[llvm-mc] - Do not crash when referencing undefined debug sections.

[grimar]

Summary

MC has code that pre-creates few debug sections:
https://github.com/llvm-mirror/llvm/blob/master/lib/MC/MCObjectFileInfo.cpp#L396

If users code has a reference to such section but does not redefine it,
MC code currently asserts, because still thinks they are normally defined.

The patch fixes the issue.

[dblaikie]

Sounds good to me, thanks!

llvm/trunk/lib/MC/ELFObjectWriter.cpp

if (Symbol.isAbsolute()) {
         if (MSD.SectionIndex >= ELF::SHN_LORESERVE)
           HasLargeSectionIndex = true;
       } else {
         MSD.SectionIndex = ELF::SHN_UNDEF;
       }
     } else {
       const MCSectionELF &Section =
           static_cast<const MCSectionELF &>(Symbol.getSection());
+
+      // We may end up with a situation when section symbol is technically
+      // defined, but should not be. That happens because we explicitly
+      // pre-create few .debug_* sections to have accessors.
+      // And if these sections were not really defined in the code, but were
+      // referenced, we simply error out.
+      if (!Section.isRegistered()) {
+        assert(static_cast<const MCSymbolELF &>(Symbol).getType() ==
+               ELF::STT_SECTION);
+        Ctx.reportError(SMLoc(),
+                        "Undefined section reference: " + Symbol.getName());
+        continue;
+      }
+
       if (Mode == NonDwoOnly && isDwoSection(Section))
         continue;
       MSD.SectionIndex = SectionIndexMap.lookup(&Section);
       assert(MSD.SectionIndex && "Invalid section index!");
       if (MSD.SectionIndex >= ELF::SHN_LORESERVE)
         HasLargeSectionIndex = true;
     }
 

llvm/trunk/test/MC/ELF/undefined-debug.s

+// RUN: not llvm-mc -filetype=obj -triple x86_64-pc-linux-gnu %s -o %t 2>&1 | FileCheck %s
+// CHECK: error: Undefined section reference: .debug_pubnames
+
+.section .foo,"",@progbits
+  .long  .debug_pubnames