthEntry();
void addTlsDescAuthEntry(const Symbol &sym);
bool addDynTlsEntry(const Symbol &sym);
bool addTlsIndex();
uint32_t getTlsDescOffset(const Symbol &sym) const;
Expand All @@ -144,6 +144,7 @@ class GotSection final : public SyntheticSection {
struct AuthEntryInfo {
size_t offset;
bool isSymbolFunc;
bool isUndefWeakNonPreemptible;
};
SmallVector<AuthEntryInfo, 0> authEntries;
};
Expand Down
48 changes: 48 additions & 0 deletions lld/test/ELF/aarch64-reloc-pauth-undef-weak-dso.s
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Original file line number Diff line number Diff line change
@@ -0,0 +1,48 @@
# REQUIRES: aarch64
# RUN: llvm-mc -filetype=obj -triple=aarch64 -mattr=+pauth %s -o %t.o
# RUN: ld.lld -shared %t.o -o %t
# RUN: llvm-readobj -r %t | FileCheck %s --check-prefix=RELA
# RUN: llvm-readelf -x.data %t | FileCheck %s --check-prefix=DATA
# RUN: llvm-readelf -x.got %t | FileCheck %s --check-prefix=GOT
# RUN: llvm-objdump -d --no-show-raw-insn %t | FileCheck %s --check-prefix=DIS

## Verify that R_AARCH64_AUTH_ABS64 against a weak undefined symbol is resolved
## to NULL (plus addend).

# RELA-LABEL: Relocations [
# RELA-NEXT: ]

# DATA-LABEL: Hex dump of section '.data':
# DATA-NEXT: 0x00030300 00000000 00000000 25000000 00000000
# DATA-NEXT: 0x00030310 00000000 00000000 25000000 00000000

# GOT-LABEL: Hex dump of section '.got':
# GOT-NEXT: 0x000202f8 00000000 00000000

# DIS-LABEL: <_start>:
# DIS-NEXT: adrp x0, 0x20000
# DIS-NEXT: ldr x0, [x0, #0x2f8]
# DIS-NEXT: mov x0, #0x0
# DIS-NEXT: nop
# DIS-NEXT: nop
# DIS-NEXT: nop

.weak undef
.hidden undef

.globl _start
_start:
adrp x0, :got_auth:undef
ldr x0, [x0, :got_auth_lo12:undef]
adrp x0, :tlsdesc_auth:undef
ldr x16, [x0, :tlsdesc_auth_lo12:undef]
add x0, x0, :tlsdesc_auth_lo12:undef
.tlsdescauthcall undef
blraa x16, x0

.data
foo:
.quad undef@AUTH(da,42)
.quad (undef + 37)@AUTH(da,42)
.quad undef
.quad (undef + 37)
47 changes: 47 additions & 0 deletions lld/test/ELF/aarch64-reloc-pauth-undef-weak-pie.s
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Original file line number Diff line number Diff line change
@@ -0,0 +1,47 @@
# REQUIRES: aarch64
# RUN: llvm-mc -filetype=obj -triple=aarch64 -mattr=+pauth %s -o %t.o
# RUN: ld.lld -pie %t.o -o %t
# RUN: llvm-readobj -r %t | FileCheck %s --check-prefix=RELA
# RUN: llvm-readelf -x.data %t | FileCheck %s --check-prefix=DATA
# RUN: llvm-readelf -x.got %t | FileCheck %s --check-prefix=GOT
# RUN: llvm-objdump -d --no-show-raw-insn %t | FileCheck %s --check-prefix=DIS

## Verify that R_AARCH64_AUTH_ABS64 against a weak undefined symbol is resolved
## to NULL (plus addend).

# RELA-LABEL: Relocations [
# RELA-NEXT: ]

# DATA-LABEL: Hex dump of section '.data':
# DATA-NEXT: 0x000302f8 00000000 00000000 25000000 00000000
# DATA-NEXT: 0x00030308 00000000 00000000 25000000 00000000

# GOT-LABEL: Hex dump of section '.got':
# GOT-NEXT: 0x000202f0 00000000 00000000

# DIS-LABEL: <_start>:
# DIS-NEXT: adrp x0, 0x20000
# DIS-NEXT: ldr x0, [x0, #0x2f0]
# DIS-NEXT: mov x0, #0x0
# DIS-NEXT: nop
# DIS-NEXT: nop
# DIS-NEXT: nop

.weak undef

.globl _start
_start:
adrp x0, :got_auth:undef
ldr x0, [x0, :got_auth_lo12:undef]
adrp x0, :tlsdesc_auth:undef
ldr x16, [x0, :tlsdesc_auth_lo12:undef]
add x0, x0, :tlsdesc_auth_lo12:undef
.tlsdescauthcall undef
blraa x16, x0

.data
foo:
.quad undef@AUTH(da,42)
.quad (undef + 37)@AUTH(da,42)
.quad undef
.quad (undef + 37)
48 changes: 48 additions & 0 deletions lld/test/ELF/aarch64-reloc-pauth-undef-weak.s
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Original file line number Diff line number Diff line change
@@ -0,0 +1,48 @@
# REQUIRES: aarch64
# RUN: llvm-mc -filetype=obj -triple=aarch64 -mattr=+pauth %s -o %t.o
# RUN: ld.lld --static %t.o -o %t
# RUN: llvm-readobj -r %t | FileCheck %s --check-prefix=RELA
# RUN: llvm-readelf -x.data %t | FileCheck %s --check-prefix=DATA
# RUN: llvm-readelf -x.got %t | FileCheck %s --check-prefix=GOT
# RUN: llvm-objdump -d --no-show-raw-insn %t | FileCheck %s --check-prefix=DIS

## Verify that R_AARCH64_AUTH_ABS64 against a weak undefined symbol is resolved
## to NULL (plus addend).

# RELA-LABEL: Relocations [
# RELA-NEXT: ]

# DATA-LABEL: Hex dump of section '.data':
# DATA-NEXT: 0x002301e8 00000000 00000000 25000000 00000000
# DATA-NEXT: 0x002301f8 00000000 00000000 25000000 00000000

# GOT-LABEL: Hex dump of section '.got':
# GOT-NEXT: 0x002201e0 00000000 00000000

# DIS-LABEL: <_start>:

# DIS-NEXT: adrp x0, 0x220000
# DIS-NEXT: ldr x0, [x0, #0x1e0]
# DIS-NEXT: mov x0, #0x0
# DIS-NEXT: nop
# DIS-NEXT: nop
# DIS-NEXT: nop

.weak undef

.globl _start
_start:
adrp x0, :got_auth:undef
ldr x0, [x0, :got_auth_lo12:undef]
adrp x0, :tlsdesc_auth:undef
ldr x16, [x0, :tlsdesc_auth_lo12:undef]
add x0, x0, :tlsdesc_auth_lo12:undef
.tlsdescauthcall undef
blraa x16, x0

.data
foo:
.quad undef@AUTH(da,42)
.quad (undef + 37)@AUTH(da,42)
.quad undef
.quad undef + 37
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