Index: ELF/Driver.cpp
===================================================================
--- ELF/Driver.cpp
+++ ELF/Driver.cpp
@@ -386,7 +386,7 @@
          s == "noexecstack" || s == "nokeep-text-section-prefix" ||
          s == "norelro" || s == "noseparate-code" || s == "notext" ||
          s == "now" || s == "origin" || s == "relro" || s == "retpolineplt" ||
-         s == "rodynamic" || s == "text" || s == "wxneeded" ||
+         s == "rodynamic" || s == "text" || s == "undefs" || s == "wxneeded" ||
          s.startswith("common-page-size") || s.startswith("max-page-size=") ||
          s.startswith("stack-size=");
 }
@@ -517,6 +517,8 @@
     case OPT_z:
       if (StringRef(arg->getValue()) == "defs")
         return errorOrWarn;
+      if (StringRef(arg->getValue()) == "undefs")
+        return UnresolvedPolicy::Ignore;
       continue;
     }
   }
Index: test/ELF/zdefs.s
===================================================================
--- test/ELF/zdefs.s
+++ test/ELF/zdefs.s
@@ -1,9 +1,15 @@
 # REQUIRES: x86
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
+
+## Unresolved references from object files are allowed by default for -shared.
 # RUN: ld.lld -shared %t.o -o %t1.so
 
+## -z defs disallows unresolved references.
 # RUN: not ld.lld -z defs -shared %t.o -o %t1.so 2>&1 | FileCheck -check-prefix=ERR %s
 # ERR: error: undefined symbol: foo
 # ERR: >>> referenced by {{.*}}:(.text+0x1)
 
+## -z undefs allows unresolved references.
+# RUN: ld.lld -z defs -z undefs -shared %t.o -o /dev/null 2>&1 | count 0
+
 callq foo@PLT