From 79ace92fcd19c357d51a414109fb1121014c0173 Mon Sep 17 00:00:00 2001
From: George Rimar <grimar@accesssoftek.com>
Date: Wed, 5 Dec 2018 10:43:58 +0000
Subject: [PATCH] Recommit r348243 - "[llvm-mc] - Do not crash when referencing
 undefined debug sections."

The patch triggered an unrelated msan issue: LayoutOrder variable was not initialized.
(http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-fast/builds/26794/steps/check-llvm%20msan/logs/stdio)
It was fixed.

Original commit message:
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.

Differential revision: https://reviews.llvm.org/D55173
----
Modified : /llvm/trunk/lib/MC/ELFObjectWriter.cpp
Added : /llvm/trunk/test/MC/ELF/undefined-debug.s

llvm-svn: 348349
---
 llvm/lib/MC/ELFObjectWriter.cpp    | 14 ++++++++++++++
 llvm/lib/MC/MCFragment.cpp         |  2 +-
 llvm/test/MC/ELF/undefined-debug.s |  5 +++++
 3 files changed, 20 insertions(+), 1 deletion(-)
 create mode 100644 llvm/test/MC/ELF/undefined-debug.s

diff --git a/llvm/lib/MC/ELFObjectWriter.cpp b/llvm/lib/MC/ELFObjectWriter.cpp
index 5b8b013ad087..89f3b30cddd6 100644
--- a/llvm/lib/MC/ELFObjectWriter.cpp
+++ b/llvm/lib/MC/ELFObjectWriter.cpp
@@ -669,6 +669,20 @@ void ELFWriter::computeSymbolTable(
     } 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);
diff --git a/llvm/lib/MC/MCFragment.cpp b/llvm/lib/MC/MCFragment.cpp
index 0ebcf21a422e..f2e08184c721 100644
--- a/llvm/lib/MC/MCFragment.cpp
+++ b/llvm/lib/MC/MCFragment.cpp
@@ -238,7 +238,7 @@ MCFragment::~MCFragment() = default;
 MCFragment::MCFragment(FragmentType Kind, bool HasInstructions,
                        MCSection *Parent)
     : Kind(Kind), HasInstructions(HasInstructions), Parent(Parent),
-      Atom(nullptr), Offset(~UINT64_C(0)) {
+      Atom(nullptr), Offset(~UINT64_C(0)), LayoutOrder(0) {
   if (Parent && !isDummy())
     Parent->getFragmentList().push_back(this);
 }
diff --git a/llvm/test/MC/ELF/undefined-debug.s b/llvm/test/MC/ELF/undefined-debug.s
new file mode 100644
index 000000000000..95ead70ef971
--- /dev/null
+++ b/llvm/test/MC/ELF/undefined-debug.s
@@ -0,0 +1,5 @@
+// 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