RFR: 8371216: oopDesc::print_value_on breaks if klass is garbage

[Thomas Stuefe]

On Fri, 7 Nov 2025 09:08:33 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).

I would not add the helper function for only one case. We already have a bunch of helpers like these. Just test for oop != NULL at the call site.

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PR Review: https://git.openjdk.org/jdk/pull/28190#pullrequestreview-3432741260