Index: llvm/trunk/lib/MC/WasmObjectWriter.cpp
===================================================================
--- llvm/trunk/lib/MC/WasmObjectWriter.cpp
+++ llvm/trunk/lib/MC/WasmObjectWriter.cpp
@@ -398,7 +398,13 @@
 // Now that the section is complete and we know how big it is, patch up the
 // section size field at the start of the section.
 void WasmObjectWriter::endSection(SectionBookkeeping &Section) {
-  uint64_t Size = W.OS.tell() - Section.PayloadOffset;
+  uint64_t Size = W.OS.tell();
+  // /dev/null doesn't support seek/tell and can report offset of 0.
+  // Simply skip this patching in that case.
+  if (!Size)
+    return;
+
+  Size -= Section.PayloadOffset;
   if (uint32_t(Size) != Size)
     report_fatal_error("section size does not fit in a uint32_t");
 
Index: llvm/trunk/test/MC/WebAssembly/null-output.s
===================================================================
--- llvm/trunk/test/MC/WebAssembly/null-output.s
+++ llvm/trunk/test/MC/WebAssembly/null-output.s
@@ -0,0 +1,10 @@
+# RUN: llvm-mc -triple=wasm32-unknown-unknown -filetype=obj -o /dev/null < %s
+
+    .text
+    .section .text.main,"",@
+    .type    main,@function
+main:
+    .functype   main (i32, i32) -> (i32)
+    end_function
+.Lfunc_end0:
+    .size main, .Lfunc_end0-main