Constant USER_ADDR_MAX 

pub const USER_ADDR_MAX: u64 = crate::memory::userslice::USER_SPACE_END; // 18_446_744_073_709_551_615u64

Maximum virtual address we accept for user-space mappings.

Strat9 userspace components are statically linked (no-pie) at ET_EXEC 0xFFFFFFFF80000000 : the higher-half window. Each user AddressSpace owns a private copy of the PML4[511] PDP (see address_space::new_user), with the kernel-image slot removed, so processes can use the full canonical higher-half range without touching kernel pages. The guard therefore only rejects non-canonical addresses.