[WebAssembly] Disallow weak undefined globals in the object format

[ncw]

Summary

This implements https://github.com/WebAssembly/tool-conventions/pull/47

---

Split out of D44028

[sbc100]

LGTM, with comments.

lib/Object/WasmObjectFile.cpp

426 ↗

(On Diff #137361)

Add and test for this error maybe?

Should the error be more specific?

Do we also want to add an error to MC/WasmObjectWriter.cpp?

[ncw]

lib/Object/WasmObjectFile.cpp

426 ↗

(On Diff #137361)

Done, on all three counts.

llvm/trunk/lib/MC/WasmObjectWriter.cpp

if (!WS.isDefined() && !WS.isComdat()) {
         wasm::WasmImport Import;
         Import.Module = WS.getModuleName();
         Import.Field = WS.getName();
         Import.Kind = wasm::WASM_EXTERNAL_FUNCTION;
         Import.SigIndex = getFunctionType(WS);
         Imports.push_back(Import);
         WasmIndices[&WS] = NumFunctionImports++;
       } else if (WS.isGlobal()) {
+        if (WS.isWeak())
+          report_fatal_error("undefined global symbol cannot be weak");
+
         wasm::WasmImport Import;
         Import.Module = WS.getModuleName();
         Import.Field = WS.getName();
         Import.Kind = wasm::WASM_EXTERNAL_GLOBAL;
         Import.Global = WS.getGlobalType();
         Imports.push_back(Import);
         WasmIndices[&WS] = NumGlobalImports++;
       }

llvm/trunk/lib/Object/WasmObjectFile.cpp

case wasm::WASM_SYMBOL_TYPE_FUNCTION:
       break;
 
     case wasm::WASM_SYMBOL_TYPE_GLOBAL:
       Info.ElementIndex = readVaruint32(Ptr);
       if (!isValidGlobalIndex(Info.ElementIndex) ||
           IsDefined != isDefinedGlobalIndex(Info.ElementIndex))
         return make_error<GenericBinaryError>("invalid global symbol index",
                                               object_error::parse_failed);
+      if (!IsDefined &&
+          (Info.Flags & wasm::WASM_SYMBOL_BINDING_MASK) ==
+              wasm::WASM_SYMBOL_BINDING_WEAK)
+        return make_error<GenericBinaryError>("undefined weak global symbol",
+                                              object_error::parse_failed);
       if (IsDefined) {
         Info.Name = readString(Ptr);
         unsigned GlobalIndex = Info.ElementIndex - NumImportedGlobals;
         wasm::WasmGlobal &Global = Globals[GlobalIndex];
         GlobalType = &Global.Type;
         if (Global.Name.empty()) {
           // Use the symbol's name to set a name for the Global, but only if
           // one hasn't already been set.

llvm/trunk/test/ObjectYAML/wasm/invalid_global_weak.yaml

+# RUN: yaml2obj < %s | not obj2yaml 2>&1 | FileCheck %s
+
+--- !WASM
+FileHeader:
+  Version:         0x00000001
+Sections:
+  - Type:            IMPORT
+    Imports:
+      - Module:          fiz
+        Field:           imported_global
+        Kind:            GLOBAL
+        GlobalType:      I32
+        GlobalMutable:   false
+  - Type:            CUSTOM
+    Name:            linking
+    SymbolTable:
+      - Index:           0
+        Kind:            GLOBAL
+        Name:            imported_global
+        Flags:           [ BINDING_WEAK, UNDEFINED ]
+        Global:          0
+...
+
+# CHECK: Error reading file: <stdin>: undefined weak global symbol

llvm/trunk/test/tools/llvm-nm/wasm/weak-symbols.yaml

Signatures:
         ParamTypes:
           - I32
   - Type:            IMPORT
     Imports:
       - Module:          env
         Field:           weak_import_func
         Kind:            FUNCTION
         SigIndex:        0
-      - Module:          env
-        Field:           weak_import_global
-        Kind:            GLOBAL
-        GlobalType:      I32
-        GlobalMutable:   false
   - Type:            FUNCTION
     FunctionTypes: [ 0 ]
   - Type:            GLOBAL
     Globals:
-      - Index:       1
+      - Index:       0
         Type:        I32
         Mutable:     false
         InitExpr:
           Opcode:          I64_CONST
           Value:           32
   - Type:            CODE
     Functions:
       - Index:           1
SymbolTable:
          Kind:            FUNCTION
          Name:            weak_defined_func
          Flags:           [ BINDING_WEAK ]
          Function:        1
        - Index:           2
          Kind:            GLOBAL
          Name:            weak_defined_global
          Flags:           [ BINDING_WEAK ]
-         Global:          1
+         Global:          0
        - Index:           3
          Kind:            DATA
          Name:            weak_import_data
          Flags:           [ BINDING_WEAK, UNDEFINED ]
        - Index:           4
          Kind:            FUNCTION
          Name:            weak_import_func
          Flags:           [ BINDING_WEAK, UNDEFINED ]
          Function:        0
-       - Index:           5
-         Kind:            GLOBAL
-         Name:            weak_import_global
-         Flags:           [ BINDING_WEAK, UNDEFINED ]
-         Global:          0
     SegmentInfo:
       - Index:           0
         Name:            .rodata.constantData
         Alignment:       1
         Flags:           [  ]
 
 
 # CHECK:      00000000 W weak_defined_data
 # CHECK-NEXT: 00000001 W weak_defined_func
-# CHECK-NEXT: 00000001 W weak_defined_global
+# CHECK-NEXT: 00000000 W weak_defined_global
 # CHECK-NEXT:          w weak_import_data
 # CHECK-NEXT:          w weak_import_func
-# CHECK-NEXT:          w weak_import_global