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

[Markus Grönlund]

On Mon, 20 Dec 2021 16:34:07 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> See the investigation in the bug.
> 
> Spot the problem:
> 
> 
> int64_t JfrChunkWriter::write_chunk_header_checkpoint(bool flushpoint) {
>    ...
>    const size_t sz_written = size_written(); // <-- returns int64_t
>    write_be_at_offset(sz_written, chunk_size_offset); // <--- template instantiation with type=size_t
>    return sz_written;
>  }
> 
> 
> This would have been nearly fine -- small `size_t` -> `int64_t` conversion is okay value-wise. But `write_be_at_offset` calculates the position for the writeout using `sizeof(T)`, which silently borks the whole thing on at least 32-bit platforms, where `sizeof(size_t)` != `sizeof(int64_t)`. 
> 
> Additional testing:
>  - [x] Linux x86_64 `jdk_jfr` (no regressions)
>  - [x] Linux x86_32 `jdk_jfr` (many failing tests now pass)

Len is only internally derived to ensure the size to be written can be accommodated by the internal buffer system, it does not affect the position of where to write on disk.

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