Since f35093f8 Linux semantics of a threads affinity functions is in use:

Minimum cpuset_t size that the Linux kernel permits in case of getaffinity()
is the maximum CPU id, present in the system / NBBY, the maximum size is not
limited. For setaffinity(), Linux does not limit the size of the user-provided
cpuset_t, internally using only the meaningful part of the set, where the upper
bound is the maximum CPU id, present in the system, no larger than the size of
the kernel cpuset_t.