[10] RFC: Relaxing access restrictions for VM annotations (original) (raw)

Vladimir Ivanov vladimir.x.ivanov at oracle.com
Tue Feb 21 14:36:26 UTC 2017

FYI I've been experimenting with relaxing restrictions on VM annotations

Right now, VM ignores any annotation unless it is declared on a class from privileged context (JDK classes or VM anonymous classes).

Some features introduced adhoc flags to relax the restriction (e.g., -XX:RestrictContended & -XX:RestrictReservedStack), but with modules there's a universal way to expose them in controlled manner: rely on annotation class accessibility checks. In order to make some annotation available in non-privileged context, it is enough to add relevant export to java.base module (e.g., --add-exports java.base/jdk.internal.vm.annotation=ALL-UNNAMED).

The problem I faced was that accessibility checks are performed on already loaded classes, but annotations are used during class parsing and the checks have to be performed against the class being loaded.

One option is to re-implement accessibility checking logic and extract all necessary information from ClassFileParser.

Another approach is to collect VM annotations during parsing and after the class is loaded apply them one-by-one doing accessibility checks. It works fine for most of the existing annotations, but problematic for @Contended - it affects instance layout which is computed during parsing.

So, after some experiments, I decided to try different approach: optimistically assume all annotations are accessible and re-parse the class if any used annotation is not available:

http://cr.openjdk.java.net/~vlivanov/vm_annotations/webrev.00

The benefit is that parsing & accessibility checking logic stay the same. Also, it enabled sanity checking logic for annotation usages almost for free.

Overall, I'm quite satisfied with the result, but stumbled upon some nuisance when implementing re-parsing (e.g., tracing framework needs access to ClassFileParser and it brings unnecessary changes into ClassFileParser::parse_instance_klass_from_stream).

Before investing into polishing the change, I'd like to discuss the approach I chose.

Any problems with class file re-parsing or annotation resolution in VM I overlooked?

Maybe there are strong arguments for other options (#1: accessibity checks in ClassFileParser & #2: apply annotations after the class is loaded)?

Thanks!

Best regards, Vladimir Ivanov

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