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

When wrapping a symbol `foo` via `-wrap=foo`, we create the symbol
`__wrap_foo` that replaces all mentions of `foo`. This feature was
implemented for wasm-ld in commit a5ca34e.

So far, no valid signature has been attached to the undefined symbol,
leading to a nullptr dereference in the logic for creating the import
section. This change adds the correct signature to the wrapped symbol,
enabling the generation of an import for it.

Author: Lukasdoe

lld/test/wasm/wrap_import.s

@@ -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
+
+# CHECK:        FunctionNames:
+# CHECK-NEXT:      - Index:           0
+# CHECK-NEXT:        Name:            __wrap_foo
+# CHECK-NEXT:      - Index:           1
+# CHECK-NEXT:        Name:            _start

lld/wasm/Driver.cpp

@@ -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);
 }
 
@@ -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

lld/wasm/SyntheticSections.cpp

@@ -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);