k-os/crates/adapter-riscv/src/boot_fs.rs

use crate::hw::virtio_blk::VirtioBlk;

const SECTOR_SIZE: usize = 512;
const MAX_FILE_SIZE: usize = 64 * 1024; // 64K max for init binary

struct BootFat16 {
    reserved_sectors: u16,
    sectors_per_fat: u16,
    root_dir_start: u32,
    root_dir_sectors: u32,
    data_start: u32,
    sectors_per_cluster: u8,
}

impl BootFat16 {
    fn parse(boot_sector: &[u8; SECTOR_SIZE]) -> Option<Self> {
        if boot_sector[510] != 0x55 || boot_sector[511] != 0xAA {
            return None;
        }

        let bytes_per_sector = u16::from_le_bytes([boot_sector[11], boot_sector[12]]);
        if bytes_per_sector != SECTOR_SIZE as u16 { return None; }

        let sectors_per_cluster = boot_sector[13];
        let reserved_sectors = u16::from_le_bytes([boot_sector[14], boot_sector[15]]);
        let num_fats = boot_sector[16];
        let root_entry_count = u16::from_le_bytes([boot_sector[17], boot_sector[18]]);
        let sectors_per_fat = u16::from_le_bytes([boot_sector[22], boot_sector[23]]);

        let fat_start = reserved_sectors as u32;
        let root_dir_start = fat_start + (num_fats as u32) * (sectors_per_fat as u32);
        let root_dir_sectors = ((root_entry_count as u32) * 32 + 511) / 512;
        let data_start = root_dir_start + root_dir_sectors;

        Some(Self {
            reserved_sectors,
            sectors_per_fat,
            root_dir_start,
            root_dir_sectors,
            data_start,
            sectors_per_cluster,
        })
    }

    fn cluster_to_sector(&self, cluster: u16) -> u32 {
        self.data_start + ((cluster as u32) - 2) * (self.sectors_per_cluster as u32)
    }
}

fn to_fat_name(name: &[u8]) -> [u8; 11] {
    let mut fat = [b' '; 11];
    let dot_pos = name.iter().position(|&b| b == b'.').unwrap_or(name.len());
    for i in 0..dot_pos.min(8) { fat[i] = name[i].to_ascii_uppercase(); }
    if dot_pos < name.len() {
        for i in 0..(name.len() - dot_pos - 1).min(3) {
            fat[8 + i] = name[dot_pos + 1 + i].to_ascii_uppercase();
        }
    }
    fat
}

pub fn read_file_from_disk(
    blk: &mut VirtioBlk,
    filename: &[u8],
    out_buf: &mut [u8],
) -> Option<usize> {
    let mut sector_buf = [0u8; SECTOR_SIZE];

    // Read boot sector
    if !blk.read_sector(0, &mut sector_buf) { return None; }
    let fs = BootFat16::parse(&sector_buf)?;

    let fat_name = to_fat_name(filename);

    // Search root directory
    let mut first_cluster: u16 = 0;
    let mut file_size: u32 = 0;
    let mut found = false;

    for s in 0..fs.root_dir_sectors {
        if !blk.read_sector((fs.root_dir_start + s) as u32, &mut sector_buf) { continue; }
        for i in 0..(SECTOR_SIZE / 32) {
            let off = i * 32;
            if sector_buf[off] == 0x00 { break; }
            if sector_buf[off] == 0xE5 { continue; }
            if sector_buf[off..off + 11] == fat_name {
                first_cluster = u16::from_le_bytes([sector_buf[off + 26], sector_buf[off + 27]]);
                file_size = u32::from_le_bytes([
                    sector_buf[off + 28], sector_buf[off + 29],
                    sector_buf[off + 30], sector_buf[off + 31],
                ]);
                found = true;
                break;
            }
        }
        if found { break; }
    }

    if !found { return None; }

    let size = (file_size as usize).min(out_buf.len());
    let mut read = 0;
    let mut cluster = first_cluster;
    let sectors_per_cluster = fs.sectors_per_cluster as u32;

    while read < size && cluster >= 2 && cluster < 0xFFF8 {
        for s in 0..sectors_per_cluster {
            if read >= size { break; }
            let sector = fs.cluster_to_sector(cluster) + s;
            if !blk.read_sector(sector, &mut sector_buf) { return Some(read); }
            let chunk = (size - read).min(SECTOR_SIZE);
            out_buf[read..read + chunk].copy_from_slice(&sector_buf[..chunk]);
            read += chunk;
        }

        // Read next cluster from FAT
        let fat_offset = (cluster as u32) * 2;
        let fat_sector = fs.reserved_sectors as u32 + fat_offset / SECTOR_SIZE as u32;
        let fat_byte = (fat_offset % SECTOR_SIZE as u32) as usize;
        if !blk.read_sector(fat_sector, &mut sector_buf) { break; }
        cluster = u16::from_le_bytes([sector_buf[fat_byte], sector_buf[fat_byte + 1]]);
    }

    Some(read)
}