my-kern/kernel/fs/fat.c

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

#include "fat.h"
#include "blockdev.h"

#include <types.h>
#include <libc/libc.h>
#include <libc/kprintf.h>
#include <mem/mem.h>

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

typedef struct {
    struct {
        u8 jump[3];
        char oem_identifier[8];
        u16 bytes_per_sector;
        u8 sectors_per_cluster;
        u16 reserved_sectors;
        u8 fats;
        u16 directories;
        u16 total_sectors;
        u8 media_descriptor;
        u16 sectors_per_fat;
        u16 sectors_per_track;
        u16 heads_sides;
        u32 hidden_sectors;
        u32 large_total_sectors;
    } __attribute((packed)) bpb;
    union {
        struct {
            u8 drive_number;
            u8 flags;
            u8 signature;
            u32 serial;
            u8 label[11];
            u8 system_id[8];
        } __attribute((packed)) ebr_12_16;
        struct {
            u32 sectors_per_fat;
            u16 flags;
            u16 fat_version;
            u32 root_directory;
            u16 fs_info;
            u16 backup_boot;
            u8 reserved[12];
            u8 drive_number;
            u8 flags_nt;
            u8 signature;
            u32 serial;
            u8 label[11];
            u8 system_id[8];
        } __attribute((packed)) ebr_32;
    };
} __attribute((packed)) fat_bpb;

typedef struct {
    u8 hours: 5;
    u8 minutes: 6;
    u8 seconds: 5;
} __attribute((packed)) time_83;

typedef struct {
    u8 hours: 5;
    u8 minutes: 6;
    u8 seconds: 5;
} __attribute((packed)) date_83;

typedef struct {
    union {
        struct {
            u8 name[11];
            u8 attributes;
            u8 reserved;
            u8 created_seconds;
            time_83 created_time;
            date_83 created_date;
            date_83 access_date;
            u16 high_first_cluster;
            time_83 last_mod_time;
            date_83 last_mod_date;
            u16 low_first_cluster;
            u32 file_size;
        } __attribute((packed)) name_83;
        struct {
            u8 order;
            u16 text_1[5];
            u8 attribute;
            u8 long_entry_type;
            u8 checksum;
            u16 text_2[6];
            u16 reserved;
            u16 text_3[2];
        } __attribute((packed)) long_name;
    };
} __attribute((packed)) fat_directory_entry;

u8 fat_check_device(const block_device *device, u8 *first_sector);

block_dev_driver fat_driver = {
        .name = "fat",
        .check_device = fat_check_device,
        .free_device = NULL, // todo
};

void fat_register_block_driver() {
    block_dev_register_driver(&fat_driver);
}

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

u8 fat_check_device(const block_device *device, u8 *first_sector) {
    fat_bpb bpb;
    memcpy((u8 *) &bpb, first_sector, sizeof(fat_bpb));
    printf("OEM: ");
    print_chars(bpb.bpb.oem_identifier, 11);
    printf("\n");

    u32 total_sectors = bpb.bpb.total_sectors == 0 ? bpb.bpb.large_total_sectors : bpb.bpb.total_sectors;
    u32 fat_size = bpb.bpb.sectors_per_fat == 0 ? bpb.ebr_32.sectors_per_fat : bpb.bpb.sectors_per_fat;
    u32 root_dir_sectors = ((bpb.bpb.directories * 32) + (bpb.bpb.bytes_per_sector - 1)) / bpb.bpb.bytes_per_sector;
    u32 first_data_sector = bpb.bpb.reserved_sectors + (bpb.bpb.fats * fat_size) + root_dir_sectors;
    u32 first_fat_sector = bpb.bpb.reserved_sectors;
    u32 data_sectors = total_sectors - (bpb.bpb.reserved_sectors + (bpb.bpb.fats * fat_size) + root_dir_sectors);
    u32 total_clusters = data_sectors / bpb.bpb.sectors_per_cluster;

    u8 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
    u32 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 {
    u32 sector;
    u32 value;
} table_result;

table_result
get_fat_table_value(u8 fat_type, const u8 *fat_table, u32 active_cluster, u32 first_fat_cluster, u32 sector_size) {
    u32 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 = *(u16 *) &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 = *(u16 *) &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 = *(u32 *) &fat_table[fat_offset % sector_size] & 0x0FFFFFFF;
            return result;
        default:
            break;
    }
    return result;
}


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