msdos_fs.h

Go to the documentation of this file.

/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
#ifndef _UAPI_LINUX_MSDOS_FS_H
#define _UAPI_LINUX_MSDOS_FS_H

/*
 * The MS-DOS filesystem constants/structures
 */

#ifndef SECTOR_SIZE
#define SECTOR_SIZE 512 /* sector size (bytes) */
#endif
#define SECTOR_BITS 9         /* log2(SECTOR_SIZE) */
#define MSDOS_DPB (MSDOS_DPS) /* dir entries per block */
#define MSDOS_DPB_BITS 4      /* log2(MSDOS_DPB) */
#define MSDOS_DPS (SECTOR_SIZE / sizeof(struct msdos_dir_entry))
#define MSDOS_DPS_BITS 4   /* log2(MSDOS_DPS) */
#define MSDOS_LONGNAME 256 /* maximum name length */

#define MSDOS_ROOT_INO 1   /* The root inode number */
#define MSDOS_FSINFO_INO 2 /* Used for managing the FSINFO block */

#define MSDOS_DIR_BITS 5 /* log2(sizeof(struct msdos_dir_entry)) */

/* directory limit */
#define FAT_MAX_DIR_ENTRIES (65536)
#define FAT_MAX_DIR_SIZE (FAT_MAX_DIR_ENTRIES << MSDOS_DIR_BITS)

#define ATTR_NONE 0x0000   /* no attribute bits */
#define ATTR_RO 0x0001   /* read-only */
#define ATTR_HIDDEN 0x0002 /* hidden */
#define ATTR_SYS 0x0004 /* system */
#define ATTR_VOLUME 0x0008 /* volume label */
#define ATTR_DIR 0x0010 /* directory */
#define ATTR_ARCH 0x0020   /* archived */

/* attribute bits that are copied "as is" */
#define ATTR_UNUSED (ATTR_VOLUME | ATTR_ARCH | ATTR_SYS | ATTR_HIDDEN)
/* bits that are used by the Windows 95/Windows NT extended FAT */
#define ATTR_EXT (ATTR_RO | ATTR_HIDDEN | ATTR_SYS | ATTR_VOLUME)

#define CASE_LOWER_BASE 8 /* base is lower case */
#define CASE_LOWER_EXT 16 /* extension is lower case */

#define DELETED_FLAG 0xe5 /* marks file as deleted when in name[0] */
#define IS_FREE(n) (!*(n) || *(n) == DELETED_FLAG)

#define FAT_LFN_LEN 255            /* maximum long name length */
#define MSDOS_NAME 11              /* maximum name length */
#define MSDOS_SLOTS 21             /* max # of slots for short and long names */
#define MSDOS_DOT ".          " /* ".", padded to MSDOS_NAME chars */
#define MSDOS_DOTDOT "..         " /* "..", padded to MSDOS_NAME chars */

/* start of data cluster's entry (number of reserved clusters) */
#define FAT_START_ENT 2

/* maximum number of clusters */
// #define MAX_FAT12 0xFF4
// #define MAX_FAT16 0xFFF4
// #define MAX_FAT32 0x0FFFFFF6

/* bad cluster mark */
#define BAD_FAT12 0xFF7
#define BAD_FAT16 0xFFF7
#define BAD_FAT32 0x0FFFFFF7

/* standard EOF */
#define EOF_FAT12 0xFFF
#define EOF_FAT16 0xFFFF
#define EOF_FAT32 0x0FFFFFFF

#define FAT_ENT_FREE (0)
#define FAT_ENT_BAD (BAD_FAT32)
#define FAT_ENT_EOF (EOF_FAT32)

#define FAT_FSINFO_SIG1 0x41615252
#define FAT_FSINFO_SIG2 0x61417272
#define IS_FSINFO(x) ((x)->signature1 == FAT_FSINFO_SIG1 && (x)->signature2 == FAT_FSINFO_SIG2)

#define FAT_STATE_DIRTY 0x01

struct __attribute__((packed)) fat_boot_sector_t {
    uint8_t jmp_boot[3];  /* Boot strap short or near jump */
    char system_id[8];  /* Name - can be used to special case partition manager volumes */
    uint16_t sector_size; /* bytes per logical sector */
    uint8_t sec_per_clus; /* sectors/cluster */
    uint16_t reserved;  /* reserved sectors */
    uint8_t num_fats;    /* number of FATs */
    uint16_t dir_entries; /* root directory entries */
    uint16_t sectors;    /* number of sectors */
    uint8_t media;        /* media code */
    uint16_t fat_length;  /* sectors/FAT */
    uint16_t secs_track;  /* sectors per track */
    uint16_t heads;       /* number of heads */
    uint32_t hidden;      /* hidden sectors (unused) */
    uint32_t total_sect;  /* number of sectors (if sectors == 0) */

    union {
        struct __attribute__((packed)) {
            /*  Extended BPB Fields for FAT16 */
            uint8_t drive_number;       /* Physical drive number */
            uint8_t state;              /* undocumented, but used for mount state. */
            uint8_t signature;          /* extended boot signature */
            uint32_t vol_id;            /* volume ID */
            char vol_label[MSDOS_NAME]; /* volume label */
            uint64_t fs_type;           /* file system type */
                                        /* other fields are not added here */
        } fat16;

        struct __attribute__((packed)) {
            /* only used by FAT32 */
            uint32_t fat_length;   /* sectors/FAT */
            uint16_t flags;        /* bit 8: fat mirroring, low 4: active fat */
            uint16_t version;     /* major, minor filesystem version */
            uint32_t root_cluster; /* first cluster in root directory */
            uint16_t info_sector;  /* filesystem info sector */
            uint16_t backup_boot;  /* backup boot sector */
            uint16_t reserved2[6]; /* Unused */
            /* Extended BPB Fields for FAT32 */
            uint8_t drive_number;       /* Physical drive number */
            uint8_t state;              /* undocumented, but used for mount state. */
            uint8_t signature;          /* extended boot signature */
            uint32_t vol_id;            /* volume ID */
            char vol_label[MSDOS_NAME]; /* volume label */
            uint64_t fs_type;           /* file system type */
                                        /* other fields are not added here */
        } fat32;

        struct __attribute__((packed)) {
            uint8_t extra_info[510 - 36];
            uint16_t signature;
        };
    };
};

static_assert(offsetof(fat_boot_sector_t, signature) == 510, "Bad fat_boot_sector_t");

struct fat_boot_fsinfo_t {
    uint32_t signature1;     /* 0x41615252L */
    uint32_t reserved1[120]; /* Nothing as far as I can tell */
    uint32_t signature2;     /* 0x61417272L */
    uint32_t free_clusters;  /* Free cluster count.  -1 if unknown */
    uint32_t next_cluster;   /* Most recently allocated cluster */
    uint16_t reserved2[7];
    uint16_t signature;
};

static_assert(offsetof(fat_boot_fsinfo_t, signature) == 510, "Bad fat_boot_fsinfo_t");

struct msdos_dir_entry_t {
    uint8_t name[MSDOS_NAME];   /* name and extension */
    uint8_t attr;               /* attribute bits */
    uint8_t lcase;              /* Case for base and extension */
    uint8_t ctime_cs;           /* Creation time, centiseconds (0-199) */
    uint16_t ctime;             /* Creation time */
    uint16_t cdate;             /* Creation date */
    uint16_t adate;             /* Last access date */
    uint16_t starthi;           /* High 16 bits of cluster in FAT32 */
    uint16_t time, date, start; /* time, date and first cluster */
    uint32_t size;              /* file size (in bytes) */
};

static_assert(sizeof(msdos_dir_entry_t) == 32, "Bad msdos_dir_entry_t");

/* Up to 13 characters of the name */
struct msdos_dir_slot_t {
    uint8_t id;             /* sequence number for slot */
    uint8_t name0_4[10];    /* first 5 characters in name */
    uint8_t attr;           /* attribute byte */
    uint8_t reserved;       /* always 0 */
    uint8_t alias_checksum; /* checksum for 8.3 alias */
    uint8_t name5_10[12];   /* 6 more characters in name */
    uint16_t start;         /* starting cluster number, 0 in long slots */
    uint8_t name11_12[4];   /* last 2 characters in name */
};

#endif /* _UAPI_LINUX_MSDOS_FS_H */