Reject unaligned futex uaddr

[jserv]

Linux's unaligned_p() rejects futex operations whose uaddr is not 4-byte aligned, since an unaligned 4-byte word loses atomicity on arm64 and can straddle a page boundary. Mirror that across the
WAIT/WAKE/REQUEUE/CMP_REQUEUE/WAKE_OP/LOCK_PI/TRYLOCK_PI/UNLOCK_PI dispatch sites by returning -EINVAL when the uaddr fails the 4-byte alignment check.

robust_list_walk now applies the same check before touching each futex word: an unaligned or out-of-range futex_gva is skipped (the list itself still advances). Without the gate, a partial cross-page guest_write_small could leave the futex word in a torn state while the subsequent wake is suppressed, leaving waiters wedged on a half-modified word.

Also handle the OWNER_DIED write failure case: log the address at debug level and suppress the wake. Waking after a failed write would just send waiters back to sleep on the unchanged word. This mirrors Linux handle_futex_death, which gives up on the entry without waking when the user-memory update faults. log_debug avoids stderr spam when a corrupted robust list trips the cap repeatedly.

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Summary by cubic

Reject unaligned futex addresses across all futex paths and tighten robust-list cleanup to avoid torn writes and stuck waiters. Matches Linux by returning -EINVAL for any 4-byte misaligned uaddr and by suppressing wakeups when OWNER_DIED updates fail.

• Bug Fixes
  • Return -EINVAL for misaligned uaddr in WAIT, WAKE, REQUEUE/CMP_REQUEUE, WAKE_OP, and PI ops (LOCK_PI, TRYLOCK_PI, UNLOCK_PI).
  • In robust_list_walk, skip out-of-range or unaligned futex words; on OWNER_DIED write failure, log debug with address and do not wake.
  • Add tests verifying -EINVAL on unaligned addresses for WAIT, WAKE, and PI ops.

^{Written for commit 16ad7b4. Summary will update on new commits.}

[cubic-dev-ai]

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