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[lld][WebAssembly] Don't apply data relocations at static constructor…
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… time

Instead, export `__wasm_apply_data_relocs` and `__wasm_call_ctors`
separately.

This is required since user code in a shared library (such as static
constructors) should not be run until relocations have been applied to
all loaded libraries.

See: emscripten-core/emscripten#17295

Differential Revision: https://reviews.llvm.org/D128515
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@sbc100
sbc100 committed Jun 27, 2022
1 parent 0103d4d commit 53217ec
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25 changes: 13 additions & 12 deletions lld/test/wasm/data-segments.ll
View file
Expand Up @@ -30,12 +30,12 @@
; Also test in combination with PIC/pie
; RUN: wasm-ld --experimental-pic -pie -no-gc-sections --no-entry --shared-memory --max-memory=131072 %t.atomics.bulk-mem.pic.o -o %t.pic.wasm
; RUN: obj2yaml %t.pic.wasm | FileCheck %s --check-prefixes PASSIVE-PIC,PASSIVE32-PIC
; RUN: llvm-objdump --disassemble-symbols=__wasm_call_ctors,__wasm_init_memory --no-show-raw-insn --no-leading-addr %t.pic.wasm | FileCheck %s --check-prefixes DIS,PIC-DIS -DPTR=i32
; RUN: llvm-objdump --disassemble-symbols=__wasm_call_ctors,__wasm_apply_data_relocs,__wasm_init_memory --no-show-raw-insn --no-leading-addr %t.pic.wasm | FileCheck %s --check-prefixes DIS,PIC-DIS -DPTR=i32

; Also test in combination with PIC/pie + wasm64
; RUN: wasm-ld -mwasm64 --experimental-pic -pie -no-gc-sections --no-entry --shared-memory --max-memory=131072 %t.atomics.bulk-mem.pic-mem64.o -o %t.pic-mem64.wasm
; RUN: obj2yaml %t.pic-mem64.wasm | FileCheck %s --check-prefixes PASSIVE-PIC,PASSIVE64-PIC
; RUN: llvm-objdump --disassemble-symbols=__wasm_call_ctors,__wasm_init_memory --no-show-raw-insn --no-leading-addr %t.pic-mem64.wasm | FileCheck %s --check-prefixes DIS,PIC-DIS -DPTR=i64
; RUN: llvm-objdump --disassemble-symbols=__wasm_call_ctors,__wasm_apply_data_relocs,__wasm_init_memory --no-show-raw-insn --no-leading-addr %t.pic-mem64.wasm | FileCheck %s --check-prefixes DIS,PIC-DIS -DPTR=i64

@a = hidden global [6 x i8] c"hello\00", align 1
@b = hidden global [8 x i8] c"goodbye\00", align 1
Expand Down Expand Up @@ -113,26 +113,26 @@
; PASSIVE-NEXT: Name: __wasm_init_memory

; PASSIVE-PIC: - Type: START
; PASSIVE-PIC-NEXT: StartFunction: 2
; PASSIVE-PIC-NEXT: StartFunction: 3
; PASSIVE-PIC-NEXT: - Type: DATACOUNT
; PASSIVE-PIC-NEXT: Count: 3
; PASSIVE-PIC-NEXT: - Type: CODE
; PASSIVE-PIC-NEXT: Functions:
; PASSIVE-PIC-NEXT: - Index: 0
; PASSIVE-PIC-NEXT: Locals: []
; PASSIVE-PIC-NEXT: Body: 10030B
; PASSIVE-PIC-NEXT: Body: 0B
; PASSIVE-PIC-NEXT: - Index: 1
; PASSIVE-PIC-NEXT: Locals: []
; PASSIVE-PIC-NEXT: Body: {{.*}}
; PASSIVE-PIC-NEXT: - Index: 2
; PASSIVE-PIC-NEXT: Locals: []
; PASSIVE-PIC-NEXT: Body: 0B
; PASSIVE-PIC-NEXT: - Index: 3
; PASSIVE-PIC-NEXT: Locals:
; PASSIVE32-PIC-NEXT: - Type: I32
; PASSIVE64-PIC-NEXT: - Type: I64
; PASSIVE-PIC-NEXT: Count: 2
; PASSIVE-PIC-NEXT: Body: {{.*}}
; PASSIVE-PIC-NEXT: - Index: 3
; PASSIVE-PIC-NEXT: Locals: []
; PASSIVE-PIC-NEXT: Body: 0B
; PASSIVE-PIC-NEXT: - Type: DATA
; PASSIVE-PIC-NEXT: Segments:
; PASSIVE-PIC-NEXT: - SectionOffset: 3
Expand All @@ -152,18 +152,19 @@
; PASSIVE-PIC-NEXT: - Index: 1
; PASSIVE-PIC-NEXT: Name: __wasm_init_tls
; PASSIVE-PIC-NEXT: - Index: 2
; PASSIVE-PIC-NEXT: Name: __wasm_init_memory
; PASSIVE-PIC-NEXT: - Index: 3
; PASSIVE-PIC-NEXT: Name: __wasm_apply_data_relocs
; PASSIVE-PIC-NEXT: - Index: 3
; PASSIVE-PIC-NEXT: Name: __wasm_init_memory

; In PIC mode __wasm_call_ctors contains a call to __wasm_apply_data_relocs
; In non-PIC mode __wasm_call_ctors is an emtpy function since there are
; no data relocations.
; DIS-LABEL: <__wasm_call_ctors>:
; DIS-EMPTY:
; PIC-DIS-NEXT: call 3
; DIS-NEXT: end

; In PIC mode __wasm_apply_data_relocs is export seperatly to __wasm_call_ctors
; PIC-DIS: <__wasm_apply_data_relocs>:
; PIC-DIS-EMPTY:

; DIS-LABEL: <__wasm_init_memory>:

; PIC-DIS: .local [[PTR]]
Expand Down
16 changes: 9 additions & 7 deletions lld/test/wasm/pie.ll
View file
@@ -1,7 +1,7 @@
; RUN: llc -relocation-model=pic -mattr=+mutable-globals -filetype=obj %s -o %t.o
; RUN: wasm-ld --no-gc-sections --experimental-pic -pie -o %t.wasm %t.o
; RUN: obj2yaml %t.wasm | FileCheck %s
; RUN: llvm-objdump --disassemble-symbols=__wasm_call_ctors --no-show-raw-insn --no-leading-addr %t.wasm | FileCheck %s --check-prefixes DISASSEM
; RUN: llvm-objdump --disassemble-symbols=__wasm_call_ctors,__wasm_apply_data_relocs --no-show-raw-insn --no-leading-addr %t.wasm | FileCheck %s --check-prefixes DISASSEM

target triple = "wasm32-unknown-emscripten"

Expand Down Expand Up @@ -91,11 +91,13 @@ declare void @external_func()
; CHECK-NEXT: Name: _start
; CHECK-NEXT: GlobalNames:

; DISASSEM: <__wasm_call_ctors>:
; DISASSEM-LABEL: <__wasm_call_ctors>:
; DISASSEM-EMPTY:
; DISASSEM-NEXT: call 2
; DISASSEM-NEXT: end

; DISASSEM-LABEL: <__wasm_apply_data_relocs>:
; DISASSEM: end

; Run the same test with extended-const support. When this is available
; we don't need __wasm_apply_global_relocs and instead rely on the add
; instruction in the InitExpr. We also, therefore, do not need these globals
Expand Down Expand Up @@ -153,10 +155,10 @@ declare void @external_func()
; SHMEM: - Type: START
; SHMEM-NEXT: StartFunction: 6

; DISASSEM-SHMEM: <__wasm_start>:
; DISASSEM-SHMEM-LABEL: <__wasm_start>:
; DISASSEM-SHMEM-EMPTY:
; DISASSEM-SHMEM-NEXT: call 5
; DISASSEM-SHMEM-NEXT: call 3
; DISASSEM-SHMEM-NEXT: call 4
; DISASSEM-SHMEM-NEXT: end

; SHMEM: FunctionNames:
Expand All @@ -167,9 +169,9 @@ declare void @external_func()
; SHMEM-NEXT: - Index: 2
; SHMEM-NEXT: Name: __wasm_init_tls
; SHMEM-NEXT: - Index: 3
; SHMEM-NEXT: Name: __wasm_init_memory
; SHMEM-NEXT: - Index: 4
; SHMEM-NEXT: Name: __wasm_apply_data_relocs
; SHMEM-NEXT: - Index: 4
; SHMEM-NEXT: Name: __wasm_init_memory
; SHMEM-NEXT: - Index: 5
; SHMEM-NEXT: Name: __wasm_apply_global_relocs
; SHMEM-NEXT: - Index: 6
Expand Down
3 changes: 3 additions & 0 deletions lld/test/wasm/shared-weak-symbols.s
View file
Expand Up @@ -62,6 +62,9 @@ call_weak:
# CHECK-NEXT: - Name: __wasm_call_ctors
# CHECK-NEXT: Kind: FUNCTION
# CHECK-NEXT: Index: 1
# CHECK-NEXT: - Name: __wasm_apply_data_relocs
# CHECK-NEXT: Kind: FUNCTION
# CHECK-NEXT: Index: 2
# CHECK-NEXT: - Name: weak_func
# CHECK-NEXT: Kind: FUNCTION
# CHECK-NEXT: Index: 3
Expand Down
1 change: 0 additions & 1 deletion lld/test/wasm/shared.s
View file
Expand Up @@ -209,7 +209,6 @@ get_local_func_address:

# DIS: <__wasm_call_ctors>:
# DIS-EMPTY:
# DIS-NEXT: call 1
# DIS-NEXT: end

# DIS: <__wasm_apply_data_relocs>:
Expand Down
1 change: 0 additions & 1 deletion lld/test/wasm/shared64.s
View file
Expand Up @@ -216,7 +216,6 @@ get_local_func_address:

# DIS: <__wasm_call_ctors>:
# DIS-EMPTY:
# DIS-NEXT: call 1
# DIS-NEXT: end

# DIS: <__wasm_apply_data_relocs>:
Expand Down
3 changes: 3 additions & 0 deletions lld/test/wasm/tls-export.s
View file
Expand Up @@ -40,6 +40,9 @@ tls1:
# CHECK-NEXT: - Name: __wasm_call_ctors
# CHECK-NEXT: Kind: FUNCTION
# CHECK-NEXT: Index: 0
# CHECK-NEXT: - Name: __wasm_apply_data_relocs
# CHECK-NEXT: Kind: FUNCTION
# CHECK-NEXT: Index: 1
# CHECK-NEXT: - Name: tls1
# CHECK-NEXT: Kind: GLOBAL
# CHECK-NEXT: Index: 2
3 changes: 3 additions & 0 deletions lld/test/wasm/tls-non-shared-memory.s
View file
Expand Up @@ -127,6 +127,9 @@ tls1:
# PIE-NEXT: - Name: memory
# PIE-NEXT: Kind: MEMORY
# PIE-NEXT: Index: 0
# PIE-NEXT: - Name: __wasm_apply_data_relocs
# PIE-NEXT: Kind: FUNCTION
# PIE-NEXT: Index: 1
# PIE-NEXT: - Type:

# .tdata and .data are combined into single segment in PIC mode.
Expand Down
11 changes: 11 additions & 0 deletions lld/wasm/Driver.cpp
View file
Expand Up @@ -656,6 +656,17 @@ static void createSyntheticSymbols() {
is64 ? i64ArgSignature : i32ArgSignature,
"__wasm_init_tls"));
}

if (config->isPic ||
config->unresolvedSymbols == UnresolvedPolicy::ImportDynamic) {
// For PIC code, or when dynamically importing addresses, we create
// synthetic functions that apply relocations. These get called from
// __wasm_call_ctors before the user-level constructors.
WasmSym::applyDataRelocs = symtab->addSyntheticFunction(
"__wasm_apply_data_relocs",
WASM_SYMBOL_VISIBILITY_DEFAULT | WASM_SYMBOL_EXPORTED,
make<SyntheticFunction>(nullSignature, "__wasm_apply_data_relocs"));
}
}

static void createOptionalSymbols() {
Expand Down
27 changes: 4 additions & 23 deletions lld/wasm/Writer.cpp
View file
Expand Up @@ -1018,17 +1018,6 @@ void Writer::createSyntheticInitFunctions() {
WasmSym::tlsBase->markLive();
}

if (config->isPic ||
config->unresolvedSymbols == UnresolvedPolicy::ImportDynamic) {
// For PIC code, or when dynamically importing addresses, we create
// synthetic functions that apply relocations. These get called from
// __wasm_call_ctors before the user-level constructors.
WasmSym::applyDataRelocs = symtab->addSyntheticFunction(
"__wasm_apply_data_relocs", WASM_SYMBOL_VISIBILITY_HIDDEN,
make<SyntheticFunction>(nullSignature, "__wasm_apply_data_relocs"));
WasmSym::applyDataRelocs->markLive();
}

if (config->isPic && out.globalSec->needsRelocations()) {
WasmSym::applyGlobalRelocs = symtab->addSyntheticFunction(
"__wasm_apply_global_relocs", WASM_SYMBOL_VISIBILITY_HIDDEN,
Expand Down Expand Up @@ -1298,8 +1287,8 @@ void Writer::createStartFunction() {

// For -shared (PIC) output, we create create a synthetic function which will
// apply any relocations to the data segments on startup. This function is
// called `__wasm_apply_data_relocs` and is added at the beginning of
// `__wasm_call_ctors` before any of the constructors run.
// called `__wasm_apply_data_relocs` and is expected to be called before
// any user code (i.e. before `__wasm_call_ctors`).
void Writer::createApplyDataRelocationsFunction() {
LLVM_DEBUG(dbgs() << "createApplyDataRelocationsFunction\n");
// First write the body's contents to a string.
Expand Down Expand Up @@ -1352,11 +1341,9 @@ void Writer::createApplyGlobalTLSRelocationsFunction() {
// Create synthetic "__wasm_call_ctors" function based on ctor functions
// in input object.
void Writer::createCallCtorsFunction() {
// If __wasm_call_ctors isn't referenced, there aren't any ctors, and we
// aren't calling `__wasm_apply_data_relocs` for Emscripten-style PIC, don't
// If __wasm_call_ctors isn't referenced, there aren't any ctors, don't
// define the `__wasm_call_ctors` function.
if (!WasmSym::callCtors->isLive() && !WasmSym::applyDataRelocs &&
initFunctions.empty())
if (!WasmSym::callCtors->isLive() && initFunctions.empty())
return;

// First write the body's contents to a string.
Expand All @@ -1365,12 +1352,6 @@ void Writer::createCallCtorsFunction() {
raw_string_ostream os(bodyContent);
writeUleb128(os, 0, "num locals");

if (WasmSym::applyDataRelocs) {
writeU8(os, WASM_OPCODE_CALL, "CALL");
writeUleb128(os, WasmSym::applyDataRelocs->getFunctionIndex(),
"function index");
}

// Call constructors
for (const WasmInitEntry &f : initFunctions) {
writeU8(os, WASM_OPCODE_CALL, "CALL");
Expand Down

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