kernel/filesystem: added function to detect fat-type

2 files changed, 47 insertions, 0 deletions

diff --git a/kernel/kernel/filesystems/fat32/filesystem.cpp b/kernel/kernel/filesystems/fat32/filesystem.cpp
index 743968fa..22e5c0ef 100644
--- a/kernel/kernel/filesystems/fat32/filesystem.cpp
+++ b/kernel/kernel/filesystems/fat32/filesystem.cpp
@@ -73,6 +73,46 @@ namespace kernel::filesystems::fat32
       kstd::println("[OS:FAT32] boot_signature: {:#06X}", boot_sector.boot_signature);
     }
 
+    enum struct fat_type
+    {
+      fat12,
+      fat16,
+      fat32
+    };
+
+    //! Derive the fat-type with the variables in the boot-sector. The differentiation is made by number of clusters in
+    //! the data-region following microsoft specification 1.03. This function doesn't validate the provided boot_sector and
+    //! necessitates that it's valid.
+    //!
+    //! @param bs populated boot_sector
+    //! @return returns fat12, fat16 or fat32
+    auto get_fat_type(boot_sector const & bs) -> fat_type
+    {
+      // is zero for FAT32 because root directory is stored in data-sector
+      auto const root_dir_size_fat16 = std::uint64_t{bs.root_entry_count} * 32; // todo maybe constant
+
+      // ceiling divide
+      auto const root_dir_sectors_fat16 = (root_dir_size_fat16 + bs.bytes_per_sector - 1) / bs.bytes_per_sector;
+
+      auto const fat_sectors = bs.fat_size_16 != 0 ? std::uint64_t{bs.fat_size_16} : bs.fat_size_32;
+      auto const total_sectors = bs.total_sectors_16 != 0 ? std::uint64_t{bs.total_sectors_16} : bs.total_sectors_32;
+
+      auto const fat_region_sectors = bs.number_of_fats * fat_sectors;
+
+      auto const data_region_sectors =
+          total_sectors - (bs.reserved_sector_count + fat_region_sectors + root_dir_sectors_fat16);
+
+      auto const data_region_clusters = data_region_sectors / bs.sectors_per_cluster;
+
+      if (data_region_clusters <= constants::max_clusters_fat12)
+        return fat_type::fat12;
+
+      if (data_region_clusters <= constants::max_clusters_fat16)
+        return fat_type::fat16;
+
+      return fat_type::fat32;
+    }
+
     //! Read the boot sector from a backing inode and check its signature.
     //!
     //! @param backing_inode The inode to read the boot sector from.
@@ -92,6 +132,10 @@ namespace kernel::filesystems::fat32
         return kstd::failure(errc::invalid_boot_signature);
       }
 
+      if (auto fat_type = get_fat_type(boot_sector); fat_type != fat_type::fat32) {
+        return kstd::failure(errc::invalid_boot_sector); // todo custom error
+      }
+
       return boot_sector;
     }
 
diff --git a/kernel/kernel/filesystems/fat32/filesystem.hpp b/kernel/kernel/filesystems/fat32/filesystem.hpp
index a392d0fd..8a173e54 100644
--- a/kernel/kernel/filesystems/fat32/filesystem.hpp
+++ b/kernel/kernel/filesystems/fat32/filesystem.hpp
@@ -26,6 +26,9 @@ namespace kernel::filesystems::fat32
     constexpr std::uint16_t inline boot_signature = 0xAA55;
     constexpr std::uint16_t inline min_bytes_per_sector = 512;
     constexpr std::uint16_t inline max_bytes_per_sector = 4096;
+
+    constexpr std::uint16_t inline max_clusters_fat12 = 4'084;
+    constexpr std::uint16_t inline max_clusters_fat16 = 65'524;
   }  // namespace constants
 
   //! The File Allocation Table 32 Filesystem (fat32)