// SPDX-License-Identifier: GPL-2.0

//! CUBELinux store reader — the kernel reading CUBE coordinates off a block device.
//!
//! The store is addressed by coordinate, and the pinned image's layout is:
//!
//! ```text
//! [MAGIC "CUBE" 4][version 1][curve tag]
//! records: [SpaceId 32][Key 24][value length u64 LE][value]
//! ```
//!
//! A store on a device is that image followed by a **write-ahead log** — the mutations since
//! the last checkpoint, in the same `op | crc32 | space | key | len | value` framing
//! `cube-store/src/wal.rs` writes. The log is a delta: the image is authoritative and
//! self-contained, and a reader that ignores the log loses only the mutations recorded after
//! the last checkpoint. `DESIGN-cubelinux-write-path.md` is why the write path looks like
//! this at all.
//!
//! This module reads both and reports the store they *describe* — the image with the log
//! applied: the record count, the total value bytes, and an FNV-1a digest over every
//! `(space, key, length, value)` in the order a checkpoint would write them. The digest exists so the kernel and userspace can be *compared*
//! rather than assumed to agree — `cube-image digest <image>` prints the same line in the
//! same field order, and the QEMU gate fails if they differ by a byte.
//!
//! # Why a reader, and only a reader
//!
//! The store's write authority has not moved yet. `PLAN-kernel-cubelinux.md` records the
//! decision (the kernel owns the store) and the hazard that makes the order matter: a
//! kernel that writes while a userspace daemon still holds the same image in memory loses
//! one of the two writers' work, silently. Until that is settled, a reader is the correct
//! amount of authority for the kernel to hold.
//!
//! # Interface
//!
//! `/dev/cubelinux` — reading it performs the read (through the kernel's own file layer)
//! and returns one line. Nothing happens at module init, so there is no ordering to get
//! wrong against the block driver that provides the device.
//!
//! ```text
//! digest curve=0 bytes=1141 records=11 value_bytes=431 fnv1a64=161113085b1573b2 errors=0
//! ```

use core::fmt::{self, Write};

use kernel::{
    alloc::AllocError,
    bindings, c_str,
    device::Device,
    fs::{File, Kiocb},
    iov::{IovIterDest, IovIterSource},
    miscdevice::{MiscDevice, MiscDeviceOptions, MiscDeviceRegistration},
    prelude::*,
    sync::aref::ARef,
};

module! {
    type: CubeStoreModule,
    name: "cubelinux_store",
    authors: ["CUBE OS"],
    description: "CUBELinux store reader (coordinates in the kernel)",
    license: "GPL",
}

/// The layout, restated here because the kernel cannot depend on the userspace crates.
/// `crates/cube-store-raw` is the source of truth; the digest comparison is what keeps
/// this copy honest.
const MAGIC: &[u8; 4] = b"CUBE";
/// The original format: magic, version, curve, then records until zero padding.
const VERSION_V1: u8 = 1;
/// The current format: the same, plus the image's byte extent and record count.
const VERSION: u8 = 2;
const HEADER_LEN_V1: usize = 6;
const HEADER_LEN_V2: usize = 6 + 8 + 8;
const SPACE_ID_LEN: usize = 32;
const RAW_KEY_LEN: usize = 24;
const RECORD_FIXED: usize = SPACE_ID_LEN + RAW_KEY_LEN + 8;

/// The device to read. A module parameter is the obvious next step; for the boot gate one
/// fixed name is honest and has one less way to be wrong.
const STORE_DEVICE: &core::ffi::CStr = c_str!("/dev/vda");

/// Refuse to pull an unbounded device into memory. The gate's images are tiny; this is the
/// guard against a wrong device name turning a read into an allocation storm.
const MAX_BYTES: usize = 64 * 1024 * 1024;

/// The log's own magic, distinct from the image's so a reader that opens the wrong one
/// cannot mistake it for the other.
const WAL_MAGIC: [u8; 4] = *b"CUBW";
const WAL_VERSION: u8 = 1;
/// Log header: magic(4) + version(1) + curve tag(1).
const WAL_HEADER_LEN: usize = 6;
/// Log entry, before the value: op(1) + crc32(4) + space(32) + key(24) + len(4).
const ENTRY_FIXED: usize = 1 + 4 + 32 + 24 + 4;
/// The log begins at the first 4 KiB boundary at or after the image. Fixed by geometry so
/// no superblock is needed to find it, and stated by the v2 header's extent.
///
/// That is the *bare* layout: an image on a device with a log after it. A device that is
/// meant to be checkpointed has a control block instead (see `Control`), because folding a
/// log into the only copy of an image is not atomic and a crash in the middle of it loses the
/// store.
const LOG_ALIGN: usize = 4096;

/// Control block magic, distinct from the image's `CUBE` and the log's `CUBW`.
const CTL_MAGIC: [u8; 4] = *b"CUBS";
const CTL_VERSION: u8 = 1;
/// One control copy, and where the two live.
const CTL_COPY_LEN: usize = 2048;
const CTL_COPY_A: usize = 0;
const CTL_COPY_B: usize = 2048;
/// Where the two image slots begin.
const CTL_DATA_OFF: u64 = 4096;
/// Bytes of a control copy covered by its checksum.
const CTL_SUMMED: usize = 44;

/// The one mutation this device accepts so far. The coordinate interface's opcodes will be
/// the language's verbs; this is the first of them.
const OP_PUT: u8 = 1;
/// Fold the log into the image.
const OP_SYNC: u8 = 3;

/// FNV-1a offset basis.
const FNV_OFFSET: u64 = 0xcbf2_9ce4_8422_2325;

/// A line of output, built in place. No allocation: the read path formats into a fixed
/// buffer, so it cannot fail for want of memory while holding a file open.
struct Line {
    buf: [u8; 256],
    len: usize,
}

impl Line {
    fn new() -> Self {
        Line {
            buf: [0; 256],
            len: 0,
        }
    }

    fn as_bytes(&self) -> &[u8] {
        &self.buf[..self.len]
    }
}

impl Write for Line {
    fn write_str(&mut self, s: &str) -> fmt::Result {
        let bytes = s.as_bytes();
        if self.len + bytes.len() > self.buf.len() {
            return Err(fmt::Error);
        }
        self.buf[self.len..self.len + bytes.len()].copy_from_slice(bytes);
        self.len += bytes.len();
        Ok(())
    }
}

/// FNV-1a 64 over a run of bytes, continuing from `h`.
fn fnv1a64(bytes: &[u8], mut h: u64) -> u64 {
    for b in bytes {
        h ^= *b as u64;
        h = h.wrapping_mul(0x0000_0100_0000_01b3);
    }
    h
}

/// Read the whole store image through the kernel's file layer.
///
/// `kernel_read` on the block device rather than a raw bio: it is the path this kernel
/// version exposes to Rust, it goes through the page cache the way any other read does, and
/// it needs no C helper. If the store ever has to be read before the VFS is up (a root
/// filesystem, say), that is the moment to reach for the block layer directly.
fn read_image(image: &mut KVVec<u8>) -> Result<()> {
    // O_RDONLY is 0 in Linux; filp_open takes the raw flags word.
    // SAFETY: STORE_DEVICE is a NUL-terminated C string literal, and filp_open either
    // returns a valid `struct file *` or an error pointer, which is checked below.
    let file = unsafe { bindings::filp_open(STORE_DEVICE.as_ptr().cast::<u8>(), 0, 0) };
    let file = kernel::error::from_err_ptr(file)?;
    if file.is_null() {
        return Err(EINVAL);
    }

    let mut pos: bindings::loff_t = 0;
    let mut chunk = KVVec::<u8>::with_capacity(4096, GFP_KERNEL)?;
    chunk.extend_from_slice(&[0u8; 4096][..], GFP_KERNEL)?;
    let mut result = Ok(());

    loop {
        // SAFETY: `file` is a live `struct file *` from filp_open; `chunk` is a 4096-byte
        // kernel buffer we own; `pos` is a valid loff_t. kernel_read reads at most
        // `chunk.len()` bytes into the buffer and does not retain either pointer.
        let n = unsafe {
            bindings::kernel_read(
                file,
                chunk.as_mut_ptr().cast::<core::ffi::c_void>(),
                chunk.len(),
                &mut pos,
            )
        };
        if n < 0 {
            result = Err(Error::from_errno(n as i32));
            break;
        }
        if n == 0 {
            break; // end of device
        }
        let n = n as usize;
        if let Err(e) = image.extend_from_slice(&chunk[..n], GFP_KERNEL) {
            result = Err(e.into());
            break;
        }
        if image.len() >= MAX_BYTES {
            break;
        }
    }

    // SAFETY: `file` came from filp_open and has not been closed; the owner argument is
    // only meaningful for locks that no one holds here.
    unsafe { bindings::filp_close(file, core::ptr::null_mut()) };

    // Less than the shortest header is not an image at all; `digest` reports the rest.
    if result.is_ok() && image.len() < HEADER_LEN_V1 {
        return Err(EINVAL);
    }
    result
}

/// Walk the records of a store image, digesting each one, exactly as
/// `cube-store-raw::iter_records` + `cube-image digest` do.
///
/// The two rules that are easy to get wrong, both from the userspace parser: a record whose
/// value runs past the buffer is a truncated record and an error, and an all-zero frame is
/// *end of records* only when every remaining byte is also zero — otherwise a real record
/// at the origin, followed by padding, would be read as an empty one.
/// What a header says, in both formats.
struct Header {
    version: u8,
    curve: u8,
    image_bytes: Option<u64>,
    record_count: Option<u64>,
}

/// Read the header, or say what is wrong with it.
fn parse_header(image: &[u8]) -> Result<Header, &'static str> {
    if image.len() < 5 || &image[0..4] != MAGIC {
        return Err("not-a-store");
    }
    let version = image[4];
    let curve = image[5];
    match version {
        VERSION_V1 => Ok(Header {
            version,
            curve,
            image_bytes: None,
            record_count: None,
        }),
        VERSION => {
            if image.len() < HEADER_LEN_V2 {
                return Err("short-v2-header");
            }
            let mut word = [0u8; 8];
            word.copy_from_slice(&image[6..14]);
            let image_bytes = u64::from_le_bytes(word);
            word.copy_from_slice(&image[14..22]);
            let record_count = u64::from_le_bytes(word);
            // A header that does not cover itself is corrupt, and guessing at the extent
            // of an image means possibly reading somebody else's bytes.
            if (image_bytes as usize) < HEADER_LEN_V2 {
                return Err("bad-extent");
            }
            Ok(Header {
                version,
                curve,
                image_bytes: Some(image_bytes),
                record_count: Some(record_count),
            })
        }
        _ => Err("unsupported-version"),
    }
}

fn digest(image: &[u8]) -> Line {
    let mut line = Line::new();

    // Where the image is, and which log belongs to it: a store device says so in its control
    // block, a bare image says so in its own header.
    let layout = match resolve_layout(image) {
        Ok(l) => l,
        Err(what) => {
            let _ = write!(line, "error={what}");
            return line;
        }
    };
    let live = &image[layout.image_off..];
    let header = match parse_header(live) {
        Ok(h) => h,
        Err(what) => {
            let _ = write!(line, "error={what}");
            return line;
        }
    };
    let end = match header.image_bytes {
        Some(n) => core::cmp::min(n as usize, live.len()),
        None => live.len(),
    };

    // With a v2 image, the store is the image *plus* whatever log follows it. The order the
    // digest is taken in is then the order a checkpoint would write: sorted by coordinate,
    // later writes winning. Without a log there is nothing to merge, and the image's own
    // order is the answer — which is also what keeps v1 images readable and unchanged.
    if header.image_bytes.is_some() {
        let log = log_window(image, &layout);
        match merged_digest(&live[..end], log, &header) {
            Ok(line) => return line,
            // A log that exists but cannot be read is worth saying out loud: the digest
            // alone would look like a shorter store.
            Err(what) if what != "no-log" => {
                let mut line = Line::new();
                let _ = write!(line, "error={what}");
                return line;
            }
            Err(_) => {}
        }
    }

    let mut h = FNV_OFFSET;
    let mut count: u64 = 0;
    let mut value_bytes: u64 = 0;
    let mut errors: u64 = 0;
    let mut off = layout.image_off
        + if header.version == VERSION_V1 {
            HEADER_LEN_V1
        } else {
            HEADER_LEN_V2
        };
    let mut remaining = header.record_count;
    let end = layout.image_off + end;

    while off + RECORD_FIXED <= end {
        if remaining == Some(0) {
            break;
        }
        let frame = &image[off..off + RECORD_FIXED];
        // Padding is not a record — but only if it is padding all the way down, and only
        // in v1, where nothing else says where the records stop. A v2 image states its
        // count, so a zero frame inside that count is a record like any other.
        if remaining.is_none()
            && frame.iter().all(|b| *b == 0)
            && image[off..].iter().all(|b| *b == 0)
        {
            break;
        }

        let space = &image[off..off + SPACE_ID_LEN];
        let key = &image[off + SPACE_ID_LEN..off + SPACE_ID_LEN + RAW_KEY_LEN];
        let mut len_bytes = [0u8; 8];
        len_bytes.copy_from_slice(&image[off + SPACE_ID_LEN + RAW_KEY_LEN..off + RECORD_FIXED]);
        let value_len = u64::from_le_bytes(len_bytes) as usize;

        let value_at = off + RECORD_FIXED;
        // A promised record that is not there is a truncated image, and saying so beats
        // returning a shorter list that looks complete.
        if value_at + value_len > end {
            errors += 1;
            break;
        }
        let value = &image[value_at..value_at + value_len];

        h = fnv1a64(space, h);
        h = fnv1a64(key, h);
        h = fnv1a64(&(value_len as u64).to_le_bytes(), h);
        h = fnv1a64(value, h);
        count += 1;
        value_bytes += value_len as u64;
        off = value_at + value_len;
        if let Some(n) = remaining.as_mut() {
            *n -= 1;
        }
    }
    // The count was a promise; not meeting it is an error the caller must see.
    if let Some(n) = remaining {
        if n > 0 {
            errors += 1;
        }
    }

    // Field order matches cube-image's `digest` line so the two diff directly.
    let _ = write!(
        line,
        "digest version={} curve={} bytes={} records={} value_bytes={} fnv1a64={:016x} errors={}",
        header.version,
        header.curve,
        image.len(),
        count,
        value_bytes,
        h,
        errors
    );
    line
}

/// What a store device's control block says: where the two image slots and the log are,
/// which slot is live, and how much of the log is in use.
#[derive(Clone, Copy)]
struct Control {
    curve: u8,
    slot_bytes: u64,
    log_capacity: u64,
    log_used: u64,
    generation: u64,
    active_slot: u8,
}

impl Control {
    /// Parse a control copy. `None` when it is not one, or its checksum does not hold — a
    /// torn update in the middle of a checkpoint is exactly what that checksum is for.
    fn decode(b: &[u8]) -> Option<Control> {
        if b.len() < CTL_SUMMED + 4 || &b[0..4] != CTL_MAGIC {
            return None;
        }
        let word = |at: usize| -> u64 {
            let mut w = [0u8; 8];
            w.copy_from_slice(&b[at..at + 8]);
            u64::from_le_bytes(w)
        };
        let mut c = [0u8; 4];
        c.copy_from_slice(&b[44..48]);
        if u32::from_le_bytes(c) != crc32(&b[..CTL_SUMMED]) {
            return None;
        }
        let control = Control {
            curve: b[5],
            slot_bytes: word(8),
            log_capacity: word(16),
            log_used: word(24),
            generation: word(32),
            active_slot: b[40],
        };
        if b[4] != CTL_VERSION || control.active_slot > 1 || control.slot_bytes == 0 {
            return None;
        }
        Some(control)
    }

    /// Encode into a control copy, checksum included.
    fn encode(&self, b: &mut [u8]) {
        for byte in b.iter_mut() {
            *byte = 0;
        }
        b[0..4].copy_from_slice(&CTL_MAGIC);
        b[4] = CTL_VERSION;
        b[5] = self.curve;
        b[8..16].copy_from_slice(&self.slot_bytes.to_le_bytes());
        b[16..24].copy_from_slice(&self.log_capacity.to_le_bytes());
        b[24..32].copy_from_slice(&self.log_used.to_le_bytes());
        b[32..40].copy_from_slice(&self.generation.to_le_bytes());
        b[40] = self.active_slot;
        let crc = crc32(&b[..CTL_SUMMED]);
        b[44..48].copy_from_slice(&crc.to_le_bytes());
    }

    fn image_off(&self) -> u64 {
        CTL_DATA_OFF + self.active_slot as u64 * self.slot_bytes
    }

    fn spare_off(&self) -> u64 {
        CTL_DATA_OFF + (1 - self.active_slot) as u64 * self.slot_bytes
    }

    fn log_off(&self) -> u64 {
        CTL_DATA_OFF + 2 * self.slot_bytes
    }
}

/// The valid control block: the copy with the higher generation. A reader that finds one
/// copy torn uses the other, which is the entire reason there are two.
fn read_control(b: &[u8]) -> Option<Control> {
    if b.len() < CTL_COPY_B + CTL_COPY_LEN {
        return None;
    }
    let a = Control::decode(&b[CTL_COPY_A..CTL_COPY_A + CTL_COPY_LEN]);
    let b_copy = Control::decode(&b[CTL_COPY_B..CTL_COPY_B + CTL_COPY_LEN]);
    match (a, b_copy) {
        (Some(x), Some(y)) => Some(if x.generation >= y.generation { x } else { y }),
        (Some(x), None) => Some(x),
        (None, Some(y)) => Some(y),
        (None, None) => None,
    }
}

/// Where the image is and what log belongs to it.
struct Layout {
    /// Byte offset of the live image.
    image_off: usize,
    /// Byte offset of its log.
    log_off: usize,
    /// Bytes of log in use.
    log_used: usize,
    /// The control block, when this is a store device rather than a bare image.
    control: Option<Control>,
}

/// Work out the layout: a store device when the control block is there, a bare image when it
/// is not. Both are readable; only the store layout can be checkpointed atomically, which is
/// why `sync` refuses on the other.
fn resolve_layout(b: &[u8]) -> Result<Layout, &'static str> {
    if b.len() >= CTL_COPY_B + CTL_COPY_LEN && &b[0..4] == CTL_MAGIC {
        let control = read_control(b).ok_or("control-block-corrupt")?;
        let log_off = control.log_off() as usize;
        let used = control.log_used as usize;
        if log_off + used > b.len() || used > control.log_capacity as usize {
            return Err("log-outside-device");
        }
        return Ok(Layout {
            image_off: control.image_off() as usize,
            log_off,
            log_used: used,
            control: Some(control),
        });
    }
    // A bare image. A v1 image has no extent and therefore no log — it is the whole device
    // as far as this module is concerned, and it is read the way it always was.
    let header = parse_header(b)?;
    let log_off = match header.image_bytes {
        Some(n) => log_offset(n),
        None => b.len(),
    };
    Ok(Layout {
        image_off: 0,
        log_off,
        // A bare device records no log length, so the log is bounded by the buffer and by
        // the entries that parse: `apply_log` stops at the first one that does not.
        log_used: b.len().saturating_sub(log_off),
        control: None,
    })
}

/// One record on its way into the merged store. Fixed size, so a `KVVec` of these sorts
/// in place; values live in a pool beside them and are referenced by offset.
#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
struct Entry {
    space: [u8; 32],
    key: [u8; 24],
    /// Order in which this entry arrived. Breaks ties between the same coordinate, so the
    /// later write wins after sorting.
    seq: u32,
    value_off: u32,
    value_len: u32,
    deleted: bool,
}

/// The store as a sorted set of records, which is what a checkpoint writes and what the
/// digest is taken over.
struct Merged {
    pool: KVVec<u8>,
    entries: KVVec<Entry>,
    seq: u32,
    /// Log entries that were seen but not applied, because they were torn or malformed.
    dropped: u64,
}

impl Merged {
    fn new() -> Result<Self, AllocError> {
        Ok(Merged {
            pool: KVVec::new(),
            entries: KVVec::new(),
            seq: 0,
            dropped: 0,
        })
    }

    /// Add a record. `deleted` marks a removal, which is kept in the list so it can
    /// override an older value for the same coordinate.
    fn add(&mut self, space: &[u8], key: &[u8], value: &[u8], deleted: bool) -> Result<(), AllocError> {
        let mut sp = [0u8; 32];
        sp.copy_from_slice(&space[..32]);
        let mut k = [0u8; 24];
        k.copy_from_slice(&key[..24]);
        let value_off = self.pool.len() as u32;
        self.pool.extend_from_slice(value, GFP_KERNEL)?;
        let seq = self.seq;
        self.seq += 1;
        self.entries.push(
            Entry {
                space: sp,
                key: k,
                seq,
                value_off,
                value_len: value.len() as u32,
                deleted,
            },
            GFP_KERNEL,
        )?;
        Ok(())
    }

    /// Put the entries in the order a checkpoint would write them: by space, then by key,
    /// with the later write of a coordinate last — so a group's final entry decides it.
    fn sort_entries(&mut self) {
        self.entries.as_mut_slice().sort_unstable();
    }

    fn value(&self, e: &Entry) -> &[u8] {
        let off = e.value_off as usize;
        &self.pool.as_slice()[off..off + e.value_len as usize]
    }
}

/// Read the log that follows the image, if there is one, and apply its entries.
///
/// The log is a delta on the image, so its entries override image records for the same
/// coordinate. Recovery is prefix-trusting, exactly as userspace does it: entries are
/// replayed from the start and the first one that is short, mis-framed or fails its
/// checksum ends the log. A fault in the middle cannot be told from a torn tail without a
/// second copy, so the log stops there and reports what it dropped instead of guessing.
fn apply_log(log: &[u8], merged: &mut Merged) -> Result<Option<u64>, &'static str> {
    if log.len() < WAL_HEADER_LEN || &log[0..4] != WAL_MAGIC {
        return Ok(None);
    }
    if log[4] != WAL_VERSION {
        return Err("unsupported-log-version");
    }

    let mut applied: u64 = 0;
    let mut off = WAL_HEADER_LEN;
    while off + ENTRY_FIXED <= log.len() {
        let start = off;
        let op = log[off];
        if op != 1 && op != 2 {
            break;
        }
        let mut word = [0u8; 4];
        word.copy_from_slice(&log[off + 1..off + 5]);
        let crc = u32::from_le_bytes(word);
        let space = &log[off + 5..off + 37];
        let key = &log[off + 37..off + 61];
        word.copy_from_slice(&log[off + 61..off + 65]);
        let len = u32::from_le_bytes(word) as usize;
        let frame_end = start + ENTRY_FIXED + len;
        if frame_end > log.len() {
            break;
        }
        // The checksum covers space, key, length and value, so a corrupted entry is
        // stopped at rather than applied.
        if crc32(&log[start + 5..frame_end]) != crc {
            break;
        }
        let value = &log[start + ENTRY_FIXED..frame_end];
        off = frame_end;
        merged
            .add(space, key, value, op == 2)
            .map_err(|_| "out-of-memory")?;
        applied += 1;
    }

    merged.dropped = (log.len() - off) as u64;
    Ok(Some(applied))
}

/// CRC-32 (IEEE 802.3), bitwise — the same polynomial and the same coverage as
/// `cube-store/src/wal.rs`, because a log written on either side must validate on both.
fn crc32(bytes: &[u8]) -> u32 {
    let mut crc = 0xFFFF_FFFFu32;
    for byte in bytes {
        crc ^= *byte as u32;
        for _ in 0..8 {
            let mask = (crc & 1).wrapping_neg();
            crc = (crc >> 1) ^ (0xEDB8_8320 & mask);
        }
    }
    !crc
}

/// The log window that belongs to the live image.
fn log_window<'a>(device: &'a [u8], layout: &Layout) -> &'a [u8] {
    let end = core::cmp::min(
        layout.log_off + WAL_HEADER_LEN + layout.log_used,
        device.len(),
    );
    if layout.log_off <= end {
        &device[layout.log_off..end]
    } else {
        &[]
    }
}

/// The store the image and its log together describe: the image's records, then the log's
/// entries applied over them. This is what a checkpoint writes and what the digest is taken
/// over, so both go through here rather than each walking the bytes its own way.
fn build_merged(image: &[u8], log: &[u8], header: &Header) -> Result<(Merged, u64), &'static str> {
    let mut merged = Merged::new().map_err(|_| "out-of-memory")?;

    let extent = header.image_bytes.ok_or("v1-has-no-extent")? as usize;
    let extent = core::cmp::min(extent, image.len());
    let mut off = if header.version == VERSION_V1 {
        HEADER_LEN_V1
    } else {
        HEADER_LEN_V2
    };
    let mut seen: u64 = 0;
    while off + RECORD_FIXED <= extent {
        if let Some(c) = header.record_count {
            if seen >= c {
                break;
            }
        }
        let space = &image[off..off + SPACE_ID_LEN];
        let key = &image[off + SPACE_ID_LEN..off + SPACE_ID_LEN + RAW_KEY_LEN];
        let mut word = [0u8; 8];
        word.copy_from_slice(&image[off + SPACE_ID_LEN + RAW_KEY_LEN..off + RECORD_FIXED]);
        let value_len = u64::from_le_bytes(word) as usize;
        let value_at = off + RECORD_FIXED;
        if value_at + value_len > extent {
            return Err("truncated-image");
        }
        merged
            .add(space, key, &image[value_at..value_at + value_len], false)
            .map_err(|_| "out-of-memory")?;
        seen += 1;
        off = value_at + value_len;
    }

    let applied = match apply_log(log, &mut merged)? {
        Some(n) => n,
        None => 0,
    };
    Ok((merged, applied))
}

/// Digest the store the image and its log together describe.
///
/// `image` is the live image's bytes alone; `log` is exactly the window of log that belongs
/// to it. Returns `Err("no-log")` when there is none, so the caller can fall back to reading
/// the image by itself.
fn merged_digest(image: &[u8], log: &[u8], header: &Header) -> Result<Line, &'static str> {
    if log.len() < WAL_HEADER_LEN || &log[0..4] != WAL_MAGIC {
        return Err("no-log");
    }
    let (mut merged, applied) = build_merged(image, log, header)?;

    merged.sort_entries();
    let entries = merged.entries.as_slice();
    let mut h = FNV_OFFSET;
    let mut count: u64 = 0;
    let mut value_bytes: u64 = 0;
    let mut i = 0;
    while i < entries.len() {
        let head = &entries[i];
        let mut last = i;
        while last + 1 < entries.len()
            && entries[last + 1].space == head.space
            && entries[last + 1].key == head.key
        {
            last += 1;
        }
        let winner = &entries[last];
        i = last + 1;
        if winner.deleted {
            continue;
        }
        let value = merged.value(winner);
        h = fnv1a64(&winner.space, h);
        h = fnv1a64(&winner.key, h);
        h = fnv1a64(&(value.len() as u64).to_le_bytes(), h);
        h = fnv1a64(value, h);
        count += 1;
        value_bytes += value.len() as u64;
    }

    // `bytes` is the size a checkpoint of this store would produce — the same number
    // userspace reports for the image it writes from the same records.
    let folded = HEADER_LEN_V2 as u64 + count * RECORD_FIXED as u64 + value_bytes;
    let mut line = Line::new();
    // The digest line is field-for-field what `cube-image digest` prints, so the two are
    // compared by diff. What the log contributed goes on its own line: a padded log region
    // makes a byte count meaningless, an entry count does not.
    let _ = write!(
        line,
        "digest version={} curve={} bytes={} records={} value_bytes={} fnv1a64={:016x} errors=0\n",
        header.version, header.curve, folded, count, value_bytes, h
    );
    let _ = write!(line, "log entries={} applied over the image\n", applied);
    Ok(line)
}

/// Where a bare image's log starts, given the image's declared extent.
fn log_offset(image_bytes: u64) -> usize {
    let n = image_bytes as usize;
    (n + LOG_ALIGN - 1) & !(LOG_ALIGN - 1)
}

/// Length of a bare log's valid prefix: the header plus every entry that parses and passes
/// its checksum. This is the append position, and the reason a torn tail is overwritten
/// rather than appended to.
fn log_valid_len(log: &[u8]) -> Result<usize, &'static str> {
    if log.len() < WAL_HEADER_LEN {
        return Err("short-log");
    }
    if &log[0..4] != WAL_MAGIC {
        return Err("no-log");
    }
    if log[4] != WAL_VERSION {
        return Err("unsupported-log-version");
    }
    let mut off = WAL_HEADER_LEN;
    while off + ENTRY_FIXED <= log.len() {
        let start = off;
        let op = log[off];
        if op != 1 && op != 2 {
            break;
        }
        let mut word = [0u8; 4];
        word.copy_from_slice(&log[off + 1..off + 5]);
        let crc = u32::from_le_bytes(word);
        word.copy_from_slice(&log[off + 61..off + 65]);
        let len = u32::from_le_bytes(word) as usize;
        // Only a validated entry moves the append point. Advancing first and checking after
        // counts a torn entry as part of the prefix, so the next append lands *after* the
        // corruption and buries it — the opposite of the rule that a torn tail is overwritten.
        let frame_end = start + ENTRY_FIXED + len;
        if frame_end > log.len() {
            break;
        }
        if crc32(&log[start + 5..frame_end]) != crc {
            break;
        }
        off = frame_end;
    }
    Ok(off)
}

/// Morton-encode a point into the store's 24-byte key — the same interleaving
/// `cube-core`'s `Curve for Morton` does, because a key written here has to be the key a
/// userspace reader decodes. Bit `i` of each axis lands at bit `3i` of the 192-bit key,
/// counting from the least significant bit, which lives in the *last* byte.
fn morton_encode(x: u64, y: u64, z: u64) -> [u8; 24] {
    let mut k = [0u8; 24];
    let put = |n: usize, v: bool, k: &mut [u8; 24]| {
        if !v {
            return;
        }
        let byte = 23 - (n / 8);
        k[byte] |= 1 << (n % 8);
    };
    for i in 0..64 {
        put(3 * i, (x >> i) & 1 != 0, &mut k);
        put(3 * i + 1, (y >> i) & 1 != 0, &mut k);
        put(3 * i + 2, (z >> i) & 1 != 0, &mut k);
    }
    k
}

/// A mutation to append: the byte-plane write the store contract calls `put`.
struct Mutation {
    space: [u8; 32],
    key: [u8; 24],
    value: KVVec<u8>,
}

/// Build a log entry: `op | crc32 | space | key | len | value`.
fn encode_entry(op: u8, space: &[u8; 32], key: &[u8; 24], value: &[u8]) -> Result<KVVec<u8>, AllocError> {
    let total = ENTRY_FIXED + value.len();
    let mut entry = KVVec::<u8>::with_capacity(total, GFP_KERNEL)?;
    entry.extend_from_slice(&[op], GFP_KERNEL)?;
    entry.extend_from_slice(&[0u8; 4][..], GFP_KERNEL)?;
    entry.extend_from_slice(space, GFP_KERNEL)?;
    entry.extend_from_slice(key, GFP_KERNEL)?;
    entry.extend_from_slice(&(value.len() as u32).to_le_bytes(), GFP_KERNEL)?;
    entry.extend_from_slice(value, GFP_KERNEL)?;
    // The checksum covers everything after the crc field.
    let crc = crc32(&entry.as_slice()[5..]);
    entry.as_mut_slice()[1..5].copy_from_slice(&crc.to_le_bytes());
    Ok(entry)
}

/// Write `bytes` at `off` and flush, or say why not. The one place the module writes.
fn write_and_sync(file: *mut bindings::file, off: u64, bytes: &[u8]) -> Result<(), Error> {
    let mut at: bindings::loff_t = off as bindings::loff_t;
    // SAFETY: the caller passes a live `struct file *`; `bytes` outlives the call; `at` is a
    // valid loff_t.
    let wrote = unsafe {
        bindings::kernel_write(
            file,
            bytes.as_ptr().cast::<core::ffi::c_void>(),
            bytes.len(),
            &mut at,
        )
    };
    if wrote < 0 {
        return Err(Error::from_errno(wrote as i32));
    }
    if wrote as usize != bytes.len() {
        return Err(EIO);
    }
    // Durability before acknowledgement: a short write that was never flushed is a write that
    // did not happen.
    // SAFETY: the caller passes a live, writable file; datasync is 0 (a full sync).
    let rc = unsafe { bindings::vfs_fsync(file, 0) };
    if rc < 0 {
        return Err(Error::from_errno(rc));
    }
    Ok(())
}

/// The control copy an update should write: the older of the two, so the current one stays
/// readable throughout.
fn older_copy(generation: u64) -> usize {
    if generation % 2 == 0 {
        CTL_COPY_A
    } else {
        CTL_COPY_B
    }
}

/// Append a mutation to the log and make it durable, returning the updated control block
/// when the layout has one.
///
/// The order is the durability contract: the entry is written and flushed to the device
/// *before* the caller is told it happened. A mutation this function has returned `Ok` for
/// survives a power cut; one it has not may or may not, which is exactly the boundary an
/// acknowledgement is supposed to mark.
///
/// On a store device the entry goes after the log bytes already in use, and the control
/// block's count is raised afterwards — entry first, count second, so a crash between them
/// loses an *unacknowledged* mutation rather than counting one that is not there. A count
/// that ran ahead would make replay read past valid data; a count that lags only forgets.
fn append(
    image: &[u8],
    layout: &Layout,
    m: &Mutation,
    op: u8,
) -> Result<Option<Control>, Error> {
    let entry = encode_entry(op, &m.space, &m.key, m.value.as_slice())?;

    // Where it goes: after the bytes in use on a store device, after the valid prefix on a
    // bare image, where nothing records the length.
    let region_at = core::cmp::min(layout.log_off, image.len());
    let region = &image[region_at..];
    let (used, capacity) = match layout.control {
        // `log_used` counts entry bytes *after* the log's header, so the header is not
        // something an append has to account for in the count.
        Some(c) => (c.log_used as usize, c.log_capacity as usize),
        None => {
            // A region with no header is a log that has never been written; it is empty, not
            // broken, and the header goes down before the first entry.
            let used = match log_valid_len(region) {
                Ok(valid) => valid - WAL_HEADER_LEN,
                Err("no-log") | Err("short-log") => 0,
                Err(what) => {
                    pr_err!("cubelinux: log region unreadable: {}\n", what);
                    return Err(EINVAL);
                }
            };
            (used, region.len())
        }
    };
    if WAL_HEADER_LEN + used + entry.len() > capacity {
        pr_err!(
            "cubelinux: the log is full ({} of {} bytes); checkpoint before appending\n",
            used,
            capacity
        );
        return Err(ENOSPC);
    }

    // SAFETY: STORE_DEVICE is a NUL-terminated literal; filp_open returns a valid file or an
    // error pointer, which is checked. O_RDWR is 2.
    let file = unsafe { bindings::filp_open(STORE_DEVICE.as_ptr().cast::<u8>(), 2, 0) };
    let file = kernel::error::from_err_ptr(file)?;
    if file.is_null() {
        return Err(EINVAL);
    }

    let mut result: Result<(), Error> = Ok(());
    // A log region that has never been written is zeros, not a log: give it a header, the
    // same thing the userspace log does when its file does not exist.
    // A log region with no header is not a log: give it one, whether this is a bare image or
    // a store device, so a reader always finds the framing it expects.
    if region.len() < WAL_HEADER_LEN || &region[0..4] != WAL_MAGIC {
        let mut hdr = [0u8; WAL_HEADER_LEN];
        hdr[0..4].copy_from_slice(&WAL_MAGIC);
        hdr[4] = WAL_VERSION;
        hdr[5] = 0; // morton
        result = write_and_sync(file, layout.log_off as u64, &hdr);
    }
    if result.is_ok() {
        result = write_and_sync(
            file,
            (region_at + WAL_HEADER_LEN + used) as u64,
            entry.as_slice(),
        );
    }

    // The count second, so replay can never be told about an entry that is not there.
    let mut updated = None;
    if result.is_ok() {
        if let Some(c) = layout.control {
            let next = Control {
                log_used: c.log_used + entry.len() as u64,
                generation: c.generation + 1,
                ..c
            };
            let mut buf = [0u8; CTL_COPY_LEN];
            next.encode(&mut buf);
            result = write_and_sync(file, older_copy(c.generation) as u64, &buf);
            if result.is_ok() {
                updated = Some(next);
            }
        }
    }

    // SAFETY: `file` came from filp_open and has not been closed.
    unsafe { bindings::filp_close(file, core::ptr::null_mut()) };
    result.map(|_| updated)
}

/// Serialize the merged store into the pinned v2 image.
///
/// The winners are collected first because the header states the image's extent and its
/// record count, and those are only known once the coalescing is done.
fn serialize_image(merged: &mut Merged, curve: u8) -> Result<KVVec<u8>, AllocError> {
    merged.sort_entries();
    let entries = merged.entries.as_slice();

    let mut winners = KVVec::<u32>::new();
    let mut values_len: u64 = 0;
    let mut i = 0;
    while i < entries.len() {
        let head = &entries[i];
        let mut last = i;
        while last + 1 < entries.len()
            && entries[last + 1].space == head.space
            && entries[last + 1].key == head.key
        {
            last += 1;
        }
        if !entries[last].deleted {
            winners.push(last as u32, GFP_KERNEL)?;
            values_len += entries[last].value_len as u64;
        }
        i = last + 1;
    }

    let image_bytes = HEADER_LEN_V2 as u64 + winners.len() as u64 * RECORD_FIXED as u64 + values_len;
    let mut out = KVVec::<u8>::with_capacity(image_bytes as usize, GFP_KERNEL)?;
    out.extend_from_slice(MAGIC, GFP_KERNEL)?;
    out.extend_from_slice(&[VERSION, curve], GFP_KERNEL)?;
    out.extend_from_slice(&image_bytes.to_le_bytes(), GFP_KERNEL)?;
    out.extend_from_slice(&(winners.len() as u64).to_le_bytes(), GFP_KERNEL)?;

    for w in winners.as_slice() {
        let e = &merged.entries.as_slice()[*w as usize];
        let value = merged.value(e);
        out.extend_from_slice(&e.space, GFP_KERNEL)?;
        out.extend_from_slice(&e.key, GFP_KERNEL)?;
        out.extend_from_slice(&(value.len() as u64).to_le_bytes(), GFP_KERNEL)?;
        out.extend_from_slice(value, GFP_KERNEL)?;
    }
    Ok(out)
}

/// Fold the log into the image: write the merged store into the spare slot, then make it
/// live.
///
/// The order is what makes this atomic. The new image goes to the slot nothing is reading and
/// is flushed; only then does the control block change, and that change is one small write to
/// the control copy that is *not* current. At every instant there is either the old image with
/// a log that still describes the mutations since it, or the new image with an empty log. A
/// crash in the middle of the copy leaves the previous store intact, which is the entire
/// reason for two slots.
fn checkpoint(ctl: &Control, merged: &mut Merged) -> Result<u64, Error> {
    let new_image = serialize_image(merged, ctl.curve)?;
    if new_image.len() as u64 > ctl.slot_bytes {
        pr_err!(
            "cubelinux: checkpoint needs {} bytes but a slot holds {}\n",
            new_image.len(),
            ctl.slot_bytes
        );
        return Err(ENOSPC);
    }

    // SAFETY: as in `append`.
    let file = unsafe { bindings::filp_open(STORE_DEVICE.as_ptr().cast::<u8>(), 2, 0) };
    let file = kernel::error::from_err_ptr(file)?;
    if file.is_null() {
        return Err(EINVAL);
    }

    let mut result = write_and_sync(file, ctl.spare_off(), new_image.as_slice());
    if result.is_ok() {
        let next = Control {
            log_used: 0,
            generation: ctl.generation + 1,
            active_slot: 1 - ctl.active_slot,
            ..*ctl
        };
        let mut buf = [0u8; CTL_COPY_LEN];
        next.encode(&mut buf);
        result = write_and_sync(file, older_copy(ctl.generation) as u64, &buf);
    }

    // SAFETY: `file` came from filp_open and has not been closed.
    unsafe { bindings::filp_close(file, core::ptr::null_mut()) };
    result.map(|_| new_image.len() as u64)
}

/// Everything a syscall needs to reach the store: read the device, resolve its layout, and
/// hand back what was asked for.
///
/// These are the `cube(2)` entry points as the Rust side exposes them. The C shim
/// (cube_syscall.c) owns the user copies and the argument validation; nothing here sees a
/// userspace pointer.
fn device_and_layout() -> Result<(KVVec<u8>, Layout), Error> {
    let mut device = KVVec::<u8>::new();
    read_image(&mut device)?;
    let layout = resolve_layout(&device).map_err(|what| {
        pr_err!("cubelinux: cannot resolve the store layout: {}\n", what);
        EINVAL
    })?;
    Ok((device, layout))
}

/// The coordinate's key, and the space, as the store addresses them.
unsafe fn coord_key(space: *const u8, x: u64, y: u64, z: u64) -> ([u8; 32], [u8; 24]) {
    let mut sp = [0u8; 32];
    // SAFETY: the caller (the syscall shim) passes a pointer to 32 bytes it has already
    // copied from userspace into kernel memory.
    unsafe { core::ptr::copy_nonoverlapping(space, sp.as_mut_ptr(), 32) };
    (sp, morton_encode(x, y, z))
}

/// `CUBE_OP_PUT`: store bytes at a coordinate.
///
/// # Safety
/// `space` must point to 32 readable bytes; `value` to `len` readable bytes when `len` is
/// non-zero.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn cubelinux_kernel_put(
    space: *const u8,
    x: u64,
    y: u64,
    z: u64,
    value: *const u8,
    len: usize,
) -> i32 {
    let (sp, key) = unsafe { coord_key(space, x, y, z) };
    let mut bytes = KVVec::<u8>::new();
    if len > 0 {
        if let Err(_) = bytes.extend_from_slice(
            // SAFETY: the shim guarantees `value` holds `len` bytes.
            unsafe { core::slice::from_raw_parts(value, len) },
            GFP_KERNEL,
        ) {
            return -12; // -ENOMEM
        }
    }
    let mutation = Mutation {
        space: sp,
        key,
        value: bytes,
    };
    let (device, layout) = match device_and_layout() {
        Ok(pair) => pair,
        Err(e) => return -(e.to_errno() as i32),
    };
    match append(&device, &layout, &mutation, 1) {
        Ok(_) => 0,
        Err(e) => -(e.to_errno() as i32),
    }
}

/// `CUBE_OP_GET`: read a coordinate.
///
/// Returns the record's length, or a negative errno. When the value does not fit in `len` the
/// bytes are *not* copied and the length is returned anyway, so the caller can size its buffer
/// and ask again — a short read that silently truncated would be worse than an error.
///
/// # Safety
/// `space` must point to 32 readable bytes; `buf` to `len` writable bytes.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn cubelinux_kernel_get(
    space: *const u8,
    x: u64,
    y: u64,
    z: u64,
    buf: *mut u8,
    len: usize,
) -> isize {
    let (sp, key) = unsafe { coord_key(space, x, y, z) };
    let (device, layout) = match device_and_layout() {
        Ok(pair) => pair,
        Err(e) => return -(e.to_errno() as isize),
    };
    let live = &device[layout.image_off..];
    let header = match parse_header(live) {
        Ok(h) => h,
        Err(what) => {
            pr_err!("cubelinux: {}\n", what);
            return -22; // -EINVAL
        }
    };
    let window = log_window(&device, &layout);
    let (mut merged, _applied) = match build_merged(
        &live[..core::cmp::min(header.image_bytes.unwrap_or(live.len() as u64) as usize, live.len())],
        window,
        &header,
    ) {
        Ok(m) => m,
        Err(what) => {
            pr_err!("cubelinux: cannot build the store: {}\n", what);
            return -22;
        }
    };
    merged.sort_entries();
    let entries = merged.entries.as_slice();

    // The last entry of the coordinate's group is the one that counts: newest write wins, and
    // a removal means there is nothing there.
    let mut found: Option<&Entry> = None;
    for e in entries {
        if e.space == sp && e.key == key {
            found = Some(e);
        }
    }
    match found {
        None => -2, // -ENOENT
        Some(e) if e.deleted => -2,
        Some(e) => {
            let value = merged.value(e);
            if value.len() > len {
                return value.len() as isize;
            }
            if !value.is_empty() {
                // SAFETY: the shim guarantees `buf` holds `len` bytes and `len >= value.len()`.
                unsafe {
                    core::ptr::copy_nonoverlapping(value.as_ptr(), buf, value.len());
                }
            }
            value.len() as isize
        }
    }
}

/// `CUBE_OP_DEL`: remove the record at a coordinate.
///
/// A removal is a log entry, not an erasure: nothing in an append-only store is rewritten in
/// place, and the checkpoint is what finally drops it.
///
/// # Safety
/// `space` must point to 32 readable bytes.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn cubelinux_kernel_del(space: *const u8, x: u64, y: u64, z: u64) -> i32 {
    let (sp, key) = unsafe { coord_key(space, x, y, z) };
    let mutation = Mutation {
        space: sp,
        key,
        value: KVVec::new(),
    };
    let (device, layout) = match device_and_layout() {
        Ok(pair) => pair,
        Err(e) => return -(e.to_errno() as i32),
    };
    // A delete is an entry with op=2; `append` writes op=1, so build it here from the same
    // framing, and let the log's own reader be the judge of it.
    match append(&device, &layout, &mutation, 2) {
        Ok(_) => 0,
        Err(e) => -(e.to_errno() as i32),
    }
}

/// `CUBE_OP_SYNC`: fold the log into the image.
#[unsafe(no_mangle)]
pub extern "C" fn cubelinux_kernel_sync() -> i32 {
    let (device, layout) = match device_and_layout() {
        Ok(pair) => pair,
        Err(e) => return -(e.to_errno() as i32),
    };
    let ctl = match layout.control {
        Some(c) => c,
        None => return -22, // -EINVAL: a bare image has no spare slot to fold into
    };
    let live = &device[layout.image_off..];
    let header = match parse_header(live) {
        Ok(h) => h,
        Err(_) => return -22,
    };
    let window = log_window(&device, &layout);
    let extent = core::cmp::min(
        header.image_bytes.unwrap_or(live.len() as u64) as usize,
        live.len(),
    );
    let mut merged = match build_merged(&live[..extent], window, &header) {
        Ok((m, _)) => m,
        Err(_) => return -22,
    };
    match checkpoint(&ctl, &mut merged) {
        Ok(_) => 0,
        Err(e) => -(e.to_errno() as i32),
    }
}

/// The module's registration; holds the misc device for as long as the module lives.
#[pin_data]
struct CubeStoreModule {
    #[pin]
    _miscdev: MiscDeviceRegistration<CubeStore>,
}

impl kernel::InPlaceModule for CubeStoreModule {
    fn init(_module: &'static ThisModule) -> impl PinInit<Self, Error> {
        pr_info!("cubelinux: store reader registered at /dev/cubelinux\n");
        try_pin_init!(Self {
            _miscdev <- MiscDeviceRegistration::register(MiscDeviceOptions {
                name: c_str!("cubelinux"),
            }),
        })
    }
}

#[pin_data]
struct CubeStore {
    dev: ARef<Device>,
}

#[vtable]
impl MiscDevice for CubeStore {
    type Ptr = Pin<KBox<Self>>;

    fn open(_file: &File, misc: &MiscDeviceRegistration<Self>) -> Result<Pin<KBox<Self>>> {
        KBox::try_pin_init(
            try_pin_init! {
                CubeStore { dev: ARef::from(misc.device()) }
            },
            GFP_KERNEL,
        )
    }

    /// Write one operation, framed as the argument block the coordinate interface will pass.
    ///
    /// ```text
    /// put  op=1 | space(32) | x(8) | y(8) | z(8) | len(4) | value[len]
    /// sync op=3
    /// ```
    ///
    /// `put` is the byte plane — bytes at a coordinate, with no header written beside them —
    /// which is what the store contract defines and what the differential gate compares
    /// against userspace's `cell put`. The header tier sits above this.
    ///
    /// `sync` folds the log into the image. It is a real operation rather than a test hook: a
    /// caller that wants the log reclaimed — a shutdown, a snapshot, a handover — is entitled
    /// to ask.
    fn write_iter(kiocb: Kiocb<'_, Self::Ptr>, iov: &mut IovIterSource<'_>) -> Result<usize> {
        let mut buf = KVVec::<u8>::new();
        let len = iov.copy_from_iter_vec(&mut buf, GFP_KERNEL)?;
        let bytes = buf.as_slice();
        if bytes.is_empty() {
            return Err(EINVAL);
        }
        let me = kiocb.file();

        // Both operations need the device: its layout says where the image and log are.
        let mut device = KVVec::<u8>::new();
        read_image(&mut device)?;
        let layout = match resolve_layout(&device) {
            Ok(l) => l,
            Err(what) => {
                dev_err!(me.dev, "cubelinux: cannot resolve the store layout: {}\n", what);
                return Err(EINVAL);
            }
        };

        match bytes[0] {
            OP_PUT => {
                if bytes.len() < 1 + 32 + 24 + 4 {
                    return Err(EINVAL);
                }
                let mut space = [0u8; 32];
                space.copy_from_slice(&bytes[1..33]);
                let axis = |at: usize| -> u64 {
                    let mut w = [0u8; 8];
                    w.copy_from_slice(&bytes[at..at + 8]);
                    u64::from_le_bytes(w)
                };
                let (x, y, z) = (axis(33), axis(41), axis(49));
                let mut w = [0u8; 4];
                w.copy_from_slice(&bytes[57..61]);
                let value_len = u32::from_le_bytes(w) as usize;
                if bytes.len() < 61 + value_len {
                    return Err(EINVAL);
                }
                let mut value = KVVec::<u8>::new();
                value.extend_from_slice(&bytes[61..61 + value_len], GFP_KERNEL)?;
                let mutation = Mutation {
                    space,
                    key: morton_encode(x, y, z),
                    value,
                };
                match append(&device, &layout, &mutation, 1) {
                    Ok(_) => {
                        dev_info!(
                            me.dev,
                            "cubelinux: appended {} bytes for a {} byte value\n",
                            ENTRY_FIXED + value_len,
                            value_len
                        );
                        Ok(len)
                    }
                    Err(e) => {
                        dev_err!(me.dev, "cubelinux: append failed: {:?}\n", e);
                        Err(e)
                    }
                }
            }
            OP_SYNC => {
                let ctl = match layout.control {
                    Some(c) => c,
                    None => {
                        // A bare image has one slot, so folding into it is not atomic and a
                        // crash in the middle would lose the store. Refusing is the honest
                        // answer; the fix is to format the device as a store.
                        dev_err!(
                            me.dev,
                            "cubelinux: cannot checkpoint a bare image (no control block)\n"
                        );
                        return Err(EINVAL);
                    }
                };
                // The store the image and its log describe, ready to be written out.
                let live = &device[layout.image_off..];
                let header = match parse_header(live) {
                    Ok(h) => h,
                    Err(what) => {
                        dev_err!(me.dev, "cubelinux: {}\n", what);
                        return Err(EINVAL);
                    }
                };
                let window = log_window(&device, &layout);
                let mut merged = match build_merged(live, window, &header) {
                    Ok(m) => m,
                    Err(what) => {
                        dev_err!(me.dev, "cubelinux: cannot build the store: {}\n", what);
                        return Err(EINVAL);
                    }
                };
                match checkpoint(&ctl, &mut merged.0) {
                    Ok(bytes) => {
                        dev_info!(me.dev, "cubelinux: checkpointed {} bytes\n", bytes);
                        Ok(len)
                    }
                    Err(e) => {
                        dev_err!(me.dev, "cubelinux: checkpoint failed: {:?}\n", e);
                        Err(e)
                    }
                }
            }
            other => {
                dev_err!(me.dev, "cubelinux: unknown operation {}\n", other);
                Err(EINVAL)
            }
        }
    }

    fn read_iter(mut kiocb: Kiocb<'_, Self::Ptr>, iov: &mut IovIterDest<'_>) -> Result<usize> {
        // The work happens here, on demand, rather than at init: by the time anything can
        // open this device, the block driver that provides the store is certainly up.
        let mut image = KVVec::<u8>::new();
        let line = match read_image(&mut image) {
            Ok(()) => digest(&image),
            Err(e) => {
                let mut line = Line::new();
                let _ = write!(line, "error={:?}", e);
                line
            }
        };
        pr_info!("cubelinux: {}\n", core::str::from_utf8(line.as_bytes()).unwrap_or("<bad utf8>"));

        let me = kiocb.file();
        dev_info!(me.dev, "cubelinux: store read complete\n");
        iov.simple_read_from_buffer(kiocb.ki_pos_mut(), line.as_bytes())
    }
}