From 6f9d49cdde65898c3fbd74a7a98e7438216b0ba5 Mon Sep 17 00:00:00 2001
From: Rui Ueyama <ruiu@google.com>
Date: Tue, 15 Jan 2019 18:30:23 +0000
Subject: [PATCH] Do not emit a corrupt symbol table entry for
 .rela_iplt_{start,end}.

If .rela.iplt does not exist, we used to emit a corrupt symbol table
that contains two symbols, .rela_iplt_{start,end}, pointing to a
nonexisting section.

This patch fixes the issue by setting section index 0 to the symbols
if .rel.iplt section does not exist.

Differential Revision: https://reviews.llvm.org/D56623

llvm-svn: 351218
---
 lld/ELF/Symbols.cpp            |  1 +
 lld/ELF/Symbols.h              |  3 ++-
 lld/ELF/Writer.cpp             | 22 ++++++++++++++++------
 lld/test/ELF/gnu-ifunc-empty.s | 16 ++++++++++++++++
 4 files changed, 35 insertions(+), 7 deletions(-)
 create mode 100644 lld/test/ELF/gnu-ifunc-empty.s

diff --git a/lld/ELF/Symbols.cpp b/lld/ELF/Symbols.cpp
index da7fdb5..a713ec5 100644
--- a/lld/ELF/Symbols.cpp
+++ b/lld/ELF/Symbols.cpp
@@ -38,6 +38,7 @@ Defined *ElfSym::GlobalOffsetTable;
 Defined *ElfSym::MipsGp;
 Defined *ElfSym::MipsGpDisp;
 Defined *ElfSym::MipsLocalGp;
+Defined *ElfSym::RelaIpltStart;
 Defined *ElfSym::RelaIpltEnd;
 
 static uint64_t getSymVA(const Symbol &Sym, int64_t &Addend) {
diff --git a/lld/ELF/Symbols.h b/lld/ELF/Symbols.h
index 803cb80..4d55405 100644
--- a/lld/ELF/Symbols.h
+++ b/lld/ELF/Symbols.h
@@ -352,7 +352,8 @@ struct ElfSym {
   static Defined *MipsGpDisp;
   static Defined *MipsLocalGp;
 
-  // __rela_iplt_end or __rel_iplt_end
+  // __rel{,a}_iplt_{start,end} symbols.
+  static Defined *RelaIpltStart;
   static Defined *RelaIpltEnd;
 };
 
diff --git a/lld/ELF/Writer.cpp b/lld/ELF/Writer.cpp
index 2f6a0bf..17f4c79 100644
--- a/lld/ELF/Writer.cpp
+++ b/lld/ELF/Writer.cpp
@@ -910,12 +910,18 @@ void PhdrEntry::add(OutputSection *Sec) {
 template <class ELFT> void Writer<ELFT>::addRelIpltSymbols() {
   if (Config->Relocatable || needsInterpSection())
     return;
-  StringRef S = Config->IsRela ? "__rela_iplt_start" : "__rel_iplt_start";
-  addOptionalRegular(S, In.RelaIplt, 0, STV_HIDDEN, STB_WEAK);
 
-  S = Config->IsRela ? "__rela_iplt_end" : "__rel_iplt_end";
-  ElfSym::RelaIpltEnd =
-      addOptionalRegular(S, In.RelaIplt, 0, STV_HIDDEN, STB_WEAK);
+  // By default, __rela_iplt_{start,end} belong to a dummy section 0
+  // because .rela.plt might be empty and thus removed from output.
+  // We'll override Out::ElfHeader with In.RelaIplt later when we are
+  // sure that .rela.plt exists in output.
+  ElfSym::RelaIpltStart = addOptionalRegular(
+      Config->IsRela ? "__rela_iplt_start" : "__rel_iplt_start",
+      Out::ElfHeader, 0, STV_HIDDEN, STB_WEAK);
+
+  ElfSym::RelaIpltEnd = addOptionalRegular(
+      Config->IsRela ? "__rela_iplt_end" : "__rel_iplt_end",
+      Out::ElfHeader, 0, STV_HIDDEN, STB_WEAK);
 }
 
 template <class ELFT>
@@ -949,8 +955,12 @@ template <class ELFT> void Writer<ELFT>::setReservedSymbolSections() {
     ElfSym::GlobalOffsetTable->Section = GotSection;
   }
 
-  if (ElfSym::RelaIpltEnd)
+  // .rela_iplt_{start,end} mark the start and the end of .rela.plt section.
+  if (ElfSym::RelaIpltStart && !In.RelaIplt->empty()) {
+    ElfSym::RelaIpltStart->Section = In.RelaIplt;
+    ElfSym::RelaIpltEnd->Section = In.RelaIplt;
     ElfSym::RelaIpltEnd->Value = In.RelaIplt->getSize();
+  }
 
   PhdrEntry *Last = nullptr;
   PhdrEntry *LastRO = nullptr;
diff --git a/lld/test/ELF/gnu-ifunc-empty.s b/lld/test/ELF/gnu-ifunc-empty.s
new file mode 100644
index 0000000..9079853
--- /dev/null
+++ b/lld/test/ELF/gnu-ifunc-empty.s
@@ -0,0 +1,16 @@
+// REQUIRES: x86
+
+// Verifies that .rela_iplt_{start,end} point to a dummy section
+// if .rela.iplt does not exist.
+
+// RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+// RUN: ld.lld -static %t.o -o %t.exe
+// RUN: llvm-objdump -syms %t.exe | FileCheck %s
+
+// CHECK: 0000000000200000 .text 00000000 .hidden __rela_iplt_end
+// CHECK: 0000000000200000 .text 00000000 .hidden __rela_iplt_start
+
+.globl _start
+_start:
+ movl $__rela_iplt_start, %edx
+ movl $__rela_iplt_end, %edx
-- 
2.7.4