[ELF] Set values for bss_start and end symbols properly, If there are two load segments with RW permissions, bss_start and end may get set inappropriate

llvm-svn: 176795

lld/lib/ReaderWriter/ELF/ExecutableWriter.h

@@ -108,10 +108,10 @@ void ExecutableWriter<ELFT>::finalizeDefaultAtomValues() {
            endAtomIter == this->_layout->absoluteAtoms().end()) &&
          "Unable to find the absolute atoms that have been added by lld");
 
-  auto phe = this->_programHeader->findProgramHeader(
-      llvm::ELF::PT_LOAD, llvm::ELF::PF_W, llvm::ELF::PF_X);
+  auto phe = this->_programHeader
+      ->findProgramHeader(llvm::ELF::PT_LOAD, llvm::ELF::PF_W, llvm::ELF::PF_X);
 
-  assert(!(phe == this->_programHeader->end()) &&
+  assert(!(phe == this->_programHeader->rend()) &&
          "Can't find a data segment in the program header!");
 
   (*bssStartAtomIter)->_virtualAddr = (*phe)->p_vaddr + (*phe)->p_filesz;

lld/lib/ReaderWriter/ELF/HeaderChunks.h

@@ -89,6 +89,7 @@ class ProgramHeader : public Chunk<ELFT> {
 public:
   typedef llvm::object::Elf_Phdr_Impl<ELFT> Elf_Phdr;
   typedef typename std::vector<Elf_Phdr *>::iterator PhIterT;
+  typedef typename std::reverse_iterator<PhIterT> ReversePhIterT;
 
   /// \brief Find a program header entry, given the type of entry that
   /// we are looking for
@@ -132,8 +133,9 @@ public:
   void write(ELFWriter *writer, llvm::FileOutputBuffer &buffer);
 
   /// \brief find a program header entry in the list of program headers
-  PhIterT findProgramHeader(uint64_t type, uint64_t flags, uint64_t flagClear) {
-    return std::find_if(_ph.begin(), _ph.end(), 
+  ReversePhIterT
+  findProgramHeader(uint64_t type, uint64_t flags, uint64_t flagClear) {
+    return std::find_if(_ph.rbegin(), _ph.rend(),
                         FindPhdr(type, flags, flagClear));
   }
 
@@ -145,6 +147,10 @@ public:
     return _ph.end();
   }
 
+  ReversePhIterT rbegin() { return _ph.rbegin(); }
+
+  ReversePhIterT rend() { return _ph.rend(); }
+
   virtual void doPreFlight() {}
 
   void finalize() {}