[lld][WebAssembly] Ensure stub symbols always get address 0

Without this extra flag we can't distingish between stub functions and
functions that happen to have address 0 (relative to __table_base).

Adding this flag bit the base symbol class actually avoids growing the
SymbolUnion struct which would not be true if we added it to the
FunctionSymbol subclass (due to bitbacking).

The previous approach of setting it's table index to zero worked for
normal static relocations but not for `-fPIC` code.

See emscripten-core/emscripten#12819

Differential Revision: https://reviews.llvm.org/D92038

lld/test/wasm/weak-undefined-pic.s

@@ -0,0 +1,90 @@
+# Checks handling of undefined weak external functions.  When the
+# static linker decides they are undefined, check GOT relocations
+# resolve to zero (i.e. a global that contains zero.).
+#
+# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown -o %t.o %s
+# RUN: wasm-ld %t.o -o %t1.wasm
+# RUN: obj2yaml %t1.wasm | FileCheck %s
+#
+# With `--unresolved-symbols=ignore-all` the behaviour should be the same
+# as the default.>
+#
+# RUN: wasm-ld --unresolved-symbols=ignore-all %t.o -o %t2.wasm
+# RUN: obj2yaml %t2.wasm | FileCheck %s
+
+.globl get_foo_addr
+get_foo_addr:
+  .functype get_foo_addr () -> (i32)
+  global.get foo@GOT
+  end_function
+
+.globl _start
+_start:
+  .functype _start () -> ()
+  call get_foo_addr
+  end_function
+
+.weak foo
+.functype foo () -> (i32)
+
+# Verify that we do not generate dynamnic relocations for the GOT entry.
+
+# CHECK-NOT: __wasm_apply_relocs
+
+# Verify that we do not generate an import for foo
+
+# CHECK-NOT:  - Type:            IMPORT
+
+#      CHECK:   - Type:            GLOBAL
+# CHECK-NEXT:     Globals:
+# CHECK-NEXT:       - Index:           0
+# CHECK-NEXT:         Type:            I32
+# CHECK-NEXT:         Mutable:         true
+# CHECK-NEXT:         InitExpr:
+# CHECK-NEXT:           Opcode:          I32_CONST
+# CHECK-NEXT:           Value:           66560
+# Global 'undefined_weak:foo' representing the GOT entry for foo
+# Unlike other internal GOT entries that need to be mutable this one
+# is immutable and not updated by `__wasm_apply_relocs`
+# CHECK-NEXT:       - Index:           1
+# CHECK-NEXT:         Type:            I32
+# CHECK-NEXT:         Mutable:         false
+# CHECK-NEXT:         InitExpr:
+# CHECK-NEXT:           Opcode:          I32_CONST
+# CHECK-NEXT:           Value:           0
+
+#      CHECK:  - Type:            CUSTOM
+# CHECK-NEXT:    Name:            name
+# CHECK-NEXT:    FunctionNames:
+# CHECK-NEXT:      - Index:           0
+# CHECK-NEXT:        Name:            'undefined_weak:foo'
+# CHECK-NEXT:      - Index:           1
+# CHECK-NEXT:        Name:            get_foo_addr
+# CHECK-NEXT:      - Index:           2
+# CHECK-NEXT:        Name:            _start
+# CHECK-NEXT:    GlobalNames:
+# CHECK-NEXT:      - Index:           0
+# CHECK-NEXT:        Name:            __stack_pointer
+# CHECK-NEXT:      - Index:           1
+# CHECK-NEXT:        Name:            'undefined_weak:foo'
+
+# With `-pie` or `-shared` the resolution should is defered to the dynamic
+# linker and the function address should be imported as GOT.func.foo.
+#
+# RUN: wasm-ld --experimental-pic -pie %t.o -o %t3.wasm
+# RUN: obj2yaml %t3.wasm | FileCheck %s --check-prefix=IMPORT
+
+#      IMPORT:  - Type:            IMPORT
+#      IMPORT:      - Module:          GOT.func
+# IMPORT-NEXT:        Field:           foo
+# IMPORT-NEXT:        Kind:            GLOBAL
+# IMPORT-NEXT:        GlobalType:      I32
+# IMPORT-NEXT:        GlobalMutable:   true
+
+#      IMPORT:     GlobalNames:
+# IMPORT-NEXT:       - Index:           0
+# IMPORT-NEXT:         Name:            __memory_base
+# IMPORT-NEXT:       - Index:           1
+# IMPORT-NEXT:         Name:            __table_base
+# IMPORT-NEXT:       - Index:           2
+# IMPORT-NEXT:         Name:            foo

lld/wasm/Driver.cpp

@@ -973,7 +973,7 @@ void LinkerDriver::link(ArrayRef<const char *> argsArr) {
       warn(Twine("symbol exported via --export not found: ") + arg->getValue());
   }
 
-  if (!config->relocatable) {
+  if (!config->relocatable && !config->isPic) {
     // Add synthetic dummies for weak undefined functions.  Must happen
     // after LTO otherwise functions may not yet have signatures.
     symtab->handleWeakUndefines();

lld/wasm/MarkLive.cpp

@@ -142,7 +142,7 @@ void MarkLive::mark() {
           reloc.Type == R_WASM_TABLE_INDEX_I32 ||
           reloc.Type == R_WASM_TABLE_INDEX_I64) {
         auto *funcSym = cast<FunctionSymbol>(sym);
-        if (funcSym->hasTableIndex() && funcSym->getTableIndex() == 0)
+        if (funcSym->isStub)
           continue;
       }
 

lld/wasm/Relocations.cpp

@@ -62,7 +62,9 @@ static void reportUndefined(Symbol *sym) {
                      << "ignoring undefined symbol: " + toString(*sym) + "\n");
           f->stubFunction = symtab->createUndefinedStub(*f->getSignature());
           f->stubFunction->markLive();
-          f->setTableIndex(0);
+          // Mark the function itself as a stub which prevents it from being
+          // assigned a table entry.
+          f->isStub = true;
         }
       }
       break;

lld/wasm/SymbolTable.cpp

@@ -673,9 +673,9 @@ InputFunction *SymbolTable::replaceWithUnreachable(Symbol *sym,
   // to be exported outside the object file.
   replaceSymbol<DefinedFunction>(sym, debugName, WASM_SYMBOL_BINDING_LOCAL,
                                  nullptr, func);
-  // Ensure it compares equal to the null pointer, and so that table relocs
-  // don't pull in the stub body (only call-operand relocs should do that).
-  func->setTableIndex(0);
+  // Ensure the stub function doesn't get a table entry.  Its address
+  // should always compare equal to the null pointer.
+  sym->isStub = true;
   return func;
 }
 

lld/wasm/Symbols.h

@@ -124,7 +124,7 @@ class Symbol {
   Symbol(StringRef name, Kind k, uint32_t flags, InputFile *f)
       : name(name), file(f), symbolKind(k), referenced(!config->gcSections),
         requiresGOT(false), isUsedInRegularObj(false), forceExport(false),
-        canInline(false), traced(false), flags(flags) {}
+        canInline(false), traced(false), isStub(false), flags(flags) {}
 
   StringRef name;
   InputFile *file;
@@ -157,6 +157,11 @@ class Symbol {
   // True if this symbol is specified by --trace-symbol option.
   bool traced : 1;
 
+  // True if this symbol is a linker-synthesized stub function (traps when
+  // called) and should otherwise be treated as missing/undefined.  See
+  // SymbolTable::replaceWithUndefined.
+  bool isStub : 1;
+
   uint32_t flags;
 };
 

lld/wasm/SyntheticSections.cpp

@@ -303,6 +303,8 @@ void GlobalSection::generateRelocationCode(raw_ostream &os) const {
       writeU8(os, opcode_ptr_const, "CONST");
       writeSleb128(os, d->getVirtualAddress(), "offset");
     } else if (auto *f = dyn_cast<FunctionSymbol>(sym)) {
+      if (f->isStub)
+        continue;
       // Get __table_base
       writeU8(os, WASM_OPCODE_GLOBAL_GET, "GLOBAL_GET");
       writeUleb128(os, WasmSym::tableBase->getGlobalIndex(), "__table_base");
@@ -329,12 +331,16 @@ void GlobalSection::writeBody() {
   // TODO(wvo): when do these need I64_CONST?
   for (const Symbol *sym : internalGotSymbols) {
     WasmGlobal global;
-    global.Type = {WASM_TYPE_I32, config->isPic};
+    // In the case of dynamic linking, internal GOT entries
+    // need to be mutable since they get updated to the correct
+    // runtime value during `__wasm_apply_relocs`.
+    bool mutable_ = config->isPic & !sym->isStub;
+    global.Type = {WASM_TYPE_I32, mutable_};
     global.InitExpr.Opcode = WASM_OPCODE_I32_CONST;
     if (auto *d = dyn_cast<DefinedData>(sym))
       global.InitExpr.Value.Int32 = d->getVirtualAddress();
     else if (auto *f = dyn_cast<FunctionSymbol>(sym))
-      global.InitExpr.Value.Int32 = f->getTableIndex();
+      global.InitExpr.Value.Int32 = f->isStub ? 0 : f->getTableIndex();
     else {
       assert(isa<UndefinedData>(sym));
       global.InitExpr.Value.Int32 = 0;
@@ -375,7 +381,10 @@ void StartSection::writeBody() {
 }
 
 void ElemSection::addEntry(FunctionSymbol *sym) {
-  if (sym->hasTableIndex())
+  // Don't add stub functions to the wasm table.  The address of all stub
+  // functions should be zero and they should they don't appear in the table.
+  // They only exist so that the calls to missing functions can validate.
+  if (sym->hasTableIndex() || sym->isStub)
     return;
   sym->setTableIndex(config->tableBase + indirectFunctions.size());
   indirectFunctions.emplace_back(sym);