Index: ELF/LinkerScript.cpp
===================================================================
--- ELF/LinkerScript.cpp
+++ ELF/LinkerScript.cpp
@@ -192,6 +192,13 @@
   return Sections;
 }
 
+static uint64_t safeAssign(Expr Expr, uint64_t Dot) {
+  uint64_t Val = Expr(Dot);
+  if (Val < Dot)
+    error("location counter may not be moved backwards");
+  return Val;
+}
+
 template <class ELFT>
 void LinkerScript<ELFT>::assignAddresses(
     ArrayRef<OutputSectionBase<ELFT> *> Sections) {
@@ -213,11 +220,10 @@
 
   for (const std::unique_ptr<BaseCommand> &Base : Opt.Commands) {
     if (auto *Cmd = dyn_cast<SymbolAssignment>(Base.get())) {
-      if (Cmd->Name == ".") {
-        Dot = Cmd->Expression(Dot);
-      } else if (Cmd->Sym) {
+      if (Cmd->Name == ".")
+        Dot = safeAssign(Cmd->Expression, Dot);
+      else if (Cmd->Sym)
         cast<DefinedRegular<ELFT>>(Cmd->Sym)->Value = Cmd->Expression(Dot);
-      }
       continue;
     }
 
@@ -230,7 +236,7 @@
         continue;
 
       if (Cmd->AddrExpr)
-        Dot = Cmd->AddrExpr(Dot);
+        Dot = safeAssign(Cmd->AddrExpr, Dot);
 
       if (Cmd->AlignExpr)
         Sec->updateAlignment(Cmd->AlignExpr(Dot));
Index: test/ELF/linkerscript/linkerscript-locationcounter-backward.s
===================================================================
--- test/ELF/linkerscript/linkerscript-locationcounter-backward.s
+++ test/ELF/linkerscript/linkerscript-locationcounter-backward.s
@@ -0,0 +1,11 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+
+# RUN: echo "SECTIONS { \
+# RUN:  . = 0x10000; \
+# RUN:  . = . - 1; \
+# RUN: }" > %t.script
+# RUN: not ld.lld -shared %t --script %t.script -o %t2 2>&1 | \
+# RUN:  FileCheck --check-prefix=ERR %s
+# ERR: location counter may not be moved backwards
+