From 9f298841a4fdb3048ef6621c6aac903da009d797 Mon Sep 17 00:00:00 2001
From: apio <blobs.trading@gmail.com>
Date: Fri, 26 May 2023 23:28:51 +0200
Subject: [PATCH] kernel: Start reading the MBR partition table from the ATAPI
 drive

---
 kernel/CMakeLists.txt               |  1 +
 kernel/src/arch/x86_64/disk/ATA.cpp |  5 +++++
 kernel/src/fs/MBR.cpp               | 29 ++++++++++++++++++++++++
 kernel/src/fs/MBR.h                 | 35 +++++++++++++++++++++++++++++
 4 files changed, 70 insertions(+)
 create mode 100644 kernel/src/fs/MBR.cpp
 create mode 100644 kernel/src/fs/MBR.h

diff --git a/kernel/CMakeLists.txt b/kernel/CMakeLists.txt
index 8c266e6d..4352da53 100644
--- a/kernel/CMakeLists.txt
+++ b/kernel/CMakeLists.txt
@@ -43,6 +43,7 @@ set(SOURCES
     src/fs/VFS.cpp
     src/fs/Pipe.cpp
     src/fs/Mount.cpp
+    src/fs/MBR.cpp
     src/fs/tmpfs/FileSystem.cpp
     src/fs/devices/DeviceRegistry.cpp
     src/fs/devices/NullDevice.cpp
diff --git a/kernel/src/arch/x86_64/disk/ATA.cpp b/kernel/src/arch/x86_64/disk/ATA.cpp
index 7e2050c0..412b1c9d 100644
--- a/kernel/src/arch/x86_64/disk/ATA.cpp
+++ b/kernel/src/arch/x86_64/disk/ATA.cpp
@@ -3,6 +3,7 @@
 #include "arch/Serial.h"
 #include "arch/Timer.h"
 #include "arch/x86_64/IO.h"
+#include "fs/MBR.h"
 #include "memory/MemoryManager.h"
 #include <luna/Alignment.h>
 #include <luna/CType.h>
@@ -336,6 +337,10 @@ namespace ATA
                     kwarnln("ata: Failed to register ATA drive %d:%d in DeviceRegistry", m_channel_index, drive);
                     continue;
                 }
+
+                // FIXME: Do not hardcode the path like this.
+                auto inode = VFS::resolve_path("/dev/cdrom", Credentials {}).value();
+                MBR::identify(inode);
             }
         }
 
diff --git a/kernel/src/fs/MBR.cpp b/kernel/src/fs/MBR.cpp
new file mode 100644
index 00000000..21bce837
--- /dev/null
+++ b/kernel/src/fs/MBR.cpp
@@ -0,0 +1,29 @@
+#include "fs/MBR.h"
+#include "Log.h"
+
+namespace MBR
+{
+    Result<bool> identify(SharedPtr<VFS::Inode> inode)
+    {
+        // Cannot read a partition table from a pipe/socket/character device! Who is even coming up with this silliness?
+        if (!VFS::is_seekable(inode)) return false;
+
+        DiskHeader hdr;
+        usize nread = TRY(inode->read((u8*)&hdr, 0, sizeof(hdr)));
+        check(nread == 512);
+
+        if (hdr.signature[0] != MBR_SIGNATURE_1 || hdr.signature[1] != MBR_SIGNATURE_2) return false;
+
+        for (int i = 0; i < 4; i++)
+        {
+            const auto& part = hdr.partitions[i];
+            if (part.partition_type == 0) continue; // Not active.
+
+            bool bootable = part.attributes & MBR_BOOTABLE;
+            kinfoln("mbr: Partition #%d is active: bootable=%d, type=%x, start=%d, sectors=%d", i, bootable,
+                    part.partition_type, part.start_lba, part.num_sectors);
+        }
+
+        return true;
+    }
+}
diff --git a/kernel/src/fs/MBR.h b/kernel/src/fs/MBR.h
new file mode 100644
index 00000000..d447cdef
--- /dev/null
+++ b/kernel/src/fs/MBR.h
@@ -0,0 +1,35 @@
+#pragma once
+
+#include "fs/VFS.h"
+#include <luna/Types.h>
+
+#define MBR_BOOTABLE 0x80
+
+#define MBR_SIGNATURE_1 0x55
+#define MBR_SIGNATURE_2 0xAA
+
+namespace MBR
+{
+    struct [[gnu::packed]] PartitionHeader
+    {
+        u8 attributes;
+        u8 chs_start[3];
+        u8 partition_type;
+        u8 chs_end[3];
+        u32 start_lba;
+        u32 num_sectors;
+    };
+
+    struct [[gnu::packed]] DiskHeader
+    {
+        u8 mbr_code[440];
+        u8 disk_id[4];
+        u8 reserved[2];
+        PartitionHeader partitions[4];
+        u8 signature[2];
+    };
+
+    static_assert(sizeof(DiskHeader) == 512ul);
+
+    Result<bool> identify(SharedPtr<VFS::Inode> inode);
+};