my-kern/kernel/fs/fat.c

//
// Created by rick on 07-02-21.
//

#include <attributes.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>

#include <myke/fs/blockdev.h>

#define FAT_TYPE_12 1
#define FAT_TYPE_16 2
#define FAT_TYPE_32 3
#define FAT_TYPE_EXFAT 4

typedef struct {
    struct {
        uint8_t jump[3];
        char oem_identifier[8];
        uint16_t bytes_per_sector;
        uint8_t sectors_per_cluster;
        uint16_t reserved_sectors;
        uint8_t fats;
        uint16_t directories;
        uint16_t total_sectors;
        uint8_t media_descriptor;
        uint16_t sectors_per_fat;
        uint16_t sectors_per_track;
        uint16_t heads_sides;
        uint32_t hidden_sectors;
        uint32_t large_total_sectors;
    } att_packed bpb;
    union {
        struct {
            uint8_t drive_number;
            uint8_t flags;
            uint8_t signature;
            uint32_t serial;
            uint8_t label[11];
            uint8_t system_id[8];
        } att_packed ebr_12_16;
        struct {
            uint32_t sectors_per_fat;
            uint16_t flags;
            uint16_t fat_version;
            uint32_t root_directory;
            uint16_t fs_info;
            uint16_t backup_boot;
            uint8_t reserved[12];
            uint8_t drive_number;
            uint8_t flags_nt;
            uint8_t signature;
            uint32_t serial;
            uint8_t label[11];
            uint8_t system_id[8];
        } att_packed ebr_32;
    };
} att_packed fat_bpb;

typedef struct {
    uint8_t hours: 5;
    uint8_t minutes: 6;
    uint8_t seconds: 5;
} att_packed time_83;

typedef struct {
    uint8_t hours: 5;
    uint8_t minutes: 6;
    uint8_t seconds: 5;
} att_packed date_83;

typedef struct {
    union {
        struct {
            uint8_t name[11];
            uint8_t attributes;
            uint8_t reserved;
            uint8_t created_seconds;
            time_83 created_time;
            date_83 created_date;
            date_83 access_date;
            uint16_t high_first_cluster;
            time_83 last_mod_time;
            date_83 last_mod_date;
            uint16_t low_first_cluster;
            uint32_t file_size;
        } att_packed name_83;
        struct {
            uint8_t order;
            uint16_t text_1[5];
            uint8_t attribute;
            uint8_t long_entry_type;
            uint8_t checksum;
            uint16_t text_2[6];
            uint16_t reserved;
            uint16_t text_3[2];
        } att_packed long_name;
    };
} att_packed fat_directory_entry;

void print_chars(char *chars, int amount) {
    for (int i = 0; i < amount; ++i) {
        if (chars[i] == 0) break;
        printf("%c", chars[i]);
    }
}

uint8_t att_used fat_check_device(const block_device_t *device, uint8_t *first_sector) {
    fat_bpb bpb;
    memcpy((uint8_t *) &bpb, first_sector, sizeof(fat_bpb));
    if (bpb.bpb.sectors_per_fat == 0 || bpb.bpb.sectors_per_cluster == 0) {
        printf("Definitely not FAT\n");
        return BLOCK_DEV_DRIVER_CHECK_NO_MATCH;
    }

    printf("OEM: ");
    print_chars(bpb.bpb.oem_identifier, 11);
    printf("\n");

    uint32_t total_sectors = bpb.bpb.total_sectors == 0 ? bpb.bpb.large_total_sectors : bpb.bpb.total_sectors;
    uint32_t fat_size = bpb.bpb.sectors_per_fat == 0 ? bpb.ebr_32.sectors_per_fat : bpb.bpb.sectors_per_fat;
    uint32_t root_dir_sectors =
            ((bpb.bpb.directories * 32) + (bpb.bpb.bytes_per_sector - 1)) / bpb.bpb.bytes_per_sector;
    uint32_t first_data_sector = bpb.bpb.reserved_sectors + (bpb.bpb.fats * fat_size) + root_dir_sectors;
    uint32_t first_fat_sector = bpb.bpb.reserved_sectors;
    uint32_t data_sectors = total_sectors - (bpb.bpb.reserved_sectors + (bpb.bpb.fats * fat_size) + root_dir_sectors);
    uint32_t total_clusters = data_sectors / bpb.bpb.sectors_per_cluster;

    uint8_t fat_type;
    if (total_clusters < 4085) {
        fat_type = FAT_TYPE_12;
    } else if (total_clusters < 65525) {
        fat_type = FAT_TYPE_16;
    } else if (total_clusters < 268435445) {
        fat_type = FAT_TYPE_32;
    } else {
        fat_type = FAT_TYPE_EXFAT;
    }

    // dump dir
    // fat12/16 only
    uint32_t first_root_dir_sector = first_data_sector - root_dir_sectors;
    // todo fat32
    fat_directory_entry *entries = malloc(root_dir_sectors * bpb.bpb.bytes_per_sector);
    device->access(device, BLOCK_DEV_DIRECTION_READ, first_root_dir_sector, root_dir_sectors, entries);
    // todo read table
    free(entries);

    return BLOCK_DEV_DRIVER_CHECK_NO_MATCH;
}

typedef struct {
    uint32_t sector;
    uint32_t value;
} table_result;

table_result
get_fat_table_value(uint8_t fat_type, const uint8_t *fat_table, uint32_t active_cluster, uint32_t first_fat_cluster,
                    uint32_t sector_size) {
    uint32_t fat_offset;
    table_result result = {
            .value = 0,
            .sector = 0,
    };
    switch (fat_type) {
        case FAT_TYPE_12:
            fat_offset = active_cluster * (active_cluster / 2);
            result.sector = first_fat_cluster + (fat_offset / sector_size);
            result.value = *(uint16_t *) &fat_table[fat_offset % sector_size];
            if (active_cluster & 0x1) {
                result.value = result.value >> 4;
            } else {
                result.value = result.value & 0x0FFF;
            }
            return result;
        case FAT_TYPE_16:
            fat_offset = active_cluster * 2;
            result.sector = first_fat_cluster + (fat_offset / sector_size);
            result.value = *(uint16_t *) &fat_table[fat_offset % sector_size];
            return result;
        case FAT_TYPE_32:
        case FAT_TYPE_EXFAT:
            fat_offset = active_cluster * 4;
            result.sector = first_fat_cluster + (fat_offset / sector_size);
            result.value = *(uint32_t *) &fat_table[fat_offset % sector_size] & 0x0FFFFFFF;
            return result;
        default:
            break;
    }
    return result;
}

BLOCK_DEV_DRIVER(900) = {
        .name = "fat",
        .check_device = fat_check_device,
        .free_device = NULL, // todo
};

// steal validation code from here https://github.com/torvalds/linux/blob/fcadab740480e0e0e9fa9bd272acd409884d431a/fs/fat/inode.c#L1456