[Apple Silicon] Thread JIT Write Protection

[ClearScript Developers]

Greetings!

V8 often calls the embedder to schedule delayed or background work. Our embedder has its own JIT compiler.

It appears that sometimes, on Apple Silicon, V8 calls the embedder with Thread JIT Write Protection in the "write, don't execute" state (see pthread_jit_write_protect_np). The embedder then blows up on any attempt to execute its own JIT-compiled code.

Our question for the V8 team: Should we report this? Would it be considered a V8 bug?

Thanks!

[Jakob Kummerow]

Feel free to report it. I'd be interested to learn more; embedder calls while being in writable mode seem a bit surprising. I can't promise we'd do anything about it, that depends in particular on how much trouble it would be.

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[ClearScript Developers]

Will do, Jakob, thanks. Since you're interested, we first saw this in 10.3, where it was very rare and treatable via "--no_wasm_async_compilation". In 10.4 and later it's ubiquitous, and "git bisect" has identified the Fast W^X feature as the culprit. That feature was reverted on the 10.3 branch but apparently relanded for 10.4 and beyond.

[ClearScript Developers]

13239 - Apple Silicon: Embedder invoked with Thread JIT Write Protection in Write state - v8 (chromium.org)