RFR: 8371216: oopDesc::print_value_on breaks if klass is garbage [v2]

[Martin Doerr]

On Fri, 7 Nov 2025 14:35:23 GMT, Paul Hübner <phubner at openjdk.org> wrote:

>> Hi all,
>> 
>> The `oopDesc::print_value_on` function checks if an oop is a string, and if so just prints the raw string. To do this, it needs to read the `klass()`. If the `klass()` reads garbage, one of many assertion errors is likely triggered.
>> 
>> For example, if G1's verification finds problematic oops, it will attempt to print them. If these oops have garbage (incorrect or racey) klasses, this will cause an assertion error, fail fast, and VM crash. G1 never finishes printing, which may make debugging more difficult. The developer can/will be made aware in other ways if the `klass()` is garbage, for example by being told that it is not in the metaspace.
>> 
>> We observed the above in Valhalla and already patched it there.
>> 
>> Testing: tiers 1-5 on Linux (x64, AArch64), macOS (x64, AArch64), Windows (x64).
>
> Paul Hübner has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Don't make a new function.

Thanks for fixing it!
I think there are more places which cause "error occurred during error reporting" due to assertions, but they can get addressed in separate issues.

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Marked as reviewed by mdoerr (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/28190#pullrequestreview-3434852888