[jdk18] RFR: 8279011: JFR: JfrChunkWriter incorrectly handles int64_t chunk size as size_t

[Aleksey Shipilev]

On Mon, 20 Dec 2021 17:03:39 GMT, Markus Grönlund <mgronlun at openjdk.org> wrote:

> But surprised there is not a compile error here (both incompatible type size and sign), if sizeof(size_t) == 4 and sizeof(int64_t) == 8:
> 
> const size_t sz_written = size_written(); // <-- returns int64_t

I believe narrowing conversions like these are implicit in C++, and do not produce warnings in modern compilers. See: https://godbolt.org/z/48oh91YE9
 
> So the value is written as a 4-byte quantity only, since it is dimensioned using sizeof(size_t), and therefore does not fill the full 8- bytes slot associated with the 'chunksize' header field. And uninitialized memory in the high 4-bytes leads to this high value?

Actually, I think it is worse than that, since `sizeof(T)*len + len` can produce the `pos` that is way off, because of multiplication by `len`:


template <typename BE, typename IE, typename WriterPolicyImpl >
template <typename T>
inline void WriterHost<BE, IE, WriterPolicyImpl>::be_write(const T* value, size_t len) {
  assert(value != NULL, "invariant");
  assert(len > 0, "invariant");
  // Might need T + 1 size
  u1* const pos = ensure_size(sizeof(T) * len + len);
  if (pos) {
    this->set_current_pos(BE::be_write(value, len, pos));
  }


So it might just store the `chunk_size` somewhere else...

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PR: https://git.openjdk.java.net/jdk18/pull/50