In arch/x86/entry/syscalls/syscall_64.tbl syscalls are numbered from 0 to 334 and then there is a gap before the syscall numbers resume at 424. The relevant portion of the source is shown below:
...333 common io_pgetevents sys_io_pgetevents334 common rseq sys_rseq# don't use numbers 387 through 423, add new calls after the last# 'common' entry424 common pidfd_send_signal sys_pidfd_send_signal425 common io_uring_setup sys_io_uring_setup...Since new syscalls get added after number 439 (as stated in the comment), why does this large number gap exist?