The ThreadSanitizer job runs on Docker nested in an unprivileged LXC container, whose kernel randomizes mmap addresses beyond the range TSan's fixed shadow mapping expects. TSan aborted at init with "unexpected memory mapping" before any test ran. Disable ASLR per-process with `setarch -R`, which needs the personality(2) syscall that Docker's default seccomp profile blocks; seccomp=unconfined on the container permits it. Verified on the runner that both are required: setarch -R alone gets EPERM, seccomp alone still aborts, both together run clean. Scoped to the tsan job, which runs only our own test binaries. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>