[MC][MachO] Emit an error for emitting relocations of the form -SYM + cst

[thegameg]

Summary

Emit an error for an unsupported relocation. Mach-o relocations can't encode the form -SYM + cst.

[pete]

LGTM. Thanks!

llvm/lib/MC/MachObjectWriter.cpp

 //===- lib/MC/MachObjectWriter.cpp - Mach-O File Writer -------------------===//
 //
 // 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 "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/Twine.h"
 #include "llvm/ADT/iterator_range.h"
 #include "llvm/BinaryFormat/MachO.h"
 #include "llvm/MC/MCAsmBackend.h"
 #include "llvm/MC/MCAsmLayout.h"
 #include "llvm/MC/MCAssembler.h"
+#include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCDirectives.h"
 #include "llvm/MC/MCExpr.h"
 #include "llvm/MC/MCFixupKindInfo.h"
 #include "llvm/MC/MCFragment.h"
 #include "llvm/MC/MCMachObjectWriter.h"
 #include "llvm/MC/MCObjectWriter.h"
 #include "llvm/MC/MCSection.h"
 #include "llvm/MC/MCSectionMachO.h"
void MachObjectWriter::writeLinkerOptionsLoadCommand(
   }
 
   // Pad to a multiple of the pointer size.
   W.OS.write_zeros(OffsetToAlignment(BytesWritten, is64Bit() ? 8 : 4));
 
   assert(W.OS.tell() - Start == Size);
 }
 
+static bool isFixupTargetValid(const MCValue &Target) {
+  // Target is (LHS - RHS + cst).
+  // We don't support the form where LHS is null: -RHS + cst
+  if (!Target.getSymA() && Target.getSymB())
+    return false;
+  return true;
+}
+
 void MachObjectWriter::recordRelocation(MCAssembler &Asm,
                                         const MCAsmLayout &Layout,
                                         const MCFragment *Fragment,
                                         const MCFixup &Fixup, MCValue Target,
                                         uint64_t &FixedValue) {
+  if (!isFixupTargetValid(Target)) {
+    Asm.getContext().reportError(Fixup.getLoc(),
+                                 "unsupported relocation expression");
+    return;
+  }
+
   TargetObjectWriter->recordRelocation(this, Asm, Layout, Fragment, Fixup,
                                        Target, FixedValue);
 }
 
 void MachObjectWriter::bindIndirectSymbols(MCAssembler &Asm) {
   // This is the point where 'as' creates actual symbols for indirect symbols
   // (in the following two passes). It would be easier for us to do this sooner
   // when we see the attribute, but that makes getting the order in the symbol

llvm/test/MC/MachO/bad-reloc.s

+// RUN: not llvm-mc -triple x86_64-apple-darwin %s -filetype=obj -o - 2>&1 | FileCheck %s
+
+.quad (0 - undef)
+
+// CHECK: error: unsupported relocation expression