Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -2611,6 +2611,8 @@
   "argument to 'section' attribute is not valid for this target: %0">;
 def warn_mismatched_section : Warning<
   "section does not match previous declaration">, InGroup<Section>;
+def warn_attribute_section_on_redeclaration : Warning<
+  "section attribute is specified on redeclared variable">, InGroup<Section>;
 
 def err_anonymous_property: Error<
   "anonymous property is not supported">;
Index: lib/CodeGen/CodeGenModule.cpp
===================================================================
--- lib/CodeGen/CodeGenModule.cpp
+++ lib/CodeGen/CodeGenModule.cpp
@@ -2431,6 +2431,12 @@
       EmitGlobalVarDefinition(D);
     }
 
+    // Emit section information for extern variables.
+    if (D->hasExternalStorage()) {
+      if (const SectionAttr *SA = D->getAttr<SectionAttr>())
+        GV->setSection(SA->getName());
+    }
+
     // Handle XCore specific ABI requirements.
     if (getTriple().getArch() == llvm::Triple::xcore &&
         D->getLanguageLinkage() == CLanguageLinkage &&
Index: lib/Sema/SemaDecl.cpp
===================================================================
--- lib/Sema/SemaDecl.cpp
+++ lib/Sema/SemaDecl.cpp
@@ -2607,6 +2607,14 @@
     }
   }
 
+  // This redeclaration adds a section attribute to a declaration that has
+  // already been ODR-used.
+  if (New->hasAttr<SectionAttr>() && !Old->hasAttr<SectionAttr>() &&
+      Old->isUsed()) {
+    Diag(New->getLocation(), diag::warn_attribute_section_on_redeclaration);
+    Diag(Old->getLocation(), diag::note_previous_declaration);
+  }
+
   if (!Old->hasAttrs())
     return;
 
Index: test/CodeGenCXX/extern-section-attribute.cpp
===================================================================
--- /dev/null
+++ test/CodeGenCXX/extern-section-attribute.cpp
@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 -emit-llvm %s -o - -triple=x86_64-pc-linux-gnu | FileCheck %s
+
+extern int aa __attribute__((section(".sdata")));
+// CHECK-DAG: @aa = external global i32, section ".sdata", align 4
+
+extern int bb __attribute__((section(".sdata"))) = 1;
+// CHECK-DAG: @bb = global i32 1, section ".sdata", align 4
+
+int foo() {
+  return aa + bb;
+}
Index: test/Sema/attr-section.c
===================================================================
--- test/Sema/attr-section.c
+++ test/Sema/attr-section.c
@@ -19,3 +19,7 @@
 void __attribute__((section("bar,zed"))) test2(void) {} // expected-warning {{section does not match previous declaration}}
 
 enum __attribute__((section("NEAR,x"))) e { one }; // expected-error {{'section' attribute only applies to functions, methods, properties, and global variables}}
+
+extern int a; // expected-note {{previous declaration is here}}
+int *b = &a;
+extern int a __attribute__((section("foo,zed"))); // expected-warning {{section attribute is specified on redeclared variable}}