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[lld][WebAssembly] Fix SEGFAULT when importing wrapped symbol #169656

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Merged
merged 3 commits into from
Nov 27, 2025
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[lld][WebAssembly] Fix SEGFAULT when importing wrapped symbol #169656

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7c377f0
Fix SEGFAULT in wasm-ld when importing wrapped symbol
Lukasdoe Nov 26, 2025
1304c26
fixes from review
Lukasdoe Nov 27, 2025
7e29af3
Merge branch 'main' into fix-wasm-ld-wrap-import-segfault
Lukasdoe Nov 27, 2025
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32 changes: 32 additions & 0 deletions lld/test/wasm/wrap_import.s
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Original file line number Diff line number Diff line change
@@ -0,0 +1,32 @@
# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown %s -o %t.o
# RUN: wasm-ld -wrap nosuchsym -wrap foo -allow-undefined -o %t.wasm %t.o
# RUN: obj2yaml %t.wasm | FileCheck %s

.globl foo
.globl _start

foo:
.functype foo () -> ()
end_function

_start:
.functype _start () -> ()
call foo
end_function

# CHECK: - Type: IMPORT
# CHECK-NEXT: Imports:
# CHECK-NEXT: - Module: env
# CHECK-NEXT: Field: __wrap_foo
# CHECK-NEXT: Kind: FUNCTION
# CHECK-NEXT SigIndex: 0

# CHECK: - Type: CODE
# CHECK-NEXT: Functions:
# CHECK-NEXT: Index: 1
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@sbc100 sbc100 Nov 26, 2025

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Is this the right value here for the relocation? Shouldn't it be function zero (i.e. the __wrap_foo import) that is called here? It is this index 1 in the symbol table? If so you we should probably show the symbol table here too. (or just drop the Relocations and emit-relocs flag completely?)

Could be a bug in --emit-relocs maybe?

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@Lukasdoe Lukasdoe Nov 27, 2025

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I just stepped through the code a bit and I think this is intended behavior.

  - Type:            CODE
    Relocations:
      - Type:            R_WASM_FUNCTION_INDEX_LEB
        Index:           1
        Offset:          0x4
    Functions:
      - Index:           1
        Locals:          []
        Body:            1080808080000B
  - Type:            CUSTOM
    Name:            linking
    Version:         2
    SymbolTable:
      - Index:           0
        Kind:            FUNCTION
        Name:            _start
        Flags:           [  ]
        Function:        1
      - Index:           1
        Kind:            FUNCTION
        Name:            __wrap_foo
        Flags:           [ UNDEFINED ]
        Function:        0
  - Type:            CUSTOM
    Name:            name
    FunctionNames:
      - Index:           0
        Name:            __wrap_foo
      - Index:           1
        Name:            _start

The symbol index is a different one than the function index. __wrap_foo has symbol index 1, which is why the relocation is emitted for index 1, while _start has function index 1, explaining why this function is shown.


# CHECK: FunctionNames:
# CHECK-NEXT: - Index: 0
# CHECK-NEXT: Name: __wrap_foo
# CHECK-NEXT: - Index: 1
# CHECK-NEXT: Name: _start
8 changes: 5 additions & 3 deletions lld/wasm/Driver.cpp
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Expand Up @@ -1173,9 +1173,10 @@ struct WrappedSymbol {
Symbol *wrap;
};

static Symbol *addUndefined(StringRef name) {
static Symbol *addUndefined(StringRef name,
const WasmSignature *signature = nullptr) {
return symtab->addUndefinedFunction(name, std::nullopt, std::nullopt,
WASM_SYMBOL_UNDEFINED, nullptr, nullptr,
WASM_SYMBOL_UNDEFINED, nullptr, signature,
false);
}

Expand All @@ -1198,7 +1199,8 @@ static std::vector<WrappedSymbol> addWrappedSymbols(opt::InputArgList &args) {
continue;

Symbol *real = addUndefined(saver().save("__real_" + name));
Symbol *wrap = addUndefined(saver().save("__wrap_" + name));
Symbol *wrap =
addUndefined(saver().save("__wrap_" + name), sym->getSignature());
v.push_back({sym, real, wrap});

// We want to tell LTO not to inline symbols to be overwritten
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4 changes: 3 additions & 1 deletion lld/wasm/SyntheticSections.cpp
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Expand Up @@ -196,7 +196,9 @@ void ImportSection::addImport(Symbol *sym) {
StringRef module = sym->importModule.value_or(defaultModule);
StringRef name = sym->importName.value_or(sym->getName());
if (auto *f = dyn_cast<FunctionSymbol>(sym)) {
ImportKey<WasmSignature> key(*(f->getSignature()), module, name);
const WasmSignature *sig = f->getSignature();
assert(sig && "imported functions must have a signature");
ImportKey<WasmSignature> key(*sig, module, name);
auto entry = importedFunctions.try_emplace(key, numImportedFunctions);
if (entry.second) {
importedSymbols.emplace_back(sym);
Expand Down