Kernel: Allow again to boot with partitioned disk

This change ensures that we don't return a zero value blindly
from DiskPartition write/read methods.

Fixes #1719.

Kernel/Devices/DiskPartition.cpp

@@ -25,6 +25,7 @@
  */
 
 #include <Kernel/Devices/DiskPartition.h>
+#include <Kernel/FileSystem/FileDescription.h>
 
 // #define OFFD_DEBUG
 
@@ -47,6 +48,34 @@ DiskPartition::~DiskPartition()
 {
 }
 
+ssize_t DiskPartition::read(FileDescription& file_description, u8* buffer, ssize_t length)
+{
+    // FIXME: This is a hacky solution, but works fine for now...
+    off_t current_offset = file_description.seek(0, SEEK_CUR);
+    auto new_offset = current_offset + (m_block_offset * m_device->block_size());
+    if ((new_offset / m_device->block_size()) > m_block_limit || ((new_offset + length) / m_device->block_size()) > m_block_limit) {
+        return 0;
+    }
+    file_description.seek(new_offset, SEEK_SET);
+    auto result = m_device->read(file_description, buffer, length);
+    file_description.seek(current_offset, SEEK_SET);
+    return result;
+}
+
+ssize_t DiskPartition::write(FileDescription& file_description, const u8* buffer, ssize_t length)
+{
+    // FIXME: This is a hacky solution, but works fine for now...
+    off_t current_offset = file_description.seek(0, SEEK_CUR);
+    auto new_offset = current_offset + (m_block_offset * m_device->block_size());
+    if ((new_offset / m_device->block_size()) > m_block_limit || ((new_offset + length) / m_device->block_size()) > m_block_limit) {
+        return 0;
+    }
+    file_description.seek(new_offset, SEEK_SET);
+    auto result = m_device->write(file_description, buffer, length);
+    file_description.seek(current_offset, SEEK_SET);
+    return result;
+}
+
 bool DiskPartition::read_blocks(unsigned index, u16 count, u8* out)
 {
 #ifdef OFFD_DEBUG

Kernel/Devices/DiskPartition.h

@@ -40,9 +40,9 @@ public:
     virtual bool write_blocks(unsigned index, u16 count, const u8*) override;
 
     // ^BlockDevice
-    virtual ssize_t read(FileDescription&, u8*, ssize_t) override { return 0; }
+    virtual ssize_t read(FileDescription&, u8*, ssize_t) override;
     virtual bool can_read(const FileDescription&) const override { return true; }
-    virtual ssize_t write(FileDescription&, const u8*, ssize_t) override { return 0; }
+    virtual ssize_t write(FileDescription&, const u8*, ssize_t) override;
     virtual bool can_write(const FileDescription&) const override { return true; }
 
 private: