RFR: 8302337: JDK crashes if lib/modules contains non-zero byte containing ATTRIBUTE_END

[Severin Gehwolf]

On Mon, 13 Feb 2023 16:57:17 GMT, Severin Gehwolf <sgehwolf at openjdk.org> wrote:

> The `jimage` location attributes are terminated with `ATTRIBUTE_END`-kinds. However,
> the byte containing `ATTRIBUTE_END` (most significant 5 bits, represent `kind`), might
> be non-zero in the lower 3 bits (values up to `0x07` represent `ATTRIBUTE_END`). The JDK code
> handles this case correctly in [`ImageLocation.decompress()`](https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/jdk/internal/jimage/ImageLocation.java#L69..L71). However, the `libjimage`
> code in `java.base` doesn't. That can lead to segfaults reading random bytes and offsets.
> 
> I propose to break the loop if `ATTRIBUTE_END` is being encountered so that reading stops.
> 
> Testing:
>  - [x] `test/jdk/tools/jimage` and `test/jdk/jdk/internal/jimage` tests.
>  - [x] Manual testing with a patched JDK to write non-zero bytes containing `ATTRIBUTE_END` into the jimage. Segfaults before, runs fine after.
>  - [x] GHA.
> 
> Thoughts?

Thanks for the reviews!

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PR: https://git.openjdk.org/jdk/pull/12539