rust-kernel

//! Trap frames: the user's registers, saved on the kernel stack.
//!
//! Every way into the kernel from ring 3 -- a `syscall`, a page fault or
//! other exception, the timer interrupt -- now saves *all* of the user's
//! registers in one layout, `TrapFrame`, at the top of the thread's kernel
//! stack, and every way back goes through one exit path (`trap_return`)
//! that reloads them and `iretq`s. Linux calls this `struct pt_regs`.
//!
//! Why bother (stage 13 used `sysretq` and the ready-made
//! `extern "x86-interrupt"` handlers)? Because the new process features
//! need to *read and change* the user's registers:
//!   * fork copies them for the child (with rax = 0, "you're the child"),
//!   * execve replaces them (new rip, new rsp, everything else zero),
//!   * a signal saves them on the user stack and points rip at the handler;
//!     rt_sigreturn puts them back,
//!   * and signals must be checked on *every* return to user mode, also
//!     after a timer interrupt (that's how Ctrl+C stops a program that
//!     never makes a syscall, like `spin`).
//!
//! Layout on the kernel stack (lowest address first; `rsp` points at r15
//! while the Rust handler runs):
//!
//!   r15 r14 r13 r12 r11 r10 r9 r8 rbp rdi rsi rdx rcx rbx rax   (we push)
//!   vector  error                                               (we push)
//!   rip cs rflags rsp ss                     (the CPU pushes, or we do)
//!
//! For a syscall `vector` is TRAP_SYSCALL and `error` holds the syscall
//! number (Linux's "orig_rax": rax gets the result, but a restarted call
//! needs the number again).

use core::arch::global_asm;

/// `vector` value of a frame built by the syscall entry.
pub const TRAP_SYSCALL: u64 = 0x100;

#[repr(C)]
#[derive(Clone, Copy, Debug, Default)]
pub struct TrapFrame {
    pub r15: u64,
    pub r14: u64,
    pub r13: u64,
    pub r12: u64,
    pub r11: u64,
    pub r10: u64,
    pub r9: u64,
    pub r8: u64,
    pub rbp: u64,
    pub rdi: u64,
    pub rsi: u64,
    pub rdx: u64,
    pub rcx: u64,
    pub rbx: u64,
    pub rax: u64,
    pub vector: u64,
    pub error: u64,
    pub rip: u64,
    pub cs: u64,
    pub rflags: u64,
    pub rsp: u64,
    pub ss: u64,
}

impl TrapFrame {
    /// Registers for starting a program: everything zero except the entry
    /// point, the stack, and RFLAGS = interrupts on (0x200, plus the
    /// always-one bit 1).
    pub fn new_user(rip: u64, rsp: u64) -> TrapFrame {
        let s = crate::gdt::selectors();
        TrapFrame {
            rip,
            rsp,
            rflags: 0x202,
            cs: s.user_code.0 as u64,
            ss: s.user_data.0 as u64,
            ..Default::default()
        }
    }

    /// Did this trap come from ring 3? (The low two bits of CS are the
    /// privilege level the CPU was running at.)
    pub fn from_user(&self) -> bool {
        self.cs & 3 == 3
    }

    pub fn is_syscall(&self) -> bool {
        self.vector == TRAP_SYSCALL
    }
}

unsafe extern "C" {
    /// Pops a TrapFrame (rsp must point at it) and `iretq`s.
    pub fn trap_return();
}

/// Leave the kernel for good: load `frame` into the registers and jump to
/// user mode. Used by a new process's first instructions (spawned or
/// forked). The frame is copied onto our own stack first; the stack below
/// it doesn't matter any more.
pub fn return_to_user(frame: TrapFrame) -> ! {
    unsafe {
        core::arch::asm!(
            "mov rsp, {frame}",
            "jmp {ret}",
            frame = in(reg) &frame as *const TrapFrame,
            ret = sym trap_return,
            options(noreturn)
        )
    }
}

// The common exit. Interrupts off: from here on we're restoring user
// registers and must not be interrupted half-way (the CPU turns them back
// on itself, because the saved RFLAGS has IF set).
global_asm!(
    ".global trap_return",
    "trap_return:",
    "    cli",
    "    pop r15",
    "    pop r14",
    "    pop r13",
    "    pop r12",
    "    pop r11",
    "    pop r10",
    "    pop r9",
    "    pop r8",
    "    pop rbp",
    "    pop rdi",
    "    pop rsi",
    "    pop rdx",
    "    pop rcx",
    "    pop rbx",
    "    pop rax",
    "    add rsp, 16", // skip vector and error
    "    iretq",
);

/// An interrupt/exception entry stub. The CPU has pushed ss, rsp, rflags,
/// cs, rip (and for some exceptions an error code). We push a fake error
/// code if needed, the vector number and all general registers, then call
/// `interrupts::trap_handler(&mut TrapFrame)` and leave through
/// `trap_return`. 22 slots of 8 bytes: the CPU aligned rsp to 16 before
/// its pushes, so it's still 16-aligned for the call.
macro_rules! trap_stub {
    ($name:literal, $vector:literal, $push_error:literal) => {
        global_asm!(
            concat!(".global ", $name),
            concat!($name, ":"),
            $push_error,
            concat!("    push ", $vector),
            "    push rax",
            "    push rbx",
            "    push rcx",
            "    push rdx",
            "    push rsi",
            "    push rdi",
            "    push rbp",
            "    push r8",
            "    push r9",
            "    push r10",
            "    push r11",
            "    push r12",
            "    push r13",
            "    push r14",
            "    push r15",
            "    mov rdi, rsp",
            "    cld", // Rust expects the direction flag clear
            "    call {handler}",
            "    jmp {ret}",
            handler = sym crate::interrupts::trap_handler,
            ret = sym trap_return,
        );
    };
}

// Exceptions that a user program can cause (and get a signal for), plus
// the local APIC timer (so a running program can be stopped by a signal).
trap_stub!("trap_divide_error", "0", "push 0"); // SIGFPE
trap_stub!("trap_invalid_opcode", "6", "push 0"); // SIGILL
trap_stub!("trap_general_protection", "13", ""); // SIGSEGV (CPU pushes an error code)
trap_stub!("trap_page_fault", "14", ""); // SIGSEGV or demand paging
trap_stub!("trap_apic_timer", "0x40", "push 0");

unsafe extern "C" {
    pub fn trap_divide_error();
    pub fn trap_invalid_opcode();
    pub fn trap_general_protection();
    pub fn trap_page_fault();
    pub fn trap_apic_timer();
}