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I don't >>> understand how it helps by pass ulimit. Did you try running your test >>> with the Binder ioctl removed? I'd guess that if it passes now, it >>> still passes with the Binder ioctl deleted. >> You're right. I ran the test you suggested and confirmed there is no >> bypass — RLIMIT_NPROC is enforced by copy_process() at clone() time, >> regardless of binder's max_threads value: >> >> // Test: uid=65534, RLIMIT_NPROC=50, with and without SET_MAX_THREADS >> [snip...] >> >> Identical. SET_MAX_THREADS has no effect on the thread creation limit. >> My original PoC was flawed — it ran as root where RLIMIT_NPROC is not >> enforced, making the binder ioctl irrelevant. >> >> I think accepting 0xFFFFFFFF for a thread pool size is arguably poor input >> validation. no sane userspace would request 4 billion threads. >> >> Would a separate patch (without Fixes tag) that caps max_threads at a >> reasonable upper bound be welcome, or is it not worth the churn? this is >> hardening, not a security fix. > > I don't think that's useful. Understood. This time, I was able to track the binder fs using my kcov-dataflow (arg/ret), which was a useful PoC. This could be helpful for anyone who wants to trace arg/ret data during Rust kernel development or testing. :) > > Alice Thanks again for the review! Yunseong