[lld][WebAssembly] Fix crash on function signature mismatch with --relocatable

[sbc100]

Summary

These stub new function were not being added to the symbol table
which in turn meant that we were crashing when trying to output
relocations against them.

Fixes: PR45645

[aheejin]

lld/wasm/SymbolTable.cpp

648

I'm not very familiar with this part of code. Does making the symbols local here prevent this from being emitted in the output object file? Why not sym->getName() bug debugName now?

[sbc100]

lld/wasm/SymbolTable.cpp

648

No, the code is still emitted, its just that its marked a local in the symbol table.

Without this change the symbol doesn't get added the symbol table at all, which will then cause the crash when wriing the relocation data.

See the assumptions being made when deciding which symbol to add to the table:
https://github.com/llvm/llvm-project/blob/10bc12588dac532fad044b2851dde8e7b9121e88/lld/wasm/Writer.cpp#L562

This code (not unreasoably) assumes that symbol are either part of the global shared symbol table, or they are local symbols in a given object. Without this bugfix this new symbol would not be local to an object or in the symbol table.

I change the name here to match the debug name so its value in the symbol table shows up as signature_mismatch:ret32' rather then just ret32. Since its a local symbol its name doesn't actually matter to the future link step.

[aheejin]

Thanks for fixing this so quickly!

lld/test/wasm/signature-mismatch.ll

 ; RUN: llc -filetype=obj %p/Inputs/ret32.ll -o %t.ret32.o
 ; RUN: llc -filetype=obj %p/Inputs/call-ret32.ll -o %t.call.o
 ; RUN: llc -filetype=obj %s -o %t.main.o
+
 ; RUN: wasm-ld --export=call_ret32 --export=ret32 -o %t.wasm %t.main.o %t.ret32.o %t.call.o 2>&1 | FileCheck %s -check-prefix=WARN
 ; RUN: obj2yaml %t.wasm | FileCheck %s -check-prefix=YAML
+
+; RUN: wasm-ld -r -o %t.reloc.o %t.main.o %t.ret32.o %t.call.o 2>&1 | FileCheck %s -check-prefix=WARN
+; RUN: obj2yaml %t.reloc.o | FileCheck %s -check-prefix=RELOC
+
 ; RUN: not wasm-ld --fatal-warnings -o %t.wasm %t.main.o %t.ret32.o %t.call.o 2>&1 | FileCheck %s -check-prefix=ERROR
 
 target triple = "wasm32-unknown-unknown"
 
 @ret32_address_main = global i32 (i32, i64, i32)* @ret32, align 4
 
 ; Function Attrs: nounwind
 define hidden void @_start() local_unnamed_addr {

 ; YAML-NEXT:       - Index:           1
 ; YAML-NEXT:         Name:            _start
 ; YAML-NEXT:       - Index:           2
 ; YAML-NEXT:         Name:            ret32
 ; YAML-NEXT:       - Index:           3
 ; YAML-NEXT:         Name:            call_ret32
 ; YAML-NEXT: ...
 
+;      RELOC:     Name:            linking
+; RELOC-NEXT:     Version:         2
+; RELOC-NEXT:     SymbolTable:
+; RELOC-NEXT:       - Index:           0
+; RELOC-NEXT:         Kind:            FUNCTION
+; RELOC-NEXT:         Name:            _start
+; RELOC-NEXT:         Flags:           [ VISIBILITY_HIDDEN ]
+; RELOC-NEXT:         Function:        1
+; RELOC-NEXT:       - Index:           1
+; RELOC-NEXT:         Kind:            FUNCTION
+; RELOC-NEXT:         Name:            ret32
+; RELOC-NEXT:         Flags:           [ VISIBILITY_HIDDEN ]
+; RELOC-NEXT:         Function:        2
+; RELOC-NEXT:       - Index:           2
+; RELOC-NEXT:         Kind:            DATA
+; RELOC-NEXT:         Name:            ret32_address_main
+; RELOC-NEXT:         Flags:           [  ]
+; RELOC-NEXT:         Segment:         0
+; RELOC-NEXT:         Size:            4
+; RELOC-NEXT:       - Index:           3
+; RELOC-NEXT:         Kind:            FUNCTION
+; RELOC-NEXT:         Name:            call_ret32
+; RELOC-NEXT:         Flags:           [ VISIBILITY_HIDDEN ]
+; RELOC-NEXT:         Function:        3
+; RELOC-NEXT:       - Index:           4
+; RELOC-NEXT:         Kind:            DATA
+; RELOC-NEXT:         Name:            ret32_address
+; RELOC-NEXT:         Flags:           [  ]
+; RELOC-NEXT:         Segment:         1
+; RELOC-NEXT:         Size:            4
+; RELOC-NEXT:       - Index:           5
+; RELOC-NEXT:         Kind:            FUNCTION
+; RELOC-NEXT:         Name:            'signature_mismatch:ret32'
+; RELOC-NEXT:         Flags:           [ BINDING_LOCAL ]
+; RELOC-NEXT:         Function:        0

lld/wasm/SymbolTable.cpp

 // equivalent of an undefined function and also handleSymbolVariants for
 // undefined functions that don't match the signature of the definition.
 InputFunction *SymbolTable::replaceWithUnreachable(Symbol *sym,
                                                    const WasmSignature &sig,
                                                    StringRef debugName) {
   auto *func = make<SyntheticFunction>(sig, sym->getName(), debugName);
   func->setBody(unreachableFn);
   syntheticFunctions.emplace_back(func);
-  replaceSymbol<DefinedFunction>(sym, sym->getName(), sym->flags, nullptr,
-                                 func);
+  // Mark new symbols as local. For relocatable output we don't want them
+  // to be exported outside the object file.
+  replaceSymbol<DefinedFunction>(sym, debugName, WASM_SYMBOL_BINDING_LOCAL,
+                                 nullptr, func);
   return func;
 }
 
 // For weak undefined functions, there may be "call" instructions that reference
 // the symbol. In this case, we need to synthesise a dummy/stub function that
 // will abort at runtime, so that relocations can still provided an operand to
 // the call instruction that passes Wasm validation.
 void SymbolTable::handleWeakUndefines() {