[lld][macho] Error out gracefully when offset is outside literal section

lld/MachO/InputSection.cpp

@@ -348,6 +348,9 @@ WordLiteralInputSection::WordLiteralInputSection(const Section &section,
 }
 
 uint64_t WordLiteralInputSection::getOffset(uint64_t off) const {
+  if (off >= data.size())
+    fatal(toString(this) + ": offset is outside the section");
+
   auto *osec = cast<WordLiteralSection>(parent);
   const uintptr_t buf = reinterpret_cast<uintptr_t>(data.data());
   switch (sectionType(getFlags())) {

lld/test/MachO/invalid/bad-offsets.s

@@ -0,0 +1,45 @@
+## Test that we properly detect and report out-of-bounds offsets in literal sections.
+## We're intentionally testing fatal errors (for malformed input files), and
+## fatal errors aren't supported for testing when main is run twice.
+# XFAIL: main-run-twice
+
+# REQUIRES: x86
+# RUN: rm -rf %t; split-file %s %t
+
+## Test WordLiteralInputSection bounds checking
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/word-literal.s -o %t/word-literal.o
+# RUN: not %lld -dylib %t/word-literal.o -o /dev/null 2>&1 | FileCheck %s --check-prefix=WORD
+
+## Test CStringInputSection bounds checking
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/cstring.s -o %t/cstring.o
+# RUN: not %lld -dylib %t/cstring.o -o /dev/null 2>&1 | FileCheck %s --check-prefix=CSTRING
+
+# WORD: error: {{.*}}word-literal.o:(__literal4): offset is outside the section
+# CSTRING: error: {{.*}}cstring.o:(__cstring): offset is outside the section
+
+#--- word-literal.s
+.section __TEXT,__literal4,4byte_literals
+L_literal:
+  .long 0x01020304
+
+.text
+.globl _main
+_main:
+  # We use a subtractor expression to force a section relocation. Symbol relocations
+  # don't trigger the error.
+  .long L_literal - _main + 4
+
+.subsections_via_symbols
+
+#--- cstring.s
+## Create a cstring section with a reference that points past the end
+.cstring
+L_str:
+  .asciz "foo"
+
+.text
+.globl _main
+_main:
+  .long L_str - _main + 4
+
+.subsections_via_symbols