1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133

//! Boot Parameter Block

use crate::{
    blockdevice::BlockCount,
    fat::{FatType, OnDiskDirEntry},
};
use byteorder::{ByteOrder, LittleEndian};

/// Represents a Boot Parameter Block. This is the first sector of a FAT
/// formatted partition, and it describes various properties of the FAT
/// filesystem.
pub struct Bpb<'a> {
    data: &'a [u8; 512],
    pub(crate) fat_type: FatType,
    cluster_count: u32,
}

impl<'a> Bpb<'a> {
    pub(crate) const FOOTER_VALUE: u16 = 0xAA55;

    /// Attempt to parse a Boot Parameter Block from a 512 byte sector.
    pub fn create_from_bytes(data: &[u8; 512]) -> Result<Bpb, &'static str> {
        let mut bpb = Bpb {
            data,
            fat_type: FatType::Fat16,
            cluster_count: 0,
        };
        if bpb.footer() != Self::FOOTER_VALUE {
            return Err("Bad BPB footer");
        }

        let root_dir_blocks =
            BlockCount::from_bytes(u32::from(bpb.root_entries_count()) * OnDiskDirEntry::LEN_U32).0;
        let non_data_blocks = u32::from(bpb.reserved_block_count())
            + (u32::from(bpb.num_fats()) * bpb.fat_size())
            + root_dir_blocks;
        let data_blocks = bpb.total_blocks() - non_data_blocks;
        bpb.cluster_count = data_blocks / u32::from(bpb.blocks_per_cluster());
        if bpb.cluster_count < 4085 {
            return Err("FAT12 is unsupported");
        } else if bpb.cluster_count < 65525 {
            bpb.fat_type = FatType::Fat16;
        } else {
            bpb.fat_type = FatType::Fat32;
        }

        match bpb.fat_type {
            FatType::Fat16 => Ok(bpb),
            FatType::Fat32 if bpb.fs_ver() == 0 => {
                // Only support FAT32 version 0.0
                Ok(bpb)
            }
            _ => Err("Invalid FAT format"),
        }
    }

    // FAT16/FAT32
    define_field!(bytes_per_block, u16, 11);
    define_field!(blocks_per_cluster, u8, 13);
    define_field!(reserved_block_count, u16, 14);
    define_field!(num_fats, u8, 16);
    define_field!(root_entries_count, u16, 17);
    define_field!(total_blocks16, u16, 19);
    define_field!(media, u8, 21);
    define_field!(fat_size16, u16, 22);
    define_field!(blocks_per_track, u16, 24);
    define_field!(num_heads, u16, 26);
    define_field!(hidden_blocks, u32, 28);
    define_field!(total_blocks32, u32, 32);
    define_field!(footer, u16, 510);

    // FAT32 only
    define_field!(fat_size32, u32, 36);
    define_field!(fs_ver, u16, 42);
    define_field!(first_root_dir_cluster, u32, 44);
    define_field!(fs_info, u16, 48);
    define_field!(backup_boot_block, u16, 50);

    /// Get the OEM name string for this volume
    pub fn oem_name(&self) -> &[u8] {
        &self.data[3..11]
    }

    // FAT16/FAT32 functions

    /// Get the Volume Label string for this volume
    pub fn volume_label(&self) -> &[u8] {
        if self.fat_type != FatType::Fat32 {
            &self.data[43..=53]
        } else {
            &self.data[71..=81]
        }
    }

    // FAT32 only functions

    /// On a FAT32 volume, return the free block count from the Info Block. On
    /// a FAT16 volume, returns None.
    pub fn fs_info_block(&self) -> Option<BlockCount> {
        if self.fat_type != FatType::Fat32 {
            None
        } else {
            Some(BlockCount(u32::from(self.fs_info())))
        }
    }

    // Magic functions that get the right FAT16/FAT32 result

    /// Get the size of the File Allocation Table in blocks.
    pub fn fat_size(&self) -> u32 {
        let result = u32::from(self.fat_size16());
        if result != 0 {
            result
        } else {
            self.fat_size32()
        }
    }

    /// Get the total number of blocks in this filesystem.
    pub fn total_blocks(&self) -> u32 {
        let result = u32::from(self.total_blocks16());
        if result != 0 {
            result
        } else {
            self.total_blocks32()
        }
    }

    /// Get the total number of clusters in this filesystem.
    pub fn total_clusters(&self) -> u32 {
        self.cluster_count
    }
}