Index: ELF/Options.td
===================================================================
--- ELF/Options.td
+++ ELF/Options.td
@@ -61,3 +61,11 @@
 
 def alias_entry : Separate<["-"], "e">,
      Alias<entry>;
+
+def no_allow_shlib_undefined
+    : Flag<["--"], "no-allow-shlib-undefined">,
+      HelpText<"This flag is ignored. --allow-shlib-undefined is always set">;
+
+def allow_shlib_undefined
+    : Flag<["--"], "allow-shlib-undefined">,
+      HelpText<"This flag is ignored. --allow-shlib-undefined is always set">;
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 +171,20 @@
 template void writeResult<ELF64LE>(SymbolTable *);
 template void writeResult<ELF64BE>(SymbolTable *);
 
+void undefError(const Twine &Symbol, const Twine &SymFile) {
+  // DSO can contain unresolved symbols
+  if (Config->Shared)
+    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
 
@@ -281,13 +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 +1,25 @@
+# --allow-shlib-undefined and --no-allow-shlib-undefined are fully
+# ignored in linker implementation. 
+# --allow-shlib-undefined is set by default
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux \
+# RUN: %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: should link with DSO containing undefined symbols in any case.
+# RUN: lld -flavor gnu2 %t1 %t.so -o %t2
+# RUN: lld -flavor gnu2 --no-allow-shlib-undefined %t1 %t.so -o %t2
+# RUN: lld -flavor gnu2 --allow-shlib-undefined %t1 %t.so -o %t2
+ 
+# DSO with undefines: 
+# should link with or without any of these options.
+# RUN: lld -shared -flavor gnu2 %t -o %t.so 
+# RUN: lld -shared --allow-shlib-undefined -flavor gnu2 %t -o %t.so 
+# RUN: lld -shared --no-allow-shlib-undefined -flavor gnu2 %t -o %t.so 
+ 
+# Executable still should not link when have undefines inside.
+# RUN: not lld -flavor gnu2 %t -o %t.so 
+
+.globl _start
+_start: 
+  call _shared