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mkfs.fat: Don't wipe the first 34 MiB if we don't need to

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We really only need to make sure that all reserved sectors, the FAT
region, and the root directory sectors/cluster are empty, and the total
disk space used by these structures is usually significantly less than
34 MiB.
This commit is contained in:
implicitfield authored and Sönke Holz committed 2025-03-13 21:59:43 +01:00
1 parent 9a66ea24e3
commit 60e054e9b7
1 file changed
+21 -28
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Userland/Utilities/mkfs.fat.cpp
+21 -28
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@@ -96,6 +96,8 @@ static constexpr u8 s_empty_32_bit_fat[12] = { 0xF8, 0xFF, 0xFF, 0x0F, 0xFF, 0xF
static constexpr u8 s_zero_buffer[4096] { 0 };
static constexpr u8 s_fat_entry_size { 32 };
enum class FATType {
FAT12,
FAT16,
@@ -192,25 +194,6 @@ static Array<u8, sizeof(Kernel::FAT32FSInfo)> serialize_fat32_fs_info(Kernel::FA
return output;
}
static ErrorOr<void> wipe_file(Core::File& file, u64 file_size)
{
VERIFY(static_cast<u64>(file_size) >= 360 * KiB);
TRY(file.seek(0, SeekMode::SetPosition));
// Wipe the entire partition or the first 34 MiB, whichever is smaller.
for (size_t i = 0; i < 34 * MiB; i += sizeof(s_zero_buffer)) {
size_t to_write = min(file_size - i, sizeof(s_zero_buffer));
TRY(file.write_until_depleted({ &s_zero_buffer, to_write }));
if (file_size - i <= sizeof(s_zero_buffer))
break;
}
TRY(file.seek(0, SeekMode::SetPosition));
return {};
}
// This algorithm only works for 512 byte sectors, which are the only ones we support anyway.
// This may also produce slightly inefficient results, using up to 2 extra sectors for FAT16 and up to 8 for FAT32.
static u32 get_sectors_per_fat(Kernel::DOS3BIOSParameterBlock const& boot_record, FATType fat_type)
@@ -238,7 +221,7 @@ static u32 get_sectors_per_fat(Kernel::DOS3BIOSParameterBlock const& boot_record
VERIFY(sector_count != 0);
u32 root_directory_sectors = ((boot_record.root_directory_entry_count * 32) + (boot_record.bytes_per_sector - 1)) / boot_record.bytes_per_sector;
u32 root_directory_sectors = ((boot_record.root_directory_entry_count * s_fat_entry_size) + (boot_record.bytes_per_sector - 1)) / boot_record.bytes_per_sector;
u32 sectors_per_container = sector_count - (boot_record.reserved_sector_count + root_directory_sectors);
u32 container_count = (256 * boot_record.sectors_per_cluster) + boot_record.fat_count;
@@ -495,8 +478,6 @@ static ErrorOr<void> format_fat_16_bit(Core::File& file, FATType fat_type, u64 f
auto boot_record = TRY(generate_dos_3_bios_parameter_block(file_size, fat_type));
auto boot_record_16_bit = generate_dos_4_bios_parameter_block(fat_type, volume_id);
TRY(wipe_file(file, file_size));
Vector<u8> mbr;
mbr.append(serialize_dos_3_bios_parameter_block(boot_record).data(), sizeof(boot_record));
mbr.append(serialize_dos_4_bios_parameter_block(boot_record_16_bit).data(), sizeof(boot_record_16_bit));
@@ -517,10 +498,14 @@ static ErrorOr<void> format_fat_16_bit(Core::File& file, FATType fat_type, u64 f
for (size_t i = 0; i < boot_record.fat_count; ++i) {
TRY(file.write_until_depleted({ FAT_sector.data(), FAT_sector.size() }));
if (i + 1 != boot_record.fat_count)
TRY(file.seek(boot_record.bytes_per_sector * (boot_record.sectors_per_fat_16bit - 1), SeekMode::FromCurrentPosition));
for (u16 j = 0; j < boot_record.sectors_per_fat_16bit - 1; ++j)
TRY(file.write_until_depleted({ &s_zero_buffer, 512 }));
}
size_t root_directory_sectors = ceil_div(boot_record.root_directory_entry_count * s_fat_entry_size, 512);
for (size_t i = 0; i < root_directory_sectors; ++i)
TRY(file.write_until_depleted({ &s_zero_buffer, 512 }));
return {};
}
@@ -531,8 +516,6 @@ static ErrorOr<void> format_fat32(Core::File& file, u64 file_size, u32 volume_id
auto fs_info = generate_fat32_fs_info(boot_record, boot_record_fat32);
s_bootcode[s_message_offset_offset] = 0x77;
TRY(wipe_file(file, file_size));
Vector<u8> mbr;
mbr.append(serialize_dos_3_bios_parameter_block(boot_record).data(), sizeof(boot_record));
mbr.append(serialize_dos_7_bios_parameter_block(boot_record_fat32).data(), sizeof(boot_record_fat32));
@@ -545,6 +528,12 @@ static ErrorOr<void> format_fat32(Core::File& file, u64 file_size, u32 volume_id
serialized_fs_info.append(serialize_fat32_fs_info(fs_info).data(), sizeof(fs_info));
VERIFY(serialized_fs_info.size() == 512);
// Wipe all the reserved sectors.
for (size_t i = 0; i < boot_record.reserved_sector_count; ++i)
TRY(file.write_until_depleted({ &s_zero_buffer, 512 }));
TRY(file.seek(0, SeekMode::SetPosition));
// Write the boot record and the FSInfo block at the start of the file, and also back them up at sectors 6 and 7 respectively.
for (size_t i = 0; i < 2; ++i) {
TRY(file.write_until_depleted({ mbr.data(), mbr.size() }));
@@ -561,10 +550,14 @@ static ErrorOr<void> format_fat32(Core::File& file, u64 file_size, u32 volume_id
TRY(file.seek(boot_record.bytes_per_sector * boot_record.reserved_sector_count, SeekMode::SetPosition));
for (size_t i = 0; i < boot_record.fat_count; ++i) {
TRY(file.write_until_depleted({ FAT_sector.data(), FAT_sector.size() }));
if (i + 1 != boot_record.fat_count)
TRY(file.seek(boot_record.bytes_per_sector * (boot_record_fat32.sectors_per_fat_32bit - 1), SeekMode::FromCurrentPosition));
for (size_t j = 0; j < boot_record_fat32.sectors_per_fat_32bit - 1; ++j)
TRY(file.write_until_depleted({ &s_zero_buffer, 512 }));
}
// Erase the root directory cluster (which we always place right after the FAT).
for (size_t i = 0; i < boot_record.sectors_per_cluster; ++i)
TRY(file.write_until_depleted({ &s_zero_buffer, 512 }));
return {};
}