feat(exec): Run userland in ring 3 with an ELF64 loader and per-process page tables

SUB-OS had a syscall table and user GDT descriptors but nothing that could
actually leave ring 0: lazybox and the shell run as kernel code, and
syscall_handler() was never reachable from a user program. This adds the
missing path end to end on x86_64.

Address spaces (arch/x86_64/mm/vmspace.c)
- The boot page tables identity-map the low 4 GB off PML4[0] and leave the
  rest empty, so userland is given its own PML4 slot (512 GiB) rather than
  holes punched in the kernel's 2 MB huge-page map. A process PML4 shares
  entry 0 with the kernel and privately owns entry 1.
- Frames outside the identity map are refused rather than written through an
  unmapped pointer; lifting that needs a recursive map or a higher-half
  direct map, which is noted in the file.

ELF loader (kernel/elf.c)
- Validates ELF64/little-endian/ET_EXEC/machine, bounds-checks every program
  header against both the image and the user address range, and lays PT_LOAD
  segments into an address space. Frames arrive zeroed, so .bss is covered.

Ring transition (arch/x86_64/cpu/usermode.asm)
- user_enter() drops to ring 3 via iretq after scrubbing every GPR, and
  user_return() unwinds back to it when the process exits, so exec looks like
  a call that blocks until the program finishes.

Syscalls
- INT 0x80 installed at DPL 3 with the Linux x86_64 register convention.
  The process gets its own kernel stack for the trap, because TSS.rsp0 must
  not point at the stack user_enter() is standing on.
- Every user pointer is range-checked before use. The process page tables are
  still live during a syscall, so without that check a program could pass a
  kernel address and have write(2) dump kernel text to the console. The new
  /sbin/init asserts both a wild pointer and a kernel pointer are rejected.
- write(2) to stdout/stderr routes through the new console_write(), which
  reuses printk's routing so output reaches serial and the framebuffer
  console rather than only the VGA text buffer.

Faults
- A ring 3 exception now kills that process and reports CR2 instead of
  panicking the machine.

Userland (userland/init/)
- /sbin/init is a real freestanding static executable, linked at the user
  base and embedded in the kernel image by scripts/mkblob.py. It reports its
  pid, verifies .bss was zeroed and .data is writable, checks the two pointer
  rejections, and exits.

aarch64 and armv8i keep building: the loader and exec path are portable, and
those ports get honest vmspace stubs that fail cleanly rather than pretending
to run a program with kernel privileges.

Verified under QEMU: init loads at 0x80004000a0, reports pid 1, .bss zeroed,
.data writable, both bad pointers rejected, and exits 0 back into the shell.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_0161Jh7uCEMLYEMNPsjC1LKf

arch/x86_64/cpu/usermode.asm

@@ -0,0 +1,89 @@
+[BITS 64]
+default rel
+
+; Ring 3 entry and exit for SUB-OS.
+;
+; user_enter() does not behave like a normal call: it drops to ring 3 and only
+; comes back when the process leaves through user_return(), which unwinds
+; straight to the frame saved here. From C it therefore looks like a call that
+; blocks until the process exits and then returns its exit code.
+
+global user_enter
+global user_return
+
+section .text
+
+; int64_t user_enter(uint64_t entry /* rdi */, uint64_t user_rsp /* rsi */)
+user_enter:
+    cli
+
+    ; Preserve the SysV callee-saved set so user_return can rebuild this frame.
+    push rbx
+    push rbp
+    push r12
+    push r13
+    push r14
+    push r15
+    mov [kernel_resume_rsp], rsp
+
+    ; Ring 3 data selectors: user data descriptor (0x20) with RPL 3.
+    mov ax, 0x23
+    mov ds, ax
+    mov es, ax
+    mov fs, ax
+    mov gs, ax
+
+    ; Build the iretq frame: SS, RSP, RFLAGS, CS, RIP.
+    push 0x23                   ; SS  = user data, RPL 3
+    push rsi                    ; RSP = top of the user stack
+    push 0x202                  ; RFLAGS with IF set, everything else clear
+    push 0x1B                   ; CS  = user code (0x18), RPL 3
+    push rdi                    ; RIP = ELF entry point
+
+    ; Do not leak kernel register contents across the privilege boundary.
+    xor rax, rax
+    xor rbx, rbx
+    xor rcx, rcx
+    xor rdx, rdx
+    xor rsi, rsi
+    xor rdi, rdi
+    xor rbp, rbp
+    xor r8,  r8
+    xor r9,  r9
+    xor r10, r10
+    xor r11, r11
+    xor r12, r12
+    xor r13, r13
+    xor r14, r14
+    xor r15, r15
+
+    iretq
+
+; void user_return(int64_t status /* rdi */) -- never returns to its caller.
+user_return:
+    cli
+    mov rsp, [kernel_resume_rsp]
+    mov rax, rdi                ; becomes user_enter's return value
+
+    ; Restore the kernel's own data selectors before touching kernel memory.
+    push rax
+    mov ax, 0x10
+    mov ds, ax
+    mov es, ax
+    mov fs, ax
+    mov gs, ax
+    pop rax
+
+    pop r15
+    pop r14
+    pop r13
+    pop r12
+    pop rbp
+    pop rbx
+    sti
+    ret
+
+section .bss
+align 8
+kernel_resume_rsp:
+    resq 1