Comment 76 for bug 190587

To answer my own question, this works:

stap -v -g -e 'probe syscall.vmsplice = kernel.function("sys_vmsplice") ? {
        name = "vmsplice"
        argstr = sprintf("%d, %p, %d, 0x%x", $fd, $iov, $nr_segs, $flags)
}

probe syscall.vmsplice {
   printf("blocking vmsplice (%s) uid %d pid %d exec %s\n", argstr, uid(),
pid(), execname())
   $nr_segs = 0
}'