isolation M1: ring 3 + a real /sbin/init, end to end

Ring 3 works: user GDT descriptors (sysret-ready layout), TSS.rsp0,
U/S-bit user mappings (W^X preserved), an int 0x80 syscall gate with a
mutable trap frame, and a setjmp-style enter/exit path. /sbin/init is a
real freestanding Zig binary built from sbin/, shipped on the ESP,
loaded by the bootloader (BootInfo.init_base/len), validated and mapped
by an in-kernel user-ELF loader, and run at CPL 3 — syscalls: exit,
ping, write. Tests: user, user-pf (U/S isolation proof, error code
0x5), init. Suite 27/27.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

build.zig

@@ -145,6 +145,31 @@ pub fn build(b: *std.Build) void {
 
     b.installArtifact(exe);
 
+    // --- /sbin/init: the first user-space program ---
+    // Its own tiny freestanding binary, linked at a fixed address inside the
+    // kernel's user region (usermode.zig) and started in ring 3 by the kernel's
+    // user-ELF loader. `.large` because the image base is above 4 GiB — small/
+    // medium code models emit 32-bit absolute relocations that can't reach.
+    // Pinned to ReleaseSmall: the user region gives it a 2 MiB budget and its
+    // size has no reason to track the kernel's optimize mode.
+    const init_exe = b.addExecutable(.{
+        .name = "init",
+        .root_module = b.createModule(.{
+            .root_source_file = b.path("sbin/init.zig"),
+            .target = kernel_target,
+            .optimize = .ReleaseSmall,
+            .code_model = .large,
+            .single_threaded = true,
+            .sanitize_c = .off,
+            .stack_check = false,
+            .stack_protector = false,
+        }),
+    });
+    init_exe.setLinkerScript(b.path("sbin/linker.ld"));
+    init_exe.entry = .{ .symbol_name = "_start" };
+    init_exe.image_base = 0x7000_0000_0000;
+    b.installArtifact(init_exe);
+
     // Boot methods live in src/boot/, one per way of getting the kernel running.
     // Each is its own binary/entry (a loader is built for its own target); today
     // that's UEFI for x86-64, with room for e.g. a device-tree path for the Pis.
@@ -203,6 +228,10 @@ pub fn build(b: *std.Build) void {
     const kernel_install = b.addInstallArtifact(exe, .{
         .dest_dir = .{ .override = .{ .custom = "esp" } },
     });
+    // The bootloader loads init from sbin/init on the same volume.
+    const init_install = b.addInstallArtifact(init_exe, .{
+        .dest_dir = .{ .override = .{ .custom = "esp/sbin" } },
+    });
 
     // The firmware needs to write NVRAM, so give it a writable copy of the vars.
     const vars_copy = b.addSystemCommand(&.{ "cp", "-f", ovmf_vars });
@@ -239,6 +268,7 @@ pub fn build(b: *std.Build) void {
     run_efi.addArgs(&.{ "-serial", b.fmt("file:{s}", .{serial_log}) });
     run_efi.step.dependOn(&efi_install.step);
     run_efi.step.dependOn(&kernel_install.step);
+    run_efi.step.dependOn(&init_install.step);
 
     const run_efi_step = b.step("run-x86-64", "Boot the x86-64 kernel in QEMU (UEFI/OVMF); serial0 is logged to zig-out/run-x86-64-serial0-<timestamp>.log");
     run_efi_step.dependOn(&run_efi.step);

docs/syscall.md

@@ -1,5 +1,11 @@
 # System Calls
-System calls (syscalls) are the bridge between your programs and the operating system's restricted core (kernel). 
+System calls (syscalls) are the bridge between your programs and the operating system's restricted core (kernel).
+
+> **Status:** danos currently has a placeholder M1 surface behind an `int 0x80`
+> gate — `0 = exit(code)`, `1 = ping(value)`, `2 = write(ptr, len)` (see
+> `src/kernel/usermode.zig`, used by `sbin/init.zig`). It exists to prove the
+> ring transition; the microkernel set below replaces it (via `syscall`/`sysret`)
+> when processes land (M3).
 
 ## The Mechanism of a Syscall
 

docs/vision.md

@@ -81,11 +81,16 @@ prerequisites.
 (with boot-services memory reclaimed), [paging](paging.md) with W^X, [exceptions and
 interrupts](interrupts.md), a [calibrated timer + ns clock](device-interrupts.md), a
 [heap](heap.md), a [fixed-priority preemptive scheduler](scheduling.md) with blocking,
-and in-kernel [IPC channels](ipc.md) — plus a [test harness](testing.md).
+in-kernel [IPC channels](ipc.md), SMP (all cores scheduling, with affinity), and
+**ring 3**: user GDT/TSS plumbing, U/S-bit mappings, an `int 0x80` syscall gate, and
+`/sbin/init` — a real user ELF built from `sbin/`, shipped on the boot volume, loaded
+by the kernel, run at CPL 3 — plus a [test harness](testing.md).
 
 - **Isolation track** — **user mode + address-space isolation** (higher-half kernel,
-  ring 3, per-process page tables). The substrate everything else needs. *Next, and a
-  prerequisite for the resilience and driver tracks.*
+  ring 3, per-process page tables). The substrate everything else needs. *In
+  progress: ring 3 + a loaded `/sbin/init` work (M1); next the higher-half move (M2),
+  then per-process address spaces + `syscall`/`sysret` + init as a real schedulable
+  process (M3), then ELF/initrd generalisation (M4).*
 - **Resilience track** — fault → kill → notify, a supervisor/reincarnation server,
   resource cleanup on death, then a restartable driver as proof. Needs isolation.
   See [resilience.md](resilience.md).

sbin/init.zig

@@ -0,0 +1,54 @@
+//! /sbin/init — the first user-space program. Built as its own freestanding
+//! binary (see build.zig), shipped on the boot volume at sbin/init, loaded by
+//! the bootloader, and started in ring 3 by the kernel's user-ELF loader
+//! (src/kernel/usermode.zig). It talks to the kernel only through the
+//! `int $0x80` syscall gate.
+//!
+//! Today it just proves the path — say hello, exit — and grows into the real
+//! init (service supervision) once processes are schedulable (M3).
+
+const std = @import("std");
+
+// The M1 syscall numbers (usermode.zig): 0 = exit(code), 2 = write(ptr, len).
+const sys_exit = 0;
+const sys_write = 2;
+
+fn syscall2(n: u64, a: u64, b: u64) u64 {
+    return asm volatile ("int $0x80"
+        : [ret] "={rax}" (-> u64),
+        : [n] "{rax}" (n),
+          [a] "{rdi}" (a),
+          [b] "{rsi}" (b),
+        : .{ .memory = true });
+}
+
+fn write(msg: []const u8) void {
+    _ = syscall2(sys_write, @intFromPtr(msg.ptr), msg.len);
+}
+
+fn exit(code: u64) noreturn {
+    _ = syscall2(sys_exit, code, 0);
+    unreachable; // the kernel never returns from exit
+}
+
+/// Entry. Naked: the kernel enters with rsp 16-aligned, but a SysV function
+/// expects rsp ≡ 8 (mod 16) on entry (as if reached by `call`) — so re-enter
+/// the ABI with an actual call. The trap after is unreachable.
+pub export fn _start() callconv(.naked) noreturn {
+    asm volatile (
+        \\call init_main
+        \\ud2
+    );
+}
+
+export fn init_main() callconv(.c) noreturn {
+    write("init: hello from user space\n");
+    exit(0);
+}
+
+/// No runtime to unwind into — report the panic as a nonzero exit code.
+pub const panic = std.debug.FullPanic(struct {
+    fn panic(_: []const u8, _: ?usize) noreturn {
+        exit(127);
+    }
+}.panic);

sbin/linker.ld

@@ -0,0 +1,45 @@
+/* /sbin/init link layout.
+ *
+ * Linked at a fixed user-space virtual base (set by `image_base` in build.zig,
+ * inside the kernel's user region). Same discipline as the kernel's script:
+ * one PT_LOAD per permission set, every section page-aligned, so the kernel's
+ * user-ELF loader can map each segment with exact W^X permissions. Note the
+ * linker also emits a read-only PT_LOAD covering the ELF headers at the image
+ * base, so the entry point comes from e_entry, not the base address.
+ */
+
+ENTRY(_start)
+
+/* FLAGS bits: 1=X, 2=W, 4=R. */
+PHDRS {
+    text   PT_LOAD FLAGS(5); /* R + X */
+    rodata PT_LOAD FLAGS(4); /* R     */
+    data   PT_LOAD FLAGS(6); /* R + W */
+}
+
+SECTIONS {
+    .text ALIGN(4K) : {
+        *(.text .text.*)
+    } :text
+
+    .rodata ALIGN(4K) : {
+        *(.rodata .rodata.*)
+    } :rodata
+
+    .data ALIGN(4K) : {
+        *(.data .data.*)
+    } :data
+
+    /* .bss occupies memory but not file space; the loader zeroes the
+     * filesz..memsz gap. */
+    .bss ALIGN(4K) : {
+        *(.bss .bss.*)
+        *(COMMON)
+    } :data
+
+    /DISCARD/ : {
+        *(.comment)
+        *(.note .note.*)
+        *(.eh_frame .eh_frame_hdr)
+    }
+}

src/boot/efi.zig

@@ -11,6 +11,10 @@ const MemoryMapSlice = uefi.tables.MemoryMapSlice;
 /// build.zig). UEFI wants a UTF-16, null-terminated path.
 const kernel_file_name = std.unicode.utf8ToUtf16LeStringLiteral("kernel");
 
+/// Path of the init program on the boot volume (UEFI paths use backslashes;
+/// the FAT driver walks the components itself, so no directory dance needed).
+const init_file_name = std.unicode.utf8ToUtf16LeStringLiteral("sbin\\init");
+
 /// Physical page size, and the sentinel UEFI uses to seek to end-of-file.
 const page_size = 4096;
 const seek_end = 0xffff_ffff_ffff_ffff;
@@ -54,6 +58,13 @@ fn boot() !noreturn {
 
     const entry = try loadKernel(bs, &boot_info);
 
+    // Best effort: a volume without sbin/init still boots (kernel-only).
+    loadInit(bs, &boot_info) catch |err| {
+        log("danos: no sbin/init (");
+        logBytes(@errorName(err));
+        log(") - booting without user space\r\n");
+    };
+
     log("danos: kernel loaded, exiting boot services\r\n");
     boot_info.memory_map = try exitBootServices(bs);
 
@@ -198,6 +209,43 @@ fn loadKernel(bs: *uefi.tables.BootServices, boot_info: *BootInfo) !usize {
     return loadElf(bs, image, boot_info);
 }
 
+/// Read the init program (sbin/init) into memory that outlives the loader and
+/// record it in the handoff. The pool buffer is deliberately NOT freed: it's
+/// LoaderData, which the memory-map conversion classifies as reserved, so the
+/// kernel identity-maps it and reads the ELF from there. The kernel does the
+/// loading itself (into ring-3 mappings) — the loader just ferries the bytes.
+fn loadInit(bs: *uefi.tables.BootServices, boot_info: *BootInfo) !void {
+    const loaded = (try bs.handleProtocol(uefi.protocol.LoadedImage, uefi.handle)) orelse
+        return error.NoLoadedImage;
+    const device = loaded.device_handle orelse return error.NoBootDevice;
+    const fs = (try bs.handleProtocol(uefi.protocol.SimpleFileSystem, device)) orelse
+        return error.NoFileSystem;
+
+    const root = try fs.openVolume();
+    defer _ = root.close() catch {};
+
+    const file = try root.open(init_file_name, .read, .{});
+    defer _ = file.close() catch {};
+
+    try file.setPosition(seek_end);
+    const size: usize = @intCast(try file.getPosition());
+    try file.setPosition(0);
+    if (size == 0) return error.EmptyFile;
+
+    const image = try bs.allocatePool(.loader_data, size); // survives the handoff
+
+    var read_total: usize = 0;
+    while (read_total < size) {
+        const n = try file.read(image[read_total..]);
+        if (n == 0) return error.UnexpectedEof;
+        read_total += n;
+    }
+
+    boot_info.init_base = @intFromPtr(image.ptr);
+    boot_info.init_len = size;
+    log("danos: sbin/init loaded\r\n");
+}
+
 /// Validate the ELF, copy every PT_LOAD segment to its physical address, and
 /// record each segment's layout so the kernel can re-map itself with the right
 /// permissions.

src/kernel/arch/x86_64/cpu.zig

@@ -76,6 +76,45 @@ pub fn unmapPage(virt: u64) void {
     paging.unmap(virt);
 }
 
+/// Map a page accessible from ring 3 (U/S bit at every level). The caller keeps
+/// W^X: code read-only + executable, data writable + no-execute.
+pub fn mapUserPage(virt: u64, phys: u64, writable: bool, executable: bool) void {
+    paging.mapUser(virt, phys, writable, executable);
+}
+
+// --- ring 3 entry/exit -----------------------------------------------------
+
+/// Drop to ring 3 at `rip` on `rsp` (defined in isr.s). Saves the kernel context,
+/// publishes the kernel stack pointer through `rsp0_slot` (this core's TSS.rsp0,
+/// so ring-3 interrupts land on a good stack), builds an iretq frame with the
+/// user selectors, and iretq's. "Returns" only when the user program triggers
+/// the exit path (user_exit_to_kernel).
+extern fn enter_user(rip: u64, rsp: u64, rsp0_slot: *align(4) u64) callconv(.c) void;
+
+/// Abandon the in-flight ring-3 trap context and resume the kernel as if
+/// `enter_user` had returned (defined in isr.s). Called by the exit syscall.
+extern fn user_exit_to_kernel() callconv(.c) noreturn;
+
+/// Run user code at `rip` with stack `rsp` on this core (`cpu` = the caller's CPU
+/// index; the arch layer can't ask the scheduler). Returns after the user program
+/// exits via syscall. Interrupts are disabled on return (the exit arrives through
+/// an interrupt gate) — the caller re-enables.
+pub fn enterUser(cpu: usize, rip: u64, rsp: u64) void {
+    enter_user(rip, rsp, tss.rsp0Ptr(cpu));
+}
+
+/// Never returns to the user program: unwind to the kernel context that called
+/// `enterUser`. For the exit syscall's handler.
+pub fn userExit() noreturn {
+    user_exit_to_kernel();
+}
+
+/// Register the handler for the ring-3 syscall gate (int 0x80, vector 128). The
+/// handler may write the trap frame (e.g. rax as the return value).
+pub fn setSyscallHandler(handler: *const fn (*idt.CpuState) void) void {
+    idt.setSyscallHandler(handler);
+}
+
 /// CR3 holds the physical address of the active top-level page table.
 pub fn readCr3() u64 {
     return asm volatile ("mov %%cr3, %[out]"
@@ -84,9 +123,11 @@ pub fn readCr3() u64 {
 }
 
 /// IA32_GS_BASE: the hidden base of the GS segment. We repurpose it as the per-CPU
-/// data pointer (there's no user mode yet, so no `swapgs` dance — GS base is always
-/// the running core's per-CPU block). Set once per core during bring-up, after the
-/// GDT is loaded (loading a GS *selector* would otherwise clobber this base).
+/// data pointer. Because it's always read back via `rdmsr` (never gs-relative
+/// addressing), no `swapgs` dance is needed even with user mode: the MSR is
+/// privileged, ring 3 can't touch it, and its value is unaffected by ring
+/// transitions. Set once per core during bring-up, after the GDT is loaded
+/// (loading a GS *selector* would otherwise clobber this base).
 const ia32_gs_base = 0xC000_0101;
 
 /// Publish this core's per-CPU data pointer so `cpuLocal` can retrieve it. Each

src/kernel/arch/x86_64/gdt.zig

@@ -1,8 +1,8 @@
 //! Global Descriptor Table. In long mode segmentation is mostly vestigial, but
 //! the CPU still needs valid code/data segment descriptors, and the IDT's gates
 //! reference a code selector — so we install our own flat GDT with known
-//! selectors (0x08 kernel code, 0x10 kernel data) rather than trusting whatever
-//! the firmware left in place.
+//! selectors (0x08/0x10 kernel code/data, 0x18/0x20 user data/code for ring 3)
+//! rather than trusting whatever the firmware left in place.
 //!
 //! The code/data descriptors are identical on every core, but the **TSS descriptor
 //! is per-core** (each core needs its own TSS — its own interrupt/fault stacks; see
@@ -14,20 +14,35 @@ const config = @import("config");
 /// Selectors into the table (index * 8). Same on every core's GDT.
 pub const kernel_code = 0x08;
 pub const kernel_data = 0x10;
-pub const tss_selector = 0x18;
+pub const user_data = 0x18;
+pub const user_code = 0x20;
+pub const tss_selector = 0x28;
+
+/// Ring-3 selectors as loaded from user mode: RPL 3 or'd in. The user *data*
+/// descriptor is load-bearing even in long mode — iretq to CPL 3 with a null SS
+/// raises #GP(0).
+pub const user_code_rpl3 = user_code | 3;
+pub const user_data_rpl3 = user_data | 3;
 
 const max_cpus = config.max_cpus;
-const entries = 5; // null, code, data, TSS-low, TSS-high
+const entries = 7; // null, kcode, kdata, udata, ucode, TSS-low, TSS-high
 
-/// The shared descriptors (slots 0-2); slots 3-4 hold this core's TSS descriptor,
+/// The shared descriptors (slots 0-4); slots 5-6 hold this core's TSS descriptor,
 /// filled in per core by `setTssFor`.
-///   code: present, ring 0, executable, readable, L=1  -> 0x00AF9A00_0000FFFF
-///   data: present, ring 0, writable                   -> 0x00CF9200_0000FFFF
+///   kernel code: present, ring 0, executable, readable, L=1 -> 0x00AF9A00_0000FFFF
+///   kernel data: present, ring 0, writable                  -> 0x00CF9200_0000FFFF
+///   user data:   present, ring 3, writable                  -> 0x00CFF200_0000FFFF
+///   user code:   present, ring 3, executable, readable, L=1 -> 0x00AFFA00_0000FFFF
+/// User data sits below user code so a future SYSRET works unchanged: it loads
+/// CS = STAR.SYSRET_CS + 16 and SS = STAR.SYSRET_CS + 8, so with SYSRET_CS = 0x10
+/// those land on 0x20 (user code) and 0x18 (user data).
 const template = [entries]u64{
     0, // null descriptor (required)
     0x00AF9A000000FFFF, // kernel code (0x08)
     0x00CF92000000FFFF, // kernel data (0x10)
-    0, // TSS descriptor low  (0x18)
+    0x00CFF2000000FFFF, // user data   (0x18)
+    0x00AFFA000000FFFF, // user code   (0x20)
+    0, // TSS descriptor low  (0x28)
     0, // TSS descriptor high
 };
 
@@ -38,13 +53,13 @@ var gdts = [_][entries]u64{template} ** max_cpus;
 /// register can point at its own TSS. Type 0x89 = present, ring 0, available 64-bit
 /// TSS. Write it into that core's GDT before it loads the TSS selector.
 pub fn setTssFor(cpu: usize, base: u64, limit: u64) void {
-    gdts[cpu][3] = (limit & 0xFFFF) |
+    gdts[cpu][5] = (limit & 0xFFFF) |
         ((base & 0xFFFF) << 16) |
         (((base >> 16) & 0xFF) << 32) |
         (@as(u64, 0x89) << 40) |
         (((limit >> 16) & 0xF) << 48) |
         (((base >> 24) & 0xFF) << 56);
-    gdts[cpu][4] = (base >> 32) & 0xFFFFFFFF;
+    gdts[cpu][6] = (base >> 32) & 0xFFFFFFFF;
 }
 
 /// The operand `lgdt` wants: table byte-length minus one, then its address.

src/kernel/arch/x86_64/idt.zig

@@ -26,6 +26,19 @@ pub fn setHandler(vector: usize, handler: Handler) void {
     handlers[vector] = handler;
 }
 
+/// The ring-3 syscall gate's vector (`int $0x80`, the classic choice — well away
+/// from the device range) and its handler. Unlike device handlers, a syscall
+/// handler gets the (mutable) trap frame: it reads its arguments from the saved
+/// user registers and writes rax as the return value, which isr_common then
+/// restores into the user context.
+pub const syscall_vector = 128;
+
+var syscall_handler: ?*const fn (*CpuState) void = null;
+
+pub fn setSyscallHandler(handler: *const fn (*CpuState) void) void {
+    syscall_handler = handler;
+}
+
 /// The register + trap frame the ISR stubs build on the stack, laid out so the
 /// lowest address (where RSP points when we call the handler) is the first field.
 /// See the push order in `isrCommon` below.
@@ -128,6 +141,13 @@ pub fn init() void {
     // Run the double-fault handler (vector 8) on IST1: a #DF usually means the
     // current stack is unusable, so it needs a guaranteed-good one. See tss.zig.
     idt[8].ist = tss.double_fault_ist;
+    // The syscall gate. Installed outside the 0..gate_count loop (stubs 48-127
+    // don't exist) and with DPL 3 — without it, `int $0x80` from ring 3 is a
+    // #GP. An interrupt gate (not trap): IF is cleared for the handler, which
+    // the ring-3 exit path relies on.
+    const syscall_stub = @extern(*const anyopaque, .{ .name = "isr128" });
+    setGate(syscall_vector, @intFromPtr(syscall_stub));
+    idt[syscall_vector].flags = 0xEE; // present, DPL 3, 64-bit interrupt gate
     loadOnThisCpu();
 }
 
@@ -145,9 +165,12 @@ pub fn loadOnThisCpu() void {
 /// Called by isr_common (isr.s) with a pointer to the trap frame. Exported so the
 /// assembly stubs can `call` it by name. Exceptions are terminal; device
 /// interrupts run their handler, get acknowledged, and return.
-export fn interruptDispatch(state: *const CpuState) callconv(.c) void {
+export fn interruptDispatch(state: *CpuState) callconv(.c) void {
     if (state.vector < 32) {
         on_fault(state); // CPU exception — never returns
+    } else if (state.vector == syscall_vector) {
+        // Software interrupt from ring 3 — no LAPIC ISR bit is set, so no EOI.
+        if (syscall_handler) |handler| handler(state);
     } else if (handlers[state.vector]) |handler| {
         // Acknowledge before running the handler: a handler that switches tasks
         // (the scheduler) may not return promptly, and the LAPIC mustn't wait on

src/kernel/arch/x86_64/isr.s

@@ -76,6 +76,96 @@ task_trampoline:
 1:  hlt                     # if the entry returns, idle (still preemptible)
     jmp 1b
 
+# --- ring 3 entry/exit ------------------------------------------------------
+
+# enter_user(rdi = user rip, rsi = user rsp, rdx = &TSS.rsp0)
+# Drop to ring 3. Saves the callee-saved registers and the kernel stack pointer
+# (so user_exit_to_kernel can unwind back here), publishes that stack pointer as
+# this core's TSS.rsp0 — everything below it is dead, so ring-3 interrupt frames
+# grow safely into it — then builds the 5-word iretq frame with the user
+# selectors (RPL 3) and drops privilege. IF is set in the pushed RFLAGS so the
+# timer keeps running in user mode.
+.global enter_user
+enter_user:
+    push %rbx
+    push %rbp
+    push %r12
+    push %r13
+    push %r14
+    push %r15
+    mov %rsp, user_saved_rsp(%rip)  # where user_exit_to_kernel unwinds to
+    mov %rsp, (%rdx)                # TSS.rsp0: ring-3 interrupts stack here
+    push $0x1B                      # user SS   (0x18 | RPL 3)
+    push %rsi                       # user RSP
+    push $0x202                     # RFLAGS: IF | reserved-1
+    push $0x23                      # user CS   (0x20 | RPL 3)
+    push %rdi                       # user RIP
+    iretq
+
+# user_exit_to_kernel: abandon the in-flight ring-3 trap frame (it lives in the
+# dead zone below user_saved_rsp) and return as if enter_user's call completed.
+# Reached from the exit syscall's handler; interrupts are off (interrupt gate)
+# and stay off — the Zig caller re-enables.
+.global user_exit_to_kernel
+user_exit_to_kernel:
+    mov user_saved_rsp(%rip), %rsp
+    pop %r15
+    pop %r14
+    pop %r13
+    pop %r12
+    pop %rbp
+    pop %rbx
+    ret
+
+.section .bss
+.balign 8
+user_saved_rsp:
+    .skip 8
+.text
+
+# --- user-mode test programs -------------------------------------------------
+# Hand-assembled ring-3 blobs, copied by the kernel onto a user-mapped page and
+# entered via enter_user. Position-independent (immediates and short jumps only).
+# In .rodata: these bytes are data to the kernel — they only execute at CPL 3
+# from the user mapping.
+.section .rodata
+
+# The hello program: ping syscall (rax=1) with a value, a delay loop long enough
+# for several timer ticks to land while at CPL 3 (proving interrupt-from-user +
+# iretq-back), a second ping, then exit (rax=0). The trailing jmp is a safety
+# net in case exit ever returns.
+.global user_prog_start
+.global user_prog_end
+user_prog_start:
+    mov $1, %rax
+    mov $0xC0DE, %rdi
+    int $0x80
+    mov $50000000, %rcx     # ~50M iterations: tens of ms even under TCG
+1:  dec %rcx
+    jnz 1b
+    mov $1, %rax
+    mov $0xBEEF, %rdi
+    int $0x80
+    mov $0, %rax
+    int $0x80
+2:  jmp 2b
+user_prog_end:
+
+# The isolation-proof program: read a kernel-only page from ring 3. The LAPIC
+# page is unconditionally mapped supervisor (paging.zig), so this must take a
+# #PF with error code 0x5 (present | user) before any access happens. The
+# address goes through %ecx (zero-extending) — an absolute `mov 0xFEE00000,%rax`
+# would sign-extend the disp32 to 0xFFFFFFFF_FEE00000 and miss the page.
+.global user_pf_start
+.global user_pf_end
+user_pf_start:
+    mov $0xFEE00000, %ecx
+    mov (%rcx), %rax
+1:  jmp 1b
+user_pf_end:
+
+.text
+
 # Stub for a vector the CPU does NOT push an error code for: push a dummy 0.
 .macro STUB_NOERR vec
 .global isr\vec
@@ -145,6 +235,10 @@ STUB_NOERR 45
 STUB_NOERR 46
 STUB_NOERR 47
 
+# The syscall gate (int $0x80 from ring 3). Same frame shape as every other
+# vector; dispatched specially in interruptDispatch.
+STUB_NOERR 128
+
 .extern interruptDispatch
 
 # Shared tail. Register push order here defines the CpuState field order.

src/kernel/arch/x86_64/paging.zig

@@ -18,6 +18,7 @@ const page_size = danos.page_size;
 // Page-table entry bits.
 const present: u64 = 1 << 0;
 const writable: u64 = 1 << 1;
+const user: u64 = 1 << 2; // U/S: accessible from ring 3 (must be set at every level)
 const no_execute: u64 = 1 << 63;
 const addr_mask: u64 = 0x000F_FFFF_FFFF_F000;
 
@@ -128,6 +129,34 @@ pub fn map(virt: u64, phys: u64, writable_page: bool) void {
     invalidate(virt);
 }
 
+/// Like `descend`, but also sets the U/S bit on the intermediate entry (new or
+/// pre-existing): ring-3 access requires U at *every* level, and `descend` leaves
+/// existing entries untouched. Only used under user-exclusive virtual ranges, so
+/// no kernel mapping's protection is widened (the leaf still governs).
+fn descendUser(entry: *u64) u64 {
+    const table = descend(entry);
+    entry.* |= user;
+    return table;
+}
+
+/// Map one 4 KiB page `virt` -> `phys` accessible from ring 3. W^X is the
+/// caller's contract: code pages are read-only + executable, data pages are
+/// writable + no-execute. `virt` must lie in a user-exclusive region (see
+/// `descendUser`).
+pub fn mapUser(virt: u64, phys: u64, writable_page: bool, executable: bool) void {
+    var flags: u64 = present | user;
+    if (writable_page) flags |= writable;
+    if (!executable) flags |= no_execute;
+    const pml4e = &tableAt(kernel_pml4)[(virt >> 39) & 0x1FF];
+    const pdpt = descendUser(pml4e);
+    const pdpte = &tableAt(pdpt)[(virt >> 30) & 0x1FF];
+    const pd = descendUser(pdpte);
+    const pde = &tableAt(pd)[(virt >> 21) & 0x1FF];
+    const pt = descendUser(pde);
+    tableAt(pt)[(virt >> 12) & 0x1FF] = (phys & addr_mask) | flags;
+    invalidate(virt);
+}
+
 /// Whether `virt` is currently mapped **executable** — present with the NX bit
 /// clear. Walks the 4-level tables (all danos mappings are 4 KiB, so no huge-page
 /// case). Returns false if unmapped. Used for W^X checks in tests.

src/kernel/arch/x86_64/tss.zig

@@ -1,9 +1,11 @@
-//! Task State Segment and its interrupt stack. In long mode the TSS's main job
-//! is the Interrupt Stack Table: an IDT gate can name an IST entry, and the CPU
-//! switches to that stack when the exception fires — no matter how broken the
-//! interrupted stack was. We use IST1 for the double-fault handler, so a fault
-//! that happens *because* the current stack is unusable still lands on solid
-//! ground instead of triple-faulting.
+//! Task State Segment and its interrupt stacks. In long mode the TSS has two
+//! jobs. First, the Interrupt Stack Table: an IDT gate can name an IST entry,
+//! and the CPU switches to that stack when the exception fires — no matter how
+//! broken the interrupted stack was. We use IST1 for the double-fault handler,
+//! so a fault that happens *because* the current stack is unusable still lands
+//! on solid ground instead of triple-faulting. Second, rsp0: the kernel stack
+//! the CPU switches to when an interrupt arrives from ring 3 (published by the
+//! user-mode entry path via `rsp0Ptr`).
 //!
 //! Each core needs **its own TSS** (its own IST stack): two cores taking a fault at
 //! once can't share one fault stack. So the TSS and its IST stack are per-core,
@@ -50,6 +52,16 @@ var ap_ist_top = [_]usize{0} ** max_cpus; // per-AP IST stack top (0 = BSP / not
 /// Loads the task register with the TSS selector. Defined in isr.s.
 extern fn load_tr(selector: u16) callconv(.c) void;
 
+/// Address of core `cpu`'s rsp0 slot — the kernel stack the CPU switches to on a
+/// ring-3 -> ring-0 interrupt. Computed as base + 4 (rsp0's architectural offset,
+/// which is why the pointer is only 4-aligned) rather than `&t.rsp0`, which on a
+/// packed struct would be an unaligned bit-pointer type. The ring-3 entry path
+/// (enter_user in isr.s) writes the current kernel stack pointer through this
+/// before dropping to user mode.
+pub fn rsp0Ptr(cpu: usize) *align(4) u64 {
+    return @ptrFromInt(@intFromPtr(&tss_table[cpu]) + 4);
+}
+
 /// Record the top of the IST stack the kernel allocated for AP `cpu`. Called on the
 /// BSP before waking that core; read by the core's own `setupThisCpu`.
 pub fn setApIstStack(cpu: usize, top: usize) void {

src/kernel/main.zig

@@ -7,6 +7,7 @@ const log = @import("log.zig");
 const pmm = @import("pmm.zig");
 const heap = @import("heap.zig");
 const scheduler = @import("scheduler.zig");
+const usermode = @import("usermode.zig");
 const platform = @import("platform");
 const tests = @import("tests.zig");
 const build_options = @import("build_options");
@@ -245,7 +246,25 @@ fn kmain(boot_info: *const BootInfo) noreturn {
     log.checkpoint(cp_running);
     status("kernel initialised.\n");
 
-    // TODO: init process
+    // Hand over to user space: run /sbin/init (read off the boot volume by the
+    // loader) in ring 3. Preemption is off for the run — the M1 user-mode path
+    // publishes *this* core's TSS.rsp0 and must not migrate (the flag is
+    // global, so the system goes cooperative meanwhile; the other cores are
+    // idle). init becomes a real schedulable process in M3.
+    if (boot_info.init_len != 0) {
+        status("starting /sbin/init...\n");
+        const image = @as([*]const u8, @ptrFromInt(boot_info.init_base))[0..boot_info.init_len];
+        scheduler.setPreemption(false);
+        const code = usermode.runInitElf(image);
+        scheduler.setPreemption(true);
+        if (code) |c| {
+            statusPrint("/sbin/init exited with code {d}.\n", .{c});
+        } else |err| {
+            statusPrint("/sbin/init failed to load: {s}\n", .{@errorName(err)});
+        }
+    } else {
+        status("no /sbin/init on the boot volume.\n");
+    }
 
     status("\nnothing left to do; halting CPU.\n");
 

src/kernel/tests.zig

@@ -17,6 +17,7 @@ const pmm = @import("pmm.zig");
 const heap = @import("heap.zig");
 const sched = @import("scheduler.zig");
 const ipc = @import("ipc.zig");
+const usermode = @import("usermode.zig");
 
 /// Formatted write straight to serial, independent of the framebuffer console.
 fn log(comptime fmt: []const u8, args: anytype) void {
@@ -92,6 +93,12 @@ pub fn run(case: []const u8, boot_info: *const BootInfo) void {
         faultNoExecute();
     } else if (eql(case, "fault-null")) {
         faultNull();
+    } else if (eql(case, "user")) {
+        userTest();
+    } else if (eql(case, "user-pf")) {
+        userPfTest();
+    } else if (eql(case, "init")) {
+        initTest(boot_info);
     } else if (eql(case, "poweroff")) {
         powerTest(.off);
     } else if (eql(case, "reboot")) {
@@ -702,6 +709,67 @@ fn smpRetryTest() void {
     result();
 }
 
+// --- ring 3 (user mode) -----------------------------------------------------
+
+/// The full ring-3 round trip: enter user mode, take syscalls and timer
+/// interrupts from CPL 3, and come back. Preemption is disabled for the run —
+/// enter_user publishes TSS.rsp0 on *this* core, so the task must not migrate
+/// (interrupts still fire and iretq back into ring 3, which is the point).
+fn userTest() void {
+    log("DANOS-TEST-BEGIN: user\n", .{});
+    sched.setPreemption(false);
+    const ran = if (usermode.run(usermode.helloBlob())) true else |err| blk: {
+        log("DANOS-USER: run failed: {s}\n", .{@errorName(err)});
+        break :blk false;
+    };
+    sched.setPreemption(true);
+
+    check("user program ran and exited (ring-3 round trip)", ran);
+    check("two ping syscalls received", usermode.ping_count == 2);
+    check("syscall args passed in registers (0xC0DE, 0xBEEF)", usermode.pings[0].value == 0xC0DE and usermode.pings[1].value == 0xBEEF);
+    check("syscalls came from CPL 3 (CS = user selector | RPL 3)", usermode.pings[0].cs == 0x23 and usermode.pings[1].cs == 0x23);
+    check("timer ticks advanced while in ring 3", usermode.pings[1].ticks > usermode.pings[0].ticks);
+    result();
+}
+
+/// Isolation: a ring-3 read of a kernel-only page (the LAPIC page — present,
+/// supervisor) must page-fault with error code 0x5 (present | user) at the user
+/// RIP. The fault report is the pass signal (matched by the harness); if the
+/// read is somehow allowed the blob spins and the harness times out.
+fn userPfTest() void {
+    log("DANOS-TEST-BEGIN: user-pf\n", .{});
+    sched.setPreemption(false);
+    _ = usermode.run(usermode.pfBlob()) catch {};
+    log("DANOS-TEST-RESULT: FAIL (user read of kernel memory did not fault)\n", .{});
+}
+
+/// The full user-binary path: the bootloader read sbin/init off the boot
+/// volume and handed it over; load it as a user ELF and run it in ring 3. The
+/// same call the normal boot path makes — here with teeth.
+fn initTest(boot_info: *const BootInfo) void {
+    log("DANOS-TEST-BEGIN: init\n", .{});
+    check("bootloader handed over sbin/init", boot_info.init_len != 0);
+    if (boot_info.init_len == 0) {
+        result();
+        return;
+    }
+    const image = @as([*]const u8, @ptrFromInt(boot_info.init_base))[0..boot_info.init_len];
+    sched.setPreemption(false); // see userTest: pins the run to this core's rsp0
+    const code = usermode.runInitElf(image);
+    sched.setPreemption(true);
+
+    if (code) |c| {
+        check("init loaded, ran, and exited (user ELF path)", true);
+        check("init exited cleanly (code 0)", c == 0);
+    } else |err| {
+        log("DANOS-INIT-ERR: {s}\n", .{@errorName(err)});
+        check("init loaded, ran, and exited (user ELF path)", false);
+    }
+    check("init's write arrived intact", eql(usermode.write_buf[0..usermode.write_len], "init: hello from user space\n"));
+    check("write came from CPL 3 (CS = user selector | RPL 3)", usermode.write_cs == 0x23);
+    result();
+}
+
 fn faultInvalidOpcode() void {
     log("DANOS-TEST-BEGIN: fault-ud\n", .{});
     asm volatile ("ud2");

src/kernel/usermode.zig

@@ -0,0 +1,298 @@
+//! Ring-3 execution — the isolation track's first rung (M1). Two entry points:
+//! `run` executes a raw code blob (the user/user-pf test programs), and
+//! `runInitElf` loads and runs a real user ELF (`/sbin/init`, handed over by
+//! the bootloader). Both run at CPL 3 in the current (global) page tables:
+//! frames are mapped user-accessible with W^X (code RO+X, data RW+NX), the CPU
+//! drops privilege via iretq, and the program talks to the kernel only through
+//! the `int $0x80` syscall gate. No processes or per-task address spaces yet —
+//! this proves the privilege mechanisms (user descriptors, TSS.rsp0, the U/S
+//! page bit, ring transitions) that M3 builds on.
+//!
+//! Caller contract (M1 limitations):
+//! - enter/exit publishes TSS.rsp0 on the *current* core only, so the caller
+//!   must prevent migration for the duration — disable preemption around the
+//!   call (rsp0-per-context-switch arrives with real user tasks in M3).
+//! - The kernel-side saved context (`user_saved_rsp` in isr.s) is a single
+//!   global: at most one core may be inside user mode at a time.
+
+const std = @import("std");
+const elf = std.elf;
+const danos = @import("danos");
+const arch = @import("arch");
+const pmm = @import("pmm.zig");
+const sched = @import("scheduler.zig");
+const log = @import("log.zig");
+
+const page_size = danos.page_size;
+
+/// User virtual addresses. PML4 index 224 — a user-exclusive region, far from
+/// the identity map (low indices) and the vmm test address (index 128), so
+/// setting the U/S bit on its intermediate tables widens no kernel mapping.
+/// An ELF image may occupy [code_virt, stack_virt); the stack page sits above.
+pub const code_virt: u64 = 0x0000_7000_0000_0000;
+pub const stack_virt: u64 = 0x0000_7000_0020_0000;
+
+// The hand-assembled user program blobs (isr.s, .rodata).
+const prog_start = @extern([*]const u8, .{ .name = "user_prog_start" });
+const prog_end = @extern([*]const u8, .{ .name = "user_prog_end" });
+const pf_start = @extern([*]const u8, .{ .name = "user_pf_start" });
+const pf_end = @extern([*]const u8, .{ .name = "user_pf_end" });
+
+/// The ring-3 hello program: ping(0xC0DE), delay loop, ping(0xBEEF), exit.
+pub fn helloBlob() []const u8 {
+    return prog_start[0 .. @intFromPtr(prog_end) - @intFromPtr(prog_start)];
+}
+
+/// The isolation-proof program: reads a kernel-only page, must #PF.
+pub fn pfBlob() []const u8 {
+    return pf_start[0 .. @intFromPtr(pf_end) - @intFromPtr(pf_start)];
+}
+
+/// What a ping syscall recorded — evidence for the test to assert on.
+pub const Ping = struct { value: u64 = 0, cs: u64 = 0, ticks: u64 = 0 };
+pub var pings = [_]Ping{.{}} ** 2;
+pub var ping_count: usize = 0;
+
+/// What write syscalls produced (accumulated), and the exit syscall's code.
+pub var write_buf: [256]u8 = undefined;
+pub var write_len: usize = 0;
+pub var write_cs: u64 = 0;
+pub var exit_code: u64 = 0;
+
+/// The M1 syscall surface, dispatched on the saved user rax:
+///   0 = exit(code)      — unwind back into the kernel context that entered
+///   1 = ping(value)     — record rdi + the caller's CS + the current tick count
+///   2 = write(ptr, len) — log bytes from user memory, prefixed DANOS-INIT:
+/// The real microkernel ABI (IPC_Call/IPC_ReplyWait/Yield, docs/syscall.md)
+/// replaces this in M3; rax is written back as the return value already, since
+/// isr_common restores user registers from the trap frame.
+fn syscall(state: *arch.CpuState) void {
+    switch (state.rax) {
+        0 => {
+            exit_code = state.rdi;
+            arch.userExit();
+        },
+        1 => {
+            if (ping_count < pings.len) {
+                pings[ping_count] = .{ .value = state.rdi, .cs = state.cs, .ticks = arch.ticks() };
+                ping_count += 1;
+            }
+            state.rax = 0;
+        },
+        2 => {
+            // The pointer must lie inside the user region (image + stack) —
+            // kernel addresses and non-canonical values fall outside it, so the
+            // kernel-side read below can't be steered at kernel data. Length is
+            // checked first so the upper-bound subtraction can't underflow.
+            // Known gap (fine for a trusted init): a pointer into an *unmapped*
+            // hole in the region passes the check and the read #PFs -> on_fault
+            // halts — a self-DoS, not an isolation break. Copy-in with fault
+            // recovery is M3+ (with SMAP, once there's a reason to enable it).
+            const ptr = state.rdi;
+            const len = state.rsi;
+            if (len <= write_buf.len and ptr >= code_virt and ptr <= stack_virt + page_size - len) {
+                const src: [*]const u8 = @ptrFromInt(ptr);
+                const n = @min(len, write_buf.len - write_len);
+                @memcpy(write_buf[write_len..][0..n], src[0..n]);
+                write_len += n;
+                write_cs = state.cs;
+                log.write("DANOS-INIT: ");
+                log.write(src[0..len]);
+                state.rax = len;
+            } else {
+                state.rax = @bitCast(@as(i64, -1));
+            }
+        },
+        else => state.rax = @bitCast(@as(i64, -1)),
+    }
+}
+
+/// Reset the recorded syscall evidence before a user-mode run.
+fn resetRecords() void {
+    ping_count = 0;
+    write_len = 0;
+    write_cs = 0;
+    exit_code = 0;
+}
+
+pub const RunError = error{ ProgramTooBig, OutOfMemory };
+
+/// Map `blob` at code_virt with a fresh user stack, drop to ring 3, and return
+/// once the program exits via syscall 0. See the migration caveat in the module
+/// doc. A program that faults instead never returns (on_fault halts the core).
+pub fn run(blob: []const u8) RunError!void {
+    if (blob.len > page_size) return error.ProgramTooBig;
+    const code_frame = pmm.alloc() orelse return error.OutOfMemory;
+    const stack_frame = pmm.alloc() orelse {
+        pmm.free(code_frame);
+        return error.OutOfMemory;
+    };
+
+    // Fill the code frame through its identity mapping (supervisor RW): the
+    // user-facing mapping is read-only, and this also sidesteps CR0.WP/SMAP.
+    // The tail is padded with int3 so a stray jump traps instead of sliding.
+    const code: [*]u8 = @ptrFromInt(code_frame);
+    @memcpy(code[0..blob.len], blob);
+    @memset(code[blob.len..page_size], 0xCC);
+
+    arch.setSyscallHandler(syscall);
+    arch.mapUserPage(code_virt, code_frame, false, true); // RO + X
+    arch.mapUserPage(stack_virt, stack_frame, true, false); // RW + NX
+    resetRecords();
+
+    arch.enterUser(sched.currentCpuIndex(), code_virt, stack_virt + page_size);
+
+    // Back via the exit syscall; the interrupt gate left IF clear.
+    arch.enableInterrupts();
+    arch.unmapPage(code_virt);
+    arch.unmapPage(stack_virt);
+    pmm.free(code_frame);
+    pmm.free(stack_frame);
+}
+
+// --- user ELF loading (/sbin/init) ------------------------------------------
+
+pub const InitError = error{
+    BadElf, // malformed/inapplicable image (magic, class, machine, type, bounds)
+    BadSegment, // PT_LOAD unaligned, out of the user region, W&X, or overlapping
+    BadEntry, // e_entry not inside an executable segment
+    ProgramTooBig, // more pages than the loader's budget
+    OutOfMemory,
+};
+
+const max_segments = 16;
+const max_pages = 256; // 1 MiB loader budget; the user region caps at 2 MiB anyway
+
+const Segment = struct {
+    vaddr: u64,
+    memsz: u64,
+    filesz: u64,
+    off: u64,
+    writable: bool,
+    executable: bool,
+
+    fn pages(self: Segment) u64 {
+        return (self.memsz + page_size - 1) / page_size;
+    }
+};
+
+/// Pages mapped for the running init image, for rollback if loading fails
+/// midway. On success the mappings stay for the system's life — there's no
+/// address-space teardown until real processes exist (M3).
+var loaded = [_]struct { virt: u64, frame: u64 }{.{ .virt = 0, .frame = 0 }} ** (max_pages + 1);
+var loaded_count: usize = 0;
+
+/// Parse and validate every PT_LOAD before touching memory. Bounds are checked
+/// against the image and the user region; segments must be page-aligned,
+/// non-overlapping, and W^X (R-only is fine — linkers may emit a headers-only
+/// segment, mapped RO+NX).
+fn parseSegments(image: []const u8, segs: *[max_segments]Segment) InitError!struct { count: usize, entry: u64 } {
+    if (image.len < @sizeOf(elf.Elf64_Ehdr)) return error.BadElf;
+    const ehdr = std.mem.bytesToValue(elf.Elf64_Ehdr, image[0..@sizeOf(elf.Elf64_Ehdr)]);
+    if (!std.mem.eql(u8, image[0..4], "\x7fELF")) return error.BadElf;
+    if (image[elf.EI_CLASS] != elf.ELFCLASS64) return error.BadElf;
+    if (ehdr.e_machine != .X86_64) return error.BadElf;
+    if (ehdr.e_type != .EXEC) return error.BadElf; // a PIE would need relocation
+    if (ehdr.e_phentsize < @sizeOf(elf.Elf64_Phdr)) return error.BadElf;
+    if (ehdr.e_phnum > max_segments) return error.BadElf;
+    const ph_bytes = @as(u64, ehdr.e_phnum) * ehdr.e_phentsize;
+    if (ehdr.e_phoff > image.len or ph_bytes > image.len - ehdr.e_phoff) return error.BadElf;
+
+    var count: usize = 0;
+    var total_pages: u64 = 0;
+    for (0..ehdr.e_phnum) |i| {
+        const off = ehdr.e_phoff + i * ehdr.e_phentsize;
+        const phdr = std.mem.bytesToValue(elf.Elf64_Phdr, image[off..][0..@sizeOf(elf.Elf64_Phdr)]);
+        if (phdr.p_type != elf.PT_LOAD) continue;
+        if (phdr.p_memsz == 0) continue;
+
+        if (phdr.p_vaddr % page_size != 0) return error.BadSegment;
+        if (phdr.p_filesz > phdr.p_memsz) return error.BadSegment;
+        if (phdr.p_offset > image.len or phdr.p_filesz > image.len - phdr.p_offset) return error.BadSegment;
+        // Inside the user image region, strictly below the stack page.
+        if (phdr.p_vaddr < code_virt) return error.BadSegment;
+        if (phdr.p_memsz > stack_virt - phdr.p_vaddr) return error.BadSegment;
+
+        const w = phdr.p_flags & elf.PF_W != 0;
+        const x = phdr.p_flags & elf.PF_X != 0;
+        if (w and x) return error.BadSegment; // W^X, even for init
+
+        const seg = Segment{
+            .vaddr = phdr.p_vaddr,
+            .memsz = phdr.p_memsz,
+            .filesz = phdr.p_filesz,
+            .off = phdr.p_offset,
+            .writable = w,
+            .executable = x,
+        };
+        // No overlap with any earlier segment (page-granular, since mapping is).
+        for (segs[0..count]) |other| {
+            const a_end = seg.vaddr + seg.pages() * page_size;
+            const b_end = other.vaddr + other.pages() * page_size;
+            if (seg.vaddr < b_end and other.vaddr < a_end) return error.BadSegment;
+        }
+        total_pages += seg.pages();
+        if (total_pages > max_pages) return error.ProgramTooBig;
+        segs[count] = seg;
+        count += 1;
+    }
+    if (count == 0) return error.BadElf;
+
+    // The entry point must land inside an executable segment.
+    for (segs[0..count]) |seg| {
+        if (seg.executable and ehdr.e_entry >= seg.vaddr and ehdr.e_entry < seg.vaddr + seg.memsz)
+            return .{ .count = count, .entry = ehdr.e_entry };
+    }
+    return error.BadEntry;
+}
+
+/// Map one page of a segment: fresh frame, zero + copy through the identity
+/// mapping (the user mapping may be read-only), then the user-visible mapping
+/// with the segment's W^X permissions. Records the page for rollback.
+fn loadPage(image: []const u8, seg: Segment, page_index: u64) InitError!void {
+    const frame = pmm.alloc() orelse return error.OutOfMemory;
+    const dst: [*]u8 = @ptrFromInt(frame);
+    @memset(dst[0..page_size], 0);
+    const page_off = page_index * page_size;
+    if (page_off < seg.filesz) {
+        const n = @min(page_size, seg.filesz - page_off);
+        @memcpy(dst[0..n], image[seg.off + page_off ..][0..n]);
+    }
+    const virt = seg.vaddr + page_off;
+    arch.mapUserPage(virt, frame, seg.writable, seg.executable);
+    loaded[loaded_count] = .{ .virt = virt, .frame = frame };
+    loaded_count += 1;
+}
+
+fn unloadAll() void {
+    for (loaded[0..loaded_count]) |p| {
+        arch.unmapPage(p.virt);
+        pmm.free(p.frame);
+    }
+    loaded_count = 0;
+}
+
+/// Load a user ELF image, run it in ring 3 from its entry point, and return its
+/// exit code. Same caller contract as `run` (preemption off, one core). On
+/// success the user mappings are left in place — teardown comes with real
+/// processes (M3); on a loading error everything is rolled back.
+pub fn runInitElf(image: []const u8) InitError!u64 {
+    var segs: [max_segments]Segment = undefined;
+    const parsed = try parseSegments(image, &segs);
+
+    loaded_count = 0;
+    errdefer unloadAll();
+    for (segs[0..parsed.count]) |seg| {
+        for (0..seg.pages()) |i| try loadPage(image, seg, i);
+    }
+    const stack_frame = pmm.alloc() orelse return error.OutOfMemory;
+    arch.mapUserPage(stack_virt, stack_frame, true, false); // RW + NX
+    loaded[loaded_count] = .{ .virt = stack_virt, .frame = stack_frame };
+    loaded_count += 1;
+
+    arch.setSyscallHandler(syscall);
+    resetRecords();
+    arch.enterUser(sched.currentCpuIndex(), parsed.entry, stack_virt + page_size);
+    arch.enableInterrupts(); // the exit arrived through an interrupt gate
+    return exit_code;
+}

src/root.zig

@@ -109,4 +109,10 @@ pub const BootInfo = extern struct {
     /// A device-tree boot path leaves this 0 and (later) fills a `device_tree_blob`
     /// field instead, so the kernel discovers devices without knowing what booted it.
     acpi_rsdp: u64 = 0,
+    /// The raw `/sbin/init` ELF image, read off the boot volume by the loader
+    /// into memory that survives the handoff (classified reserved, so the kernel
+    /// identity-maps it and never allocates over it). 0/0 = no init found — the
+    /// kernel boots without user space. Grows into a full initrd handoff later.
+    init_base: u64 = 0,
+    init_len: u64 = 0,
 };

test/qemu_test.py

@@ -57,6 +57,8 @@ ARCHES = {
         ],
         "efi_app": ("EFI/BOOT/BOOTX64.efi", "BOOTX64.efi"),  # (dest in ESP, name in zig-out/bin)
         "kernel": ("kernel", "kernel"),
+        # Further files shipped on the ESP: the init user program.
+        "extra": [("sbin/init", "init")],
         # Built as a function so we can splice in per-run paths.
         "qemu_args": lambda a, esp, vars_fd, serial: [
             "-machine", "q35", "-m", "128M",
@@ -148,6 +150,21 @@ CASES = [
      "expect": r"page fault \(vector 14\)",
      "fail":   r"NX not enforced"},
     {"name": "fault-null", "expect": r"page fault \(vector 14\)"},
+    # Ring 3: a user program runs at CPL 3, makes int 0x80 syscalls, survives
+    # timer interrupts, and exits back into the kernel.
+    {"name": "user",
+     "expect": r"DANOS-TEST-RESULT: PASS",
+     "fail":   r"DANOS-TEST-RESULT: FAIL"},
+    # Isolation: a ring-3 read of a kernel-only page must #PF with error code
+    # 0x5 (present|user) at the user RIP. ([\s\S] spans lines; `.` doesn't.)
+    {"name": "user-pf",
+     "expect": r"page fault \(vector 14\)[\s\S]*error code : 0x5[\s\S]*RIP\s*: 0x00007000000000",
+     "fail":   r"DANOS-TEST-RESULT: FAIL"},
+    # The real user binary: the bootloader ships sbin/init off the ESP, the
+    # kernel loads the ELF and runs it in ring 3, and it writes + exits cleanly.
+    {"name": "init",
+     "expect": r"DANOS-TEST-RESULT: PASS",
+     "fail":   r"DANOS-TEST-RESULT: FAIL"},
     # The ACPI power path succeeds by QEMU *exiting* (S5 off / reset), so match the
     # pre-transition marker; the FAIL line only appears if the transition didn't take.
     {"name": "poweroff",
@@ -179,6 +196,9 @@ def make_esp(arch):
     os.makedirs(os.path.join(esp, os.path.dirname(efi_dest)), exist_ok=True)
     shutil.copy(os.path.join(REPO, "zig-out", "bin", efi_name), os.path.join(esp, efi_dest))
     shutil.copy(os.path.join(REPO, "zig-out", "bin", kern_name), os.path.join(esp, kern_dest))
+    for dest, name in arch.get("extra", []):
+        os.makedirs(os.path.join(esp, os.path.dirname(dest)), exist_ok=True)
+        shutil.copy(os.path.join(REPO, "zig-out", "bin", name), os.path.join(esp, dest))
     return esp