[lld-macho] Error on encountering undefined symbols

[int3]

Summary

... instead of silently emitting a reference to the zero address.

Depends on D80049.

[smeenai]

LGTM, thanks for fixing!

LLD ELF does this as part of its scanRelocations loop. That might be nicer cos we'd bail out with undefined symbol errors earlier. It doesn't matter a whole lot right now, but we could add a TODO.

[int3]

move to scanRelocations

lld/MachO/InputSection.cpp

 //===- InputSection.cpp ---------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "InputSection.h"
 #include "OutputSegment.h"
 #include "Symbols.h"
 #include "Target.h"
 #include "lld/Common/Memory.h"
 #include "llvm/Support/Endian.h"
 
+using namespace llvm;
 using namespace llvm::MachO;
 using namespace llvm::support;
 using namespace lld;
 using namespace lld::macho;
 
 std::vector<InputSection *> macho::inputSections;
 
 uint64_t InputSection::getFileOffset() const {

lld/MachO/Writer.cpp

 private:
   // Recent versions of Darwin won't run any binary that has dyld at a
   // different location.
   const StringRef path = "/usr/lib/dyld";
 };
 } // namespace
 
 void Writer::scanRelocations() {
-  for (InputSection *sect : inputSections)
-    for (Reloc &r : sect->relocs)
-      if (auto *s = r.target.dyn_cast<Symbol *>())
-        if (auto *dylibSymbol = dyn_cast<DylibSymbol>(s))
+  for (InputSection *isec : inputSections) {
+    for (Reloc &r : isec->relocs) {
+      if (auto *s = r.target.dyn_cast<Symbol *>()) {
+        if (isa<Undefined>(s))
+          error("undefined symbol " + s->getName() + ", referenced from " +
+                sys::path::filename(isec->file->getName()));
+        else if (auto *dylibSymbol = dyn_cast<DylibSymbol>(s))
           target->prepareDylibSymbolRelocation(*dylibSymbol, r.type);
       }
+    }
+  }
+}
 
 void Writer::createLoadCommands() {
   headerSection->addLoadCommand(
       make<LCDyldInfo>(bindingSection, lazyBindingSection, exportSection));
   headerSection->addLoadCommand(
       make<LCSymtab>(symtabSection, stringTableSection));
   headerSection->addLoadCommand(make<LCDysymtab>());
 

lld/test/MachO/invalid/undefined-symbol.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %s -o %t.o
+# RUN: not lld -flavor darwinnew -Z -o %t %t.o 2>&1 | FileCheck %s -DBASENAME=%basename_t
+# CHECK: error: undefined symbol _foo, referenced from [[BASENAME]]
+
+.globl _main
+.text
+_main:
+  callq _foo
+  movq $0, %rax
+  retq