[re-send with smaller CC list] This adds the ability for threads to request seccomp filter synchronization across their thread group (at filter attach time). For example, for Chrome to make sure graphic driver threads are fully confined after seccomp filters have been attached. To support this, locking on seccomp changes is introduced, along with refactoring of no_new_privs. Races with thread creation/death are handled via tasklist_lock. This includes a new syscall (instead of adding a new prctl option), as suggested by Andy Lutomirski and Michael Kerrisk. Thanks! -Kees v6: - switch from seccomp-specific lock to thread-group lock to gain atomicity - implement seccomp syscall across all architectures with seccomp filter - clean up sparse warnings around locking v5: - move includes around (drysdale) - drop set_nnp return value (luto) - use smp_load_acquire/store_release (luto) - merge nnp changes to seccomp always, fewer ifdef (luto) v4: - cleaned up locking further, as noticed by David Drysdale v3: - added SECCOMP_EXT_ACT_FILTER for new filter install options v2: - reworked to avoid clone races
