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Kernel/FATFS: Fix reading from large 12-bit FATs

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12-bit FATs aren't necessarily block-aligned, so in the worst case
we'll have to reach into the next block to perform the read properly.
This commit is contained in:
implicitfield 2024-04-21 00:51:19 +04:00 • committed by Tim Schumacher
commit a6a1508601
Notes: sideshowbarker 2024-07-17 18:49:10 +09:00
1 changed files with 14 additions and 4 deletions
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18
Kernel/FileSystem/FATFS/FileSystem.cpp
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@ -340,16 +340,26 @@ ErrorOr<u32> FATFS::fat_read(u32 cluster)
{ {
dbgln_if(FAT_DEBUG, "FATFS: Reading FAT entry for cluster {}", cluster); dbgln_if(FAT_DEBUG, "FATFS: Reading FAT entry for cluster {}", cluster);
auto fat_sector = TRY(KBuffer::try_create_with_size("FATFS: FAT read buffer"sv, m_device_block_size));
auto fat_sector_buffer = UserOrKernelBuffer::for_kernel_buffer(fat_sector->data());
u32 fat_offset = fat_offset_for_cluster(cluster); u32 fat_offset = fat_offset_for_cluster(cluster);
u32 fat_sector_index = m_parameter_block->common_bpb()->reserved_sector_count + (fat_offset / m_device_block_size); u32 fat_sector_index = m_parameter_block->common_bpb()->reserved_sector_count + (fat_offset / m_device_block_size);
u32 entry_offset = fat_offset % m_device_block_size; u32 entry_offset = fat_offset % m_device_block_size;
// NOTE: On FAT12, FATs aren't necessarily block aligned, so in the worst case we have to read
// an extra byte from the next block.
bool read_extra_block = m_fat_version == FATVersion::FAT12 && entry_offset == m_device_block_size - 1;
size_t buffer_size = m_device_block_size;
if (read_extra_block)
buffer_size += m_device_block_size;
auto fat_sector = TRY(KBuffer::try_create_with_size("FATFS: FAT read buffer"sv, buffer_size));
auto fat_sector_buffer = UserOrKernelBuffer::for_kernel_buffer(fat_sector->data());
MutexLocker locker(m_lock); MutexLocker locker(m_lock);
TRY(read_block(fat_sector_index, &fat_sector_buffer, m_device_block_size)); if (read_extra_block)
TRY(read_blocks(fat_sector_index, 2, fat_sector_buffer));
else
TRY(read_block(fat_sector_index, &fat_sector_buffer, m_device_block_size));
// Look up the next cluster to read, or read End of Chain marker from table. // Look up the next cluster to read, or read End of Chain marker from table.
return cluster_number(*fat_sector, cluster, entry_offset); return cluster_number(*fat_sector, cluster, entry_offset);