[WebAssembly][lld] Fix crash when applying relocations to debug sections

[sbc100]

Summary

Debug sections are special in that they can contain relocations against
symbols that are not present in the final output (i.e. not live).
However it is also possible to have R_WASM_TABLE_INDEX relocations
against symbols that don't have a table index assigned (since they are
not address taken by actual code.

[ruiu]

LGTM

lld/test/wasm/debuginfo-relocs.s

+# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown -o %t.o %s
+# RUN: wasm-ld %t.o -o %t.wasm
+# RUN: obj2yaml %t.wasm | FileCheck %s
+
+bar:
+    .functype bar () -> ()
+    end_function
+
+    .globl _start
+_start:
+    .functype _start () -> ()
+    call bar
+    end_function
+
+    .section .debug_info,"",@
+    .int32 bar
+
+# Even though `bar` is live in the final binary it doesn't have a table entry
+# since its not address taken in the code.  In this case any relocations in the
+# debug sections see a address of zero.
+
+# CHECK:         Name:            .debug_info
+# CHECK-NEXT:    Payload:         '00000000'

lld/wasm/InputChunks.cpp

void InputChunk::writeTo(uint8_t *buf) const {
   // Apply relocations
   if (relocations.empty())
     return;
 
 #ifndef NDEBUG
   verifyRelocTargets();
 #endif
 
-  LLVM_DEBUG(dbgs() << "applying relocations: " << getName()
+  LLVM_DEBUG(dbgs() << "applying relocations: " << toString(this)
                     << " count=" << relocations.size() << "\n");
   int32_t off = outputOffset - getInputSectionOffset();
 
   for (const WasmRelocation &rel : relocations) {
     uint8_t *loc = buf + rel.Offset + off;
     uint32_t value = file->calcNewValue(rel);
     LLVM_DEBUG(dbgs() << "apply reloc: type=" << relocTypeToString(rel.Type));
     if (rel.Type != R_WASM_TYPE_INDEX_LEB)

lld/wasm/InputFiles.cpp

if (reloc.Type != R_WASM_TYPE_INDEX_LEB) {
     if ((isa<FunctionSymbol>(sym) || isa<DataSymbol>(sym)) && !sym->isLive())
       return 0;
   }
 
   switch (reloc.Type) {
   case R_WASM_TABLE_INDEX_I32:
   case R_WASM_TABLE_INDEX_SLEB:
   case R_WASM_TABLE_INDEX_REL_SLEB:
-    if (config->isPic && !getFunctionSymbol(reloc.Index)->hasTableIndex())
+    if (!getFunctionSymbol(reloc.Index)->hasTableIndex())
       return 0;
     return getFunctionSymbol(reloc.Index)->getTableIndex();
   case R_WASM_MEMORY_ADDR_SLEB:
   case R_WASM_MEMORY_ADDR_I32:
   case R_WASM_MEMORY_ADDR_LEB:
   case R_WASM_MEMORY_ADDR_REL_SLEB:
     if (isa<UndefinedData>(sym))
       return 0;

llvm/lib/MC/MCParser/WasmAsmParser.cpp

auto Kind = StringSwitch<Optional<SectionKind>>(Name)
                     .StartsWith(".data", SectionKind::getData())
                     .StartsWith(".rodata", SectionKind::getReadOnly())
                     .StartsWith(".text", SectionKind::getText())
                     .StartsWith(".custom_section", SectionKind::getMetadata())
                     .StartsWith(".bss", SectionKind::getBSS())
                     // See use of .init_array in WasmObjectWriter and
                     // TargetLoweringObjectFileWasm
                     .StartsWith(".init_array", SectionKind::getData())
+                    .StartsWith(".debug_", SectionKind::getMetadata())
                     .Default(Optional<SectionKind>());
     if (!Kind.hasValue())
       return Parser->Error(Lexer->getLoc(), "unknown section kind: " + Name);
 
     MCSectionWasm *Section = getContext().getWasmSection(Name, Kind.getValue());
 
     // Update section flags if present in this .section directive
     bool Passive = false;