Index: ELF/Config.h
===================================================================
--- ELF/Config.h
+++ ELF/Config.h
@@ -25,6 +25,7 @@
   bool DiscardNone = false;
   bool ExportDynamic = false;
   bool NoInhibitExec = false;
+  bool AllowShlibUndefines = false;
 };
 
 extern Configuration *Config;
Index: ELF/Driver.cpp
===================================================================
--- ELF/Driver.cpp
+++ ELF/Driver.cpp
@@ -79,8 +79,10 @@
   if (!RPaths.empty())
     Config->RPath = llvm::join(RPaths.begin(), RPaths.end(), ":");
 
-  if (Args.hasArg(OPT_shared))
+  if (Args.hasArg(OPT_shared)) {
     Config->Shared = true;
+    Config->AllowShlibUndefines = true;
+  }
 
   if (Args.hasArg(OPT_discard_all))
     Config->DiscardAll = true;
@@ -97,6 +99,12 @@
   if (Args.hasArg(OPT_noinhibit_exec))
     Config->NoInhibitExec = true;
 
+  if (auto *arg = Args.getLastArg(OPT_allow_shlib_undefined,
+                                  OPT_no_allow_shlib_undefined)) {
+    Config->AllowShlibUndefines =
+        arg->getOption().getID() == OPT_allow_shlib_undefined;
+  }
+
   // Create a list of input files.
   std::vector<MemoryBufferRef> Inputs;
 
Index: ELF/InputFiles.cpp
===================================================================
--- ELF/InputFiles.cpp
+++ ELF/InputFiles.cpp
@@ -11,6 +11,7 @@
 #include "InputSection.h"
 #include "Error.h"
 #include "Symbols.h"
+#include "Writer.h"
 #include "llvm/ADT/STLExtras.h"
 
 using namespace llvm;
@@ -227,13 +228,13 @@
   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;
-
+    if (Sym.isUndefined()) {
+      undefError(Name, getName());
+      continue;
+    }
     SymbolBodies.emplace_back(Name, Sym);
   }
 }
Index: ELF/Options.td
===================================================================
--- ELF/Options.td
+++ ELF/Options.td
@@ -37,3 +37,12 @@
 
 def export_dynamic : Flag<["--"], "export-dynamic">,
      HelpText<"Put symbols in the dynamic symbol table">;
+
+def no_allow_shlib_undefined
+    : Flag<["--"], "no-allow-shlib-undefined">,
+      HelpText<"Do not allow undefined symbols from dynamic"
+               " library when creating executables">;
+
+def allow_shlib_undefined : Flag<["--"], "allow-shlib-undefined">,
+                            HelpText<"Allow undefined symbols from dynamic"
+                                     " library when creating executables">;
Index: ELF/Writer.h
===================================================================
--- ELF/Writer.h
+++ ELF/Writer.h
@@ -10,6 +10,10 @@
 #ifndef LLD_ELF_WRITER_H
 #define LLD_ELF_WRITER_H
 
+namespace llvm {
+class Twine;
+}
+
 namespace lld {
 namespace elf2 {
 
@@ -18,6 +22,8 @@
 template <class ELFT>
 void writeResult(SymbolTable *Symtab);
 
+void undefError(const llvm::Twine &Symbol, const llvm::Twine &SymFile);
+
 }
 }
 
Index: ELF/Writer.cpp
===================================================================
--- ELF/Writer.cpp
+++ ELF/Writer.cpp
@@ -171,6 +171,19 @@
 template void writeResult<ELF64LE>(SymbolTable *);
 template void writeResult<ELF64BE>(SymbolTable *);
 
+void undefError(const Twine &Symbol, const Twine &SymFile) {
+  if (Config->AllowShlibUndefines)
+    return;
+
+  std::string Message = Twine("undefined symbol: " + Symbol).str();
+  if (!SymFile.isTriviallyEmpty())
+    Message = Twine(Message.c_str() + Twine(" in ") + SymFile).str();
+  if (Config->NoInhibitExec)
+    warning(Message);
+  else
+    error(Message);
+}
+
 } // namespace elf2
 } // namespace lld
 
@@ -282,13 +295,7 @@
       SymFile = F.get();
   }
 
-  std::string Message = "undefined symbol: " + Sym.getName().str();
-  if (SymFile)
-    Message += " in " + SymFile->getName().str();
-  if (Config->NoInhibitExec)
-    warning(Message);
-  else
-    error(Message);
+  undefError(Sym.getName(), SymFile ? SymFile->getName() : "");
 }
 
 // Create output section objects and add them to OutputSections.
Index: test/elf2/Inputs/allow-shlib-undefined.s
===================================================================
--- test/elf2/Inputs/allow-shlib-undefined.s
+++ test/elf2/Inputs/allow-shlib-undefined.s
@@ -0,0 +1,3 @@
+.globl _shared
+_shared: 
+  call _unresolved
Index: test/elf2/allow-shlib-undefined.s
===================================================================
--- test/elf2/allow-shlib-undefined.s
+++ test/elf2/allow-shlib-undefined.s
@@ -0,0 +1,18 @@
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %p/Inputs/allow-shlib-undefined.s -o %t
+# RUN: lld -shared -flavor gnu2 %t -o %t.so 
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t1
+
+# Executable: does not link by default and with 
+# --no-allow-shlib-undefined but links with --allow-shlib-undefined
+# RUN: not lld -flavor gnu2 %t1 %t.so -o %t2
+# RUN: not lld -flavor gnu2 --no-allow-shlib-undefined %t1 %t.so -o %t2
+# RUN: lld -flavor gnu2 --allow-shlib-undefined %t1 %t.so -o %t2
+ 
+# Shared library: links successfully by default and with 
+# --allow-shlib-undefined but does not link with --no-allow-shlib-undefined
+# RUN: lld -shared --allow-shlib-undefined -flavor gnu2 %t -o %t.so 
+# RUN: not lld -shared --no-allow-shlib-undefined -flavor gnu2 %t -o %t.so 
+ 
+.globl _start
+_start: 
+  call _shared