Kernel: Implement MBR partition loader (#168)

This implements a basic MBR partition loader, which removes the reliance
on a hard-coded filesystem offset in the stage2 init.

2 files changed, 108 insertions, 0 deletions

diff --git a/Kernel/Devices/MBRPartitionTable.cpp b/Kernel/Devices/MBRPartitionTable.cpp
new file mode 100644
index 0000000000..369be452e1
--- /dev/null
+++ b/Kernel/Devices/MBRPartitionTable.cpp
@@ -0,0 +1,61 @@
+#include <AK/ByteBuffer.h>
+#include <Kernel/Devices/MBRPartitionTable.h>
+
+#define MBR_DEBUG
+
+MBRPartitionTable::MBRPartitionTable(Retained<DiskDevice>&& device)
+    : m_device(move(device))
+{
+}
+
+MBRPartitionTable::~MBRPartitionTable()
+{
+}
+
+const MBRPartitionHeader& MBRPartitionTable::header() const
+{
+    return *reinterpret_cast<const MBRPartitionHeader*>(m_cached_header);
+}
+
+bool MBRPartitionTable::initialize()
+{
+    if (!m_device->read_block(0, m_cached_header)) {
+        return false;
+    }
+
+    auto& header = this->header();
+
+#ifdef MBR_DEBUG
+    kprintf("MBRPartitionTable::initialize: mbr_signature=%#x\n", header.mbr_signature);
+#endif
+
+    if (header.mbr_signature != MBR_SIGNATURE) {
+        kprintf("MBRPartitionTable::initialize: bad mbr signature %#x\n", header.mbr_signature);
+        return false;
+    }
+
+    return true;
+}
+
+RetainPtr<DiskPartition> MBRPartitionTable::partition(unsigned index)
+{
+    ASSERT(index >= 1 && index <= 4);
+
+    auto& header = this->header();
+    auto& entry = header.entry[index - 1];
+
+    if (header.mbr_signature != MBR_SIGNATURE) {
+        kprintf("MBRPartitionTable::initialize: bad mbr signature - not initalized? %#x\n", header.mbr_signature);
+        return nullptr;
+    }
+
+#ifdef MBR_DEBUG
+    kprintf("MBRPartitionTable::partition: status=%#x offset=%#x\n", entry.status, entry.offset);
+#endif
+
+    if (entry.status == 0x00) {
+        return nullptr;
+    }
+
+    return DiskPartition::create(m_device.copy_ref(), entry.offset);
+}
diff --git a/Kernel/Devices/MBRPartitionTable.h b/Kernel/Devices/MBRPartitionTable.h
new file mode 100644
index 0000000000..a3ad937b2c
--- /dev/null
+++ b/Kernel/Devices/MBRPartitionTable.h
@@ -0,0 +1,47 @@
+#pragma once
+
+#include <AK/RetainPtr.h>
+#include <AK/Vector.h>
+#include <Kernel/Devices/DiskDevice.h>
+#include <Kernel/Devices/DiskPartition.h>
+
+#define MBR_SIGNATURE 0xaa55
+
+struct MBRPartitionEntry {
+    byte status;
+    byte chs1[3];
+    byte type;
+    byte chs2[3];
+    dword offset;
+    dword length;
+} __attribute__((packed));
+
+struct MBRPartitionHeader {
+    byte code1[218];
+    word ts_zero;
+    byte ts_drive, ts_seconds, ts_minutes, ts_hours;
+    byte code2[216];
+    dword disk_signature;
+    word disk_signature_zero;
+    MBRPartitionEntry entry[4];
+    word mbr_signature;
+} __attribute__((packed));
+
+class MBRPartitionTable {
+    AK_MAKE_ETERNAL
+
+public:
+    MBRPartitionTable(Retained<DiskDevice>&& device);
+    ~MBRPartitionTable();
+
+    bool initialize();
+    RetainPtr<DiskPartition> partition(unsigned index);
+
+private:
+    Retained<DiskDevice> m_device;
+
+    ByteBuffer read_header() const;
+    const MBRPartitionHeader& header() const;
+
+    byte m_cached_header[512];
+};