Fixed a roll-over on size_t in getNewUninitMemBuffer()

[slydiman]

Summary

This patch fixes a roll-over on size_t in WritableMemoryBuffer::getNewUninitMemBuffer().

[serge-sans-paille]

Thanks for spotting this. At first I was a bit surprised by usage of *rollover* term instead of *overflow* but I guess it's an established term.

llvm/lib/Support/MemoryBuffer.cpp

WritableMemoryBuffer::getNewUninitMemBuffer(size_t Size, const Twine &BufferName) {
   // Allocate space for the MemoryBuffer, the data and the name. It is important
   // that MemoryBuffer and data are aligned so PointerIntPair works with them.
   // TODO: Is 16-byte alignment enough?  We copy small object files with large
   // alignment expectations into this buffer.
   SmallString<256> NameBuf;
   StringRef NameRef = BufferName.toStringRef(NameBuf);
   size_t AlignedStringLen = alignTo(sizeof(MemBuffer) + NameRef.size() + 1, 16);
   size_t RealLen = AlignedStringLen + Size + 1;
+  if (RealLen <= Size) // Check for rollover.
+    return nullptr;
   char *Mem = static_cast<char*>(operator new(RealLen, std::nothrow));
   if (!Mem)
     return nullptr;
 
   // The name is stored after the class itself.
   CopyStringRef(Mem + sizeof(MemBuffer), NameRef);
 
   // The buffer begins after the name and must be aligned.

llvm/unittests/Support/MemoryBufferTest.cpp

TEST_F(MemoryBufferTest, make_new) {
   for (size_t i = 0; i < 321; ++i)
     EXPECT_EQ(0, Three->getBufferStart()[0]);
 
   // 0-initialized buffer with name
   OwningBuffer Four(WritableMemoryBuffer::getNewMemBuffer(123, "zeros"));
   EXPECT_NE(nullptr, Four.get());
   for (size_t i = 0; i < 123; ++i)
     EXPECT_EQ(0, Four->getBufferStart()[0]);
+
+  // uninitialized buffer with rollover size
+  OwningBuffer Five(
+      WritableMemoryBuffer::getNewUninitMemBuffer(SIZE_MAX, "huge"));
+  EXPECT_EQ(nullptr, Five.get());
 }
 
 void MemoryBufferTest::testGetOpenFileSlice(bool Reopen) {
   // Test that MemoryBuffer::getOpenFile works properly when no null
   // terminator is requested and the size is large enough to trigger
   // the usage of memory mapping.
   int TestFD;
   SmallString<64> TestPath;