Index: ELF/Driver.cpp
===================================================================
--- ELF/Driver.cpp
+++ ELF/Driver.cpp
@@ -247,5 +247,6 @@
     Config->OutputFile = "a.out";
 
   // Write the result to the file.
+  Symtab.finalize();
   writeResult<ELFT>(&Symtab);
 }
Index: ELF/InputFiles.h
===================================================================
--- ELF/InputFiles.h
+++ ELF/InputFiles.h
@@ -173,6 +173,7 @@
   typedef typename llvm::object::ELFFile<ELFT>::Elf_Sym_Range Elf_Sym_Range;
 
   std::vector<SharedSymbol<ELFT>> SymbolBodies;
+  std::vector<StringRef> Undefs;
   StringRef SoName;
 
 public:
@@ -181,6 +182,8 @@
     return SymbolBodies;
   }
 
+  llvm::ArrayRef<StringRef> getUndefinedSymbols() { return Undefs; }
+
   static bool classof(const InputFile *F) {
     return F->kind() == Base::SharedKind &&
            cast<ELFFileBase<ELFT>>(F)->getELFKind() == Base::getStaticELFKind();
Index: ELF/InputFiles.cpp
===================================================================
--- ELF/InputFiles.cpp
+++ ELF/InputFiles.cpp
@@ -301,14 +301,14 @@
   uint32_t NumSymbols = std::distance(Syms.begin(), Syms.end());
   SymbolBodies.reserve(NumSymbols);
   for (const Elf_Sym &Sym : Syms) {
-    if (Sym.isUndefined())
-      continue;
-
     ErrorOr<StringRef> NameOrErr = Sym.getName(this->StringTable);
     error(NameOrErr.getError());
     StringRef Name = *NameOrErr;
 
-    SymbolBodies.emplace_back(this, Name, Sym);
+    if (Sym.isUndefined())
+      Undefs.push_back(Name);
+    else
+      SymbolBodies.emplace_back(this, Name, Sym);
   }
 }
 
Index: ELF/SymbolTable.h
===================================================================
--- ELF/SymbolTable.h
+++ ELF/SymbolTable.h
@@ -52,6 +52,7 @@
   void addSyntheticSym(StringRef Name, OutputSection<ELFT> &Section,
                        typename llvm::object::ELFFile<ELFT>::uintX_t Value);
   void addIgnoredSym(StringRef Name);
+  void finalize();
 
 private:
   Symbol *insert(SymbolBody *New);
@@ -61,6 +62,7 @@
   void checkCompatibility(std::unique_ptr<InputFile> &File);
   void init(uint16_t EMachine);
   void resolve(SymbolBody *Body);
+  SymbolBody *find(StringRef Name);
   void reportConflict(const Twine &Message, const SymbolBody &Old,
                       const SymbolBody &New, bool Warning);
 
Index: ELF/SymbolTable.cpp
===================================================================
--- ELF/SymbolTable.cpp
+++ ELF/SymbolTable.cpp
@@ -181,6 +181,13 @@
   return Sym;
 }
 
+template <class ELFT> SymbolBody *SymbolTable<ELFT>::find(StringRef Name) {
+  auto It = Symtab.find(Name);
+  if (It == Symtab.end())
+    return nullptr;
+  return It->second->Body;
+}
+
 template <class ELFT> void SymbolTable<ELFT>::addLazy(Lazy *New) {
   Symbol *Sym = insert(New);
   if (Sym->Body == New)
@@ -231,6 +238,20 @@
   addFile(std::move(File));
 }
 
+template <class ELFT> void SymbolTable<ELFT>::finalize() {
+  // This code takes care of the case in which shared libraries depend on
+  // the user program (not the other way, which is usual). Shared libraries
+  // may have undefined symbols, expecting that the user program provides
+  // the definitions for them. An example is BSD's __progname symbol.
+  // We need to put such symbols to the main program's .dynsym so that
+  // shared libraries can find them.
+  for (std::unique_ptr<SharedFile<ELFT>> &File : SharedFiles)
+    for (StringRef U : File->getUndefinedSymbols())
+      if (SymbolBody *Sym = find(U))
+        if (Sym->isDefined())
+          Sym->setUsedInDynamicReloc();
+}
+
 template class lld::elf2::SymbolTable<ELF32LE>;
 template class lld::elf2::SymbolTable<ELF32BE>;
 template class lld::elf2::SymbolTable<ELF64LE>;
Index: test/elf2/progname.s
===================================================================
--- /dev/null
+++ test/elf2/progname.s
@@ -0,0 +1,20 @@
+// RUN: llvm-mc -filetype=obj -triple=i686-unknown-linux %s -o %t.o
+// RUN: echo '.global __progname' > %t2.s
+// RUN: llvm-mc -filetype=obj -triple=i686-unknown-linux %t2.s -o %t2.o
+// RUN: ld.lld2 -shared %t2.o -o %t2.so
+// RUN: ld.lld2 -o %t %t.o %t2.so
+// RUN: llvm-readobj -dyn-symbols %t | FileCheck %s
+
+// CHECK:      Name:     __progname@ (58)
+// CHECK-NEXT: Value:    0x11000
+// CHECK-NEXT: Size:     0
+// CHECK-NEXT: Binding:  Global (0x1)
+// CHECK-NEXT: Type:     None (0x0)
+// CHECK-NEXT: Other:    0
+// CHECK-NEXT: Section:  .text (0x4)
+// CHECK-NEXT: }
+
+.global _start, __progname
+_start:
+__progname:
+  nop