m/p/fat32: add fat32 package
The fat32 package is a write-only implementation of the FAT32
filesystem. It works quite unlike a normal file system by first
determining the entire disk layout and then sequentially writing
out everything. This allows it to have a fully streaming output without
needing to seek at all.
Because all IO is sequential the implementation is extremely fast and
can potentially even leverage things like the copy_file_range syscall.
This means however that all files and readers need to be prepared ahead
of time, it is not possible to make decisions during the writing
process.
It is also possible to generate "right-sized" filesystems by not
specifying an explicit block count. In that case the resulting image
will contain exactly as many clusters as needed.
Change-Id: I49bf2ce09b26a7d628a39a0dd0745bca61c1c4da
Reviewed-on: https://review.monogon.dev/c/monogon/+/841
Tested-by: Jenkins CI
Reviewed-by: Sergiusz Bazanski <serge@monogon.tech>
diff --git a/metropolis/pkg/fat32/structs.go b/metropolis/pkg/fat32/structs.go
new file mode 100644
index 0000000..396361d
--- /dev/null
+++ b/metropolis/pkg/fat32/structs.go
@@ -0,0 +1,133 @@
+package fat32
+
+const (
+ // FAT32 entries are only 28 bits
+ fatMask = 0x0fffffff
+ // Free entries are 0
+ fatFree = 0x0
+ // Entry at the end of a cluster chain
+ fatEOF = 0x0ffffff8
+)
+
+// FAT32 Boot Sector and BIOS Parameter Block. This structure is 512 bytes long,
+// even if the logical block size is longer. The rest should be filled up with
+// zeroes.
+type bootSector struct {
+ // Jump instruction to boot code.
+ JmpInstruction [3]byte
+ // Creator name. "MSWIN4.1" recommended for compatibility.
+ OEMName [8]byte
+ // Count of bytes per block (i.e. logical block size)
+ // Must be one of 512, 1024, 2048 or 4096
+ BlockSize uint16
+ // Number of blocks per allocation unit (cluster).
+ // Must be a power of 2 that is greater than 0.
+ BlocksPerCluster uint8
+ // Number of reserved blocks in the reserved region of the volume starting
+ // at the first block of the volume. This field must not be 0.
+ ReservedBlocks uint16
+ // The count of FAT data structures on the volume. This field should always
+ // contain the value of 2 for any FAT volume of any type.
+ NumFATs uint8
+ _ [4]byte
+ // Legacy value for media determination, must be 0xf8.
+ MediaCode uint8
+ _ [2]byte
+ // Number of sectors per track for 0x13 interrupts.
+ SectorsPerTrack uint16
+ // Number of heads for 0x13 interrupts.
+ NumHeads uint16
+ // Count of hidden blocks preceding the partition that contains this FAT
+ // volume.
+ HiddenBlocks uint32
+ // Total count of blocks on the volume.
+ TotalBlocks uint32
+ // Count of blocks per FAT.
+ BlocksPerFAT uint32
+ // Flags for FAT32
+ Flags uint16
+ _ [2]byte
+ // Cluster number of the first cluster of the root directory. Usually 2.
+ RootClusterNumber uint32
+ // Block number of the FSINFO structure in the reserved area.
+ FSInfoBlock uint16
+ // Block number of the copy of the boot record in the reserved area.
+ BackupStartBlock uint16
+ _ [12]byte
+ // Drive number for 0x13 interrupts.
+ DriveNumber uint8
+ _ [1]byte
+ BootSignature uint8
+ // ID of this filesystem
+ ID uint32
+ // Human-readable label of this filesystem, padded with spaces (0x20)
+ Label [11]byte
+ // Always set to ASCII "FAT32 "
+ Type [8]byte
+ _ [420]byte
+ // Always 0x55, 0xAA
+ Signature [2]byte
+}
+
+// Special block (usually at block 1) containing additional metadata,
+// specifically the number of free clusters and the next free cluster.
+// Always 512 bytes, rest of the block should be padded with zeroes.
+type fsinfo struct {
+ // Validates that this is an FSINFO block. Always 0x52, 0x52, 0x61, 0x41
+ LeadSignature [4]byte
+ _ [480]byte
+ // Another signature. Always 0x72, 0x72, 0x41, 0x61
+ StructSignature [4]byte
+ // Last known number of free clusters on the volume.
+ FreeCount uint32
+ // Next free cluster hint. All 1's is interpreted as undefined.
+ NextFreeCluster uint32
+ _ [14]byte
+ // One more signature. Always 0x55, 0xAA.
+ TrailingSignature [2]byte
+}
+
+// Directory entry
+type dirEntry struct {
+ // DOS 8.3 file name.
+ DOSName [11]byte
+ // Attribtes of the file or directory, 0x0f reserved to mark entry as a
+ // LFN entry (see lfnEntry below)
+ Attributes uint8
+ _ byte
+ CreationTenMilli uint8 // Actually 10ms units, 0-199 range
+ CreationTime uint16
+ CreationDate uint16
+ _ [2]byte
+ FirstClusterHigh uint16
+ LastWrittenToTime uint16
+ LastWrittenToDate uint16
+ FirstClusterLow uint16
+ FileSize uint32
+}
+
+const (
+ // lastSequenceNumberFlag is logically-ORed with the sequence number of the
+ // last Long File Name entry to mark it as such.
+ lastSequenceNumberFlag = 0x40
+ // codepointsPerEntry is the number of UTF-16 codepoints that fit into a
+ // single Long File Name entry.
+ codepointsPerEntry = 5 + 6 + 2
+)
+
+// VFAT long file name prepended entry
+type lfnEntry struct {
+ SequenceNumber uint8
+ // First 5 UTF-16 code units
+ NamePart1 [5]uint16
+ // Attributes (must be 0x0f)
+ Attributes uint8
+ _ byte
+ // Checksum of the 8.3 name.
+ Checksum uint8
+ // Next 6 UTF-16 code units
+ NamePart2 [6]uint16
+ _ [2]byte
+ // Next 2 UTF-16 code units
+ NamePart3 [2]uint16
+}