diff --git a/kernel/src/loader/mod.rs b/kernel/src/loader/mod.rs
index deb308799aa..cfa90ffcbd2 100644
--- a/kernel/src/loader/mod.rs
+++ b/kernel/src/loader/mod.rs
@@ -15,8 +15,7 @@ mod symbols;
 mod tls;
 
 pub use start::{build_child_handles, PendingHandles, SLOT_PAIR_LEN};
-pub(crate) use start::alloc_kernel_stack;
-pub(crate) use crate::arch::entry::{kernel_start, process_start, thread_start};
+pub(crate) use start::{alloc_kernel_stack, Start};
 pub use tls::{TlsBlock, DTV_INITIAL_CAPACITY, VARIANT as TLS_VARIANT};
 
 use alloc::string::String;
@@ -568,7 +567,12 @@ pub fn spawn<H>(
         return Err(SyscallError::ResourceExhausted.into());
     };
 
-    let entry = (image_start + layout.entry().get()).raw();
+    // Inside the image, which `rebase_base` placed inside the user half.
+    let Some(entry) = toyos_userbound::Entry::new((image_start + layout.entry().get()).raw())
+    else {
+        log!("spawn: {}: the entry is outside the user half", path);
+        return Err(SyscallError::InvalidArgument.into());
+    };
     let image_end = (image_start + layout.span()).raw();
     let sp = user_stack.write_argv(argv);
     let t_tls = crate::clock::nanos_since_boot();
@@ -585,7 +589,7 @@ pub fn spawn<H>(
         bias: base,
     });
 
-    let (ks_alloc, ks_sp) = match alloc_kernel_stack(process_start, entry, sp, 0) {
+    let (ks_alloc, ks_sp) = match alloc_kernel_stack(Start::Process { entry, sp }) {
         Some(ks) => ks,
         None => {
             log!("spawn: {}: failed to allocate kernel stack", path);
@@ -683,7 +687,7 @@ pub fn spawn<H>(
 
     let t3 = crate::clock::nanos_since_boot();
     log!("spawn: {} pid={} tid={} dst={} base={:#x} entry={:#x} root={:#x} (layout={}ms relocs={}ms deps={}ms tls={}ms total={}ms)",
-        path, pid, tid, dst.0, base, entry, child_pt.lock().root().phys(),
+        path, pid, tid, dst.0, base, entry.addr(), child_pt.lock().root().phys(),
         (t1 - t0) / 1_000_000, (t2 - t1) / 1_000_000, (t_deps - t2) / 1_000_000,
         (t_tls - t_deps) / 1_000_000, (t3 - t0) / 1_000_000);
 
diff --git a/kernel/src/loader/start.rs b/kernel/src/loader/start.rs
index 5d42b4c7ae8..4d67e159b35 100644
--- a/kernel/src/loader/start.rs
+++ b/kernel/src/loader/start.rs
@@ -1,7 +1,9 @@
 //! Loads a built process onto a CPU, builds the handle table it starts with
 //! and puts its spawner's handle to it in the spawner's table. The frame a new
 //! stack starts from and the trampolines it returns into are the
-//! architecture's (`arch::entry`).
+//! architecture's (`arch::entry`), and [`Start`] is the only way to one: a
+//! trampoline that returns to userland is reached with an
+//! [`Entry`](toyos_userbound::Entry) and with nothing else.
 
 use alloc::sync::Arc;
 use alloc::vec::Vec;
@@ -21,13 +23,24 @@ use toyos_abi::syscall::{
 /// One `[child_slot, parent_handle]` pair of `SpawnArgs::slot_map_ptr`, in bytes.
 pub const SLOT_PAIR_LEN: usize = 8;
 
+/// Where a new context first runs.
+pub(crate) enum Start {
+    /// A program's first instruction, on the stack its loader wrote.
+    Process { entry: toyos_userbound::Entry, sp: u64 },
+    /// A thread's, on a stack its process chose, with its argument.
+    Thread { entry: toyos_userbound::Entry, sp: u64, arg: u64 },
+    /// A kernel thread's body, which never returns to userland.
+    Kernel { body: extern "C" fn(u64) -> !, arg: u64 },
+}
+
 /// Allocate a kernel stack and lay out the frame `context_switch` will restore.
-pub(crate) fn alloc_kernel_stack(
-    trampoline: unsafe extern "C" fn(),
-    user_entry: u64,
-    user_sp: u64,
-    arg: u64,
-) -> Option<(OwnedAlloc, u64)> {
+pub(crate) fn alloc_kernel_stack(start: Start) -> Option<(OwnedAlloc, u64)> {
+    use crate::arch::entry::{kernel_start, process_start, thread_start};
+    let (trampoline, user_entry, user_sp, arg): (unsafe extern "C" fn(), _, _, _) = match start {
+        Start::Process { entry, sp } => (process_start, entry.addr(), sp, 0),
+        Start::Thread { entry, sp, arg } => (thread_start, entry.addr(), sp, arg),
+        Start::Kernel { body, arg } => (kernel_start, body as usize as u64, 0, arg),
+    };
     let alloc = OwnedAlloc::new(KERNEL_STACK_SIZE, 4096)?;
     scheduler::write_stack_canary(&alloc);
     let top = alloc.ptr() as u64 + KERNEL_STACK_SIZE as u64;
diff --git a/kernel/src/process.rs b/kernel/src/process.rs
index 78cfa1ded34..79b7a85a7a4 100644
--- a/kernel/src/process.rs
+++ b/kernel/src/process.rs
@@ -21,7 +21,7 @@ use crate::sched::payload::ThreadSched;
 use crate::time::{Deadline, Duration};
 use crate::{elf, pipe, scheduler};
 use crate::UserAddr;
-use crate::loader::{alloc_kernel_stack, thread_start, TlsBlock};
+use crate::loader::{alloc_kernel_stack, Start, TlsBlock};
 
 pub use toyos_abi::{Pid, Tid};
 pub use crate::scheduler::TaskId;
@@ -923,7 +923,12 @@ impl Drop for Admission {
 }
 
 /// Spawn a thread within the current process.
-pub fn spawn_thread(entry: u64, stack_ptr: u64, arg: u64, stack_base: u64) -> Option<Tid> {
+pub fn spawn_thread(
+    entry: toyos_userbound::Entry,
+    stack_ptr: u64,
+    arg: u64,
+    stack_base: u64,
+) -> Option<Tid> {
     // Phase 1: parent's data + address space (table lock dropped after).
     let parent_process = current_process();
     let (parent_addr_space, process_data_arc) = {
@@ -962,7 +967,7 @@ pub fn spawn_thread(entry: u64, stack_ptr: u64, arg: u64, stack_base: u64) -> Op
     };
     let tls_alloc_tcb = tls_alloc.ptr().wrapping_add(tp_offset);
 
-    let (ks_alloc, ks_sp) = match alloc_kernel_stack(thread_start, entry, stack_ptr, arg) {
+    let (ks_alloc, ks_sp) = match alloc_kernel_stack(Start::Thread { entry, sp: stack_ptr, arg }) {
         Some(ks) => ks,
         None => {
             tls_alloc.release(&parent_addr_space);
diff --git a/kernel/src/sched/kthread.rs b/kernel/src/sched/kthread.rs
index e63843293c5..55a7119020a 100644
--- a/kernel/src/sched/kthread.rs
+++ b/kernel/src/sched/kthread.rs
@@ -1,5 +1,5 @@
 //! Kernel threads: ordinary tasks that name `mm::paging::kernel` as their
-//! address space, enter through `loader::kernel_start`, and hold a process-table
+//! address space, enter through `arch::entry::kernel_start`, and hold a process-table
 //! entry. One is preempted or stolen only at a preemption point its body reaches,
 //! and a Ring 0 loop reaches none. [`ROWS`] holds every one.
 
@@ -74,13 +74,8 @@ pub fn is_kernel_task(id: TaskId) -> bool {
 
 /// Start a kernel thread running `body(arg)` on its own kernel stack and return its scheduler faces.
 pub fn spawn(name: &str, body: extern "C" fn(u64) -> !, arg: u64) -> ThreadSched {
-    let (stack, entry_sp) = crate::loader::alloc_kernel_stack(
-        crate::loader::kernel_start,
-        body as usize as u64,
-        0,
-        arg,
-    )
-    .unwrap_or_else(|| panic!("kthread: no kernel stack for {name}"));
+    let (stack, entry_sp) = crate::loader::alloc_kernel_stack(crate::loader::Start::Kernel { body, arg })
+        .unwrap_or_else(|| panic!("kthread: no kernel stack for {name}"));
 
     // Before the table lock: a panic holding the process table hangs the machine.
     let claim = Claim::take(name);
diff --git a/kernel/src/syscall/proc.rs b/kernel/src/syscall/proc.rs
index e856d42a1c1..3251510eac1 100644
--- a/kernel/src/syscall/proc.rs
+++ b/kernel/src/syscall/proc.rs
@@ -128,11 +128,16 @@ pub(super) fn sys_endowments(out: &mut crate::user_ptr::UserBytesMut) -> u64 {
     needed as u64
 }
 
-/// Spawn a thread; refuses a `stack_base` above `stack_ptr` (no stack to clamp to).
+/// Spawn a thread; refuses a `stack_base` above `stack_ptr` (no stack to clamp to)
+/// and an `entry` outside the user half, which the return to it would fault on
+/// in the kernel's ring. The stack pointer is the thread's own to fault on.
 pub(super) fn sys_thread_spawn(entry: u64, stack_ptr: u64, arg: u64, stack_base: u64) -> u64 {
     if stack_base > stack_ptr {
         return SyscallError::InvalidArgument.to_u64();
     }
+    let Some(entry) = toyos_userbound::Entry::new(entry) else {
+        return SyscallError::InvalidArgument.to_u64();
+    };
     // A None here is a resource failure or teardown race, never a bad argument.
     process::spawn_thread(entry, stack_ptr, arg, stack_base)
         .map_or(SyscallError::ResourceExhausted.to_u64(), |t| t.raw() as u64)
diff --git a/tests/toyos-rust-tests/src/bin/abuse_thread_table.rs b/tests/toyos-rust-tests/src/bin/abuse_thread_table.rs
index 679420e8313..912ce62d2c1 100644
--- a/tests/toyos-rust-tests/src/bin/abuse_thread_table.rs
+++ b/tests/toyos-rust-tests/src/bin/abuse_thread_table.rs
@@ -1,6 +1,7 @@
 //! A process's threads are bounded at `MAX_THREADS`, exited ones not yet
 //! joined among them: past it a spawn is refused by name, and a join is room
-//! for one.
+//! for one. An entry outside the user half is refused by name whatever room
+//! there is.
 
 use std::sync::atomic::{AtomicU32, Ordering};
 use std::time::{Duration, Instant};
@@ -81,6 +82,17 @@ fn main() {
     let ran = AtomicU32::new(0);
     let mut stacks = Stacks { chunk: 0, used: 0 };
 
+    // Canonical under neither 48-bit nor 57-bit addressing; the first address
+    // past the user half; the first of the kernel half.
+    let (base, top) = stacks.next();
+    for entry in [0x0100_0000_0000_0000, 0x0000_8000_0000_0000, 0xFFFF_8000_0000_0000] {
+        assert_eq!(
+            SyscallError::from_u64(unsafe { syscall::thread_spawn(entry, top, 0, base) }),
+            Some(SyscallError::InvalidArgument),
+            "a thread entry of {entry:#x} was not refused",
+        );
+    }
+
     let mut first = None;
     let mut spawned = 0usize;
     let mut refusal = None;
diff --git a/toyos-abi/src/syscall.rs b/toyos-abi/src/syscall.rs
index 3e3260d780e..259fe56cfce 100644
--- a/toyos-abi/src/syscall.rs
+++ b/toyos-abi/src/syscall.rs
@@ -990,7 +990,8 @@ pub fn mark_tty(handle: RawHandle) {
 pub const MAX_THREADS: usize = 4096;
 
 /// Spawn a new thread with the given entry point, stack pointer, argument, and stack base.
-/// `stack_base` is the bottom of the user stack (for stack info queries).
+/// `stack_base` is the bottom of the user stack (for stack info queries). An
+/// `entry` outside the user half is `InvalidArgument`.
 ///
 /// # Safety
 /// `entry` must be a valid function pointer and `stack`/`stack_base` must
diff --git a/toyos-userbound/src/lib.rs b/toyos-userbound/src/lib.rs
index bf63df0e219..fe80d070c22 100644
--- a/toyos-userbound/src/lib.rs
+++ b/toyos-userbound/src/lib.rs
@@ -38,6 +38,6 @@ pub use place::{PageSpan, Window};
 pub use port::{port_access, IoBitmap, PortAccess, Ports, Reserved, Undeclared, IO_PORTS};
 pub use segment::{pieces, segments, Pinned, Pins, Segment};
 pub use span::{
-    align_2m_checked, in_user_half, is_user_addr, is_user_object, rebase_base, Access, PAGE_2M,
-    PAGE_4K, USER_TOP,
+    align_2m_checked, in_user_half, is_user_addr, is_user_object, rebase_base, Access, Entry,
+    PAGE_2M, PAGE_4K, USER_TOP,
 };
diff --git a/toyos-userbound/src/place.rs b/toyos-userbound/src/place.rs
index 992d7c7fd72..274ddb78939 100644
--- a/toyos-userbound/src/place.rs
+++ b/toyos-userbound/src/place.rs
@@ -17,7 +17,7 @@
 //! What a region already registered says about itself is the kernel's own
 //! ledger and is not re-checked: a wrap there is a kernel bug and traps.
 
-use crate::span::{align_2m_checked, PAGE_2M, USER_TOP};
+use crate::span::{align_2m_checked, PAGE_2M, PLACED_TOP};
 
 /// The part of the user half anonymous ranges are placed in, and the guard
 /// left above each. Built in a `const`, so a window that breaks
@@ -44,15 +44,15 @@ impl PageSpan {
 }
 
 impl Window {
-    /// Every bound 2 MiB-aligned, inside the user half, with room for at least
-    /// the guard: the kernel's own layout, so a window that breaks any of them
+    /// Every bound 2 MiB-aligned, at or below [`PLACED_TOP`], with room for at
+    /// least the guard: the kernel's own layout, so a window that breaks any of them
     /// is a kernel bug.
     pub const fn new(floor: u64, ceiling: u64, guard: u64) -> Self {
         assert!(
             floor.is_multiple_of(PAGE_2M) && ceiling.is_multiple_of(PAGE_2M) && guard.is_multiple_of(PAGE_2M),
             "a placement window's bounds and guard are whole 2 MiB pages"
         );
-        assert!(ceiling <= USER_TOP, "a placement window is inside the user half");
+        assert!(ceiling <= PLACED_TOP, "a placement window stops below the last page of the user half");
         assert!(floor < ceiling && ceiling - floor >= guard, "a placement window holds its guard");
         Self { floor, ceiling, guard }
     }
@@ -108,6 +108,7 @@ impl Window {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::span::USER_TOP;
 
     /// The kernel's own window, by value: `vma::WINDOW`.
     const FLOOR: u64 = 0x0002_0000_0000;
@@ -132,6 +133,9 @@ mod tests {
         }
     }
 
+    /// The highest window there is compiles; the test below is one page past it.
+    const _: Window = Window::new(FLOOR, PLACED_TOP, PAGE_2M);
+
     #[test]
     fn a_zero_length_is_refused() {
         assert_eq!(WINDOW.span(0), None);
@@ -210,9 +214,9 @@ mod tests {
     }
 
     #[test]
-    #[should_panic(expected = "inside the user half")]
-    fn a_window_past_the_user_half_is_a_kernel_bug() {
-        let _ = Window::new(FLOOR, USER_TOP + PAGE_2M, PAGE_2M);
+    #[should_panic(expected = "below the last page of the user half")]
+    fn a_window_reaching_the_last_page_of_the_user_half_is_a_kernel_bug() {
+        let _ = Window::new(FLOOR, USER_TOP, PAGE_2M);
     }
 
     /// `find_gap` no longer pre-filters by the ceiling; `gap` is the one
diff --git a/toyos-userbound/src/span.rs b/toyos-userbound/src/span.rs
index e7ef60af548..e4a458950a3 100644
--- a/toyos-userbound/src/span.rs
+++ b/toyos-userbound/src/span.rs
@@ -40,6 +40,34 @@ pub fn is_user_addr(addr: u64) -> bool {
     addr < USER_TOP
 }
 
+/// An instruction pointer the kernel may return to userland at.
+///
+/// A return to an address that is not canonical faults in the returning
+/// instruction itself, in the kernel's ring and not the thread's, so one that
+/// crossed the trust boundary is refused before any frame carries it. No
+/// other constructor: a first return to userland takes this and nothing else.
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+pub struct Entry(u64);
+
+impl Entry {
+    /// `None` for an address outside the user half; what is mapped at one
+    /// inside it is the thread's own fault to take.
+    pub fn new(addr: u64) -> Option<Self> {
+        is_user_addr(addr).then_some(Self(addr))
+    }
+
+    pub const fn addr(self) -> u64 {
+        self.0
+    }
+}
+
+/// One past the highest address the kernel places anything at, an image or a
+/// [`Window`](crate::Window)'s range: the user half less its last page. An
+/// instruction that ends at [`USER_TOP`] leaves that address, no canonical
+/// one, as where a syscall or an interrupt taken after it returns to, so
+/// nothing executable is placed where one could end there.
+pub(crate) const PLACED_TOP: u64 = USER_TOP - PAGE_2M;
+
 /// Whether `[ptr, ptr + len)` is entirely in the user half.
 ///
 /// Also the bound `sys_mmap` applies to a range it will install rather than
@@ -54,12 +82,13 @@ pub fn in_user_half(ptr: u64, len: u64) -> bool {
 
 /// The load base an `ET_DYN` image rebases to `vm_base` (`vm_base - vaddr_min`),
 /// or `None` when it cannot: a `vaddr_min` above `vm_base` underflows the
-/// subtraction, and a `span` reaching from `vm_base` past the user half does not
+/// subtraction, and a `span` reaching from `vm_base` past [`PLACED_TOP`] does not
 /// fit. The ELF spec leaves an `ET_DYN` `p_vaddr` unconstrained, so the kernel
 /// that picks `vm_base` is the only place that can refuse one.
 pub fn rebase_base(vm_base: u64, vaddr_min: u64, span: u64) -> Option<u64> {
     let base = vm_base.checked_sub(vaddr_min)?;
-    in_user_half(vm_base, span).then_some(base)
+    let end = vm_base.checked_add(span)?;
+    (end <= PLACED_TOP).then_some(base)
 }
 
 /// Whether the kernel may read or write a `size`-byte value of alignment
@@ -111,6 +140,19 @@ mod tests {
         assert!(is_user_addr(0));
     }
 
+    /// The last address of the user half is one, and the first that is not
+    /// canonical, the first of the kernel half and the last of all are not.
+    #[test]
+    fn an_entry_is_an_address_of_the_user_half_and_nothing_else() {
+        assert_eq!(Entry::new(USER_TOP - 1).map(Entry::addr), Some(USER_TOP - 1));
+        assert_eq!(Entry::new(0).map(Entry::addr), Some(0));
+        assert_eq!(Entry::new(USER_TOP), None);
+        assert_eq!(Entry::new(0x0100_0000_0000_0000), None);
+        assert_eq!(Entry::new(0xFFFF_7FFF_FFFF_FFFF), None);
+        assert_eq!(Entry::new(0xFFFF_8000_0000_0000), None);
+        assert_eq!(Entry::new(u64::MAX), None);
+    }
+
     #[test]
     fn a_range_ending_past_the_bound_is_refused_and_a_wrapping_one_too() {
         assert!(in_user_half(USER_TOP - 8, 8));
@@ -185,9 +227,11 @@ mod tests {
     }
 
     #[test]
-    fn an_image_that_rebases_past_the_user_half_is_refused() {
-        assert_eq!(rebase_base(USER_VM_BASE, 0, USER_TOP - USER_VM_BASE), Some(USER_VM_BASE));
-        assert_eq!(rebase_base(USER_VM_BASE, 0, USER_TOP - USER_VM_BASE + 1), None);
+    fn an_image_that_reaches_the_last_page_of_the_user_half_is_refused() {
+        assert_eq!(rebase_base(USER_VM_BASE, 0, PLACED_TOP - USER_VM_BASE), Some(USER_VM_BASE));
+        assert_eq!(rebase_base(USER_VM_BASE, 0, PLACED_TOP - USER_VM_BASE + 1), None);
+        assert_eq!(rebase_base(USER_VM_BASE, 0, USER_TOP - USER_VM_BASE), None);
         assert_eq!(rebase_base(USER_VM_BASE, 0, u64::MAX), None);
+        assert_eq!(rebase_base(u64::MAX, 0, 1), None);
     }
 }