[WebAssembly] Fix overflow for input without version

[JDevlieghere]

Summary

ASAN detected a heap buffer overflow in the WasmObjectFile ctor when reading an file with a missing or incomplete version number.

[dschuff]

Thanks!

[sbc100]

Thanks! I should really have included better handling of EOF in general within this file.

[JDevlieghere]

Running the tests identified an off-by-one error: must be > rather than >=

llvm/trunk/lib/Object/WasmObjectFile.cpp

WasmObjectFile::WasmObjectFile(MemoryBufferRef Buffer, Error &Err)
 
   ErrorAsOutParameter ErrAsOutParam(&Err);
   Header.Magic = getData().substr(0, 4);
   if (Header.Magic != StringRef("\0asm", 4)) {
     Err = make_error<StringError>("Bad magic number",
                                   object_error::parse_failed);
     return;
   }
+
+  const uint8_t *Eof = getPtr(getData().size());
   const uint8_t *Ptr = getPtr(4);
+
+  if (Ptr + 4 > Eof) {
+    Err = make_error<StringError>("Missing version number",
+                                  object_error::parse_failed);
+    return;
+  }
+
   Header.Version = readUint32(Ptr);
   if (Header.Version != wasm::WasmVersion) {
     Err = make_error<StringError>("Bad version number",
                                   object_error::parse_failed);
     return;
   }
 
-  const uint8_t *Eof = getPtr(getData().size());
   WasmSection Sec;
   while (Ptr < Eof) {
     if ((Err = readSection(Sec, Ptr, getPtr(0))))
       return;
     if ((Err = parseSection(Sec)))
       return;
 
     Sections.push_back(Sec);

llvm/trunk/test/Object/wasm-missing-version.test

+# RUN: not llvm-objdump -h %p/Inputs/WASM/missing-version.wasm 2>&1 | FileCheck %s
+# CHECK: {{.*}}: Missing version number