Stub scheduler policy syscalls

[jserv]

Add sched_{setparam,setscheduler,getscheduler,getparam,get_priority_min, get_priority_max,rr_get_interval}. Newer musl and glibc pthread_create probe the parent thread's policy via sched_getscheduler before spawning a worker; returning ENOSYS broke thread startup for guests that exercised that path.

Pid arguments are TIDs in Linux scheduler ABI, not TGIDs, so a worker calling sched_*(gettid()) must reach its own thread entry. Live TIDs are matched through thread_tid_alive(); unknown pids return ESRCH.

Setter errno ordering tracks Linux 6.x kernel/sched/syscalls.c: copy_from_user runs before find_process_by_pid, so a bad sched_param pointer plus an unknown pid yields EFAULT, not ESRCH.

Any policy transition away from SCHED_OTHER is refused. SCHED_BATCH and SCHED_IDLE return EPERM at priority 0 rather than being silently accepted while the stub keeps reporting SCHED_OTHER. SCHED_FIFO and SCHED_RR collapse to EINVAL when priority is outside [1, 99] and to EPERM when valid; SCHED_DEADLINE through the legacy entry is EINVAL because the syscall cannot supply deadline attributes (real Linux requires sched_setattr). SCHED_RESET_ON_FORK (0x40000000) is parsed off the policy before validation; any other high bit is EINVAL.

sched_rr_get_interval reports a 100 ms slice, matching the sched_rr_timeslice default and a typical CFS quantum, instead of a zero timespec that would mislead callers branching on the value.

sched_getaffinity also runs through the per-thread pid gate so unknown pids return ESRCH instead of silently succeeding.

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Summary by cubic

Stubbed Linux scheduler policy syscalls with Linux-like TID semantics and errno order to unblock musl/glibc pthread_create and tighten PID checks. Also updated sched_getaffinity and added a targeted test for per-thread TIDs and policy validation.

• New Features
  • Implemented sched_setparam, sched_setscheduler, sched_getscheduler, sched_getparam, sched_get_priority_min, sched_get_priority_max, sched_rr_get_interval.
  • Enforced TID-based pid args (pid 0 allowed; unknown pids return -ESRCH) and Linux-like errno order for setters/getters.
  • Modeled only SCHED_OTHER; reject other policies with Linux-like -EINVAL/-EPERM rules, strip SCHED_RESET_ON_FORK, and return a 100ms slice from sched_rr_get_interval.
  • sched_getaffinity now uses the same per-thread PID gate; added test-sched-policy and a Makefile target.

^{Written for commit d8873a2. Summary will update on new commits.}

[cubic-dev-ai]

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