RFR: 8217920: Lookup.defineClass injects a class that can access private members of any class in its own module [v2]

[Alan Bateman]

On Thu, 26 Jan 2023 22:27:36 GMT, Mandy Chung <mchung at openjdk.org> wrote:

>> Currently, a `Lookup` object with `PACKAGE` access can be used to inject a class in the runtime package of the Lookup's lookup class via `Lookup::defineClass`.   The classes that are injected have the same access as other members in the module and can access private members of all types in the module via reflection.
>> 
>> However, changing `Lookup.defineClass` to require full privilege access (`PRIVATE` + `MODULE`) is an incompatible change that would break existing frameworks which use `privateLookupIn` and `Lookup::defineClass` to inject auxiliary classes in a module.   A module authorizes the framework by opening a package for it to access and `Lookup::defineClass` was the supported replacement for `setAccessible` on `ClassLoader::defineClass` hack in JDK 9.    
>> 
>> This PR proposes to keep existing behavior and provide better documentation to help developers to beware of the permissions given out when opening a package to another module. A class injected in a module has the same privilege as other module members.
>
> Mandy Chung has updated the pull request incrementally with one additional commit since the last revision:
> 
>   review feedback

src/java.base/share/classes/java/lang/Module.java line 605:

> 603:      * {@link java.lang.invoke.MethodHandles.Lookup Lookup} object that is allowed to
> 604:      * {@link java.lang.invoke.MethodHandles.Lookup#defineClass(byte[]) define classes}
> 605:      * in package {@code p}. </p>

I wonder if this should be an apiNote rather method description. There is also the no-arg isOpen method and maybe we should add a note there too. What would you think about linking "deep reflection" to AccessibleObject.setAccesssible(boolean) ?

src/java.base/share/classes/java/lang/invoke/MethodHandles.java line 240:

> 238:      * of {@code targetClass}. Extreme caution should be taken when opening a package
> 239:      * to another module as such defined classes have the same full privilege
> 240:      * access as other members in the target module.

"define classes" instead of inject classes is good.

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PR: https://git.openjdk.org/jdk/pull/12236