kernel/Ext2: Read the root inode metadata from the disk

initrd/etc/init/02-ext2fs

@@ -0,0 +1,3 @@
+Name=ext2fs
+Script=/sbin/test-ext2fs
+Wait=true

initrd/sbin/test-ext2fs

@@ -0,0 +1,4 @@
+# (for testing) Remove this automatic mount once ext2 is working.
+
+mkdir /mnt
+mount -t ext2 /dev/cd0p2 /mnt

kernel/CMakeLists.txt

@@ -48,6 +48,7 @@ set(SOURCES
     src/fs/tmpfs/FileSystem.cpp
     src/fs/tmpfs/Inode.cpp
     src/fs/ext2/FileSystem.cpp
+    src/fs/ext2/Inode.cpp
     src/fs/devices/DeviceRegistry.cpp
     src/fs/devices/NullDevice.cpp
     src/fs/devices/ZeroDevice.cpp

kernel/src/fs/VFS.h

@@ -36,6 +36,11 @@ namespace VFS
 
         virtual Result<void> set_mount_dir(SharedPtr<Inode> parent) = 0;
 
+        virtual bool is_readonly() const
+        {
+            return false;
+        }
+
         virtual u64 handles() const
         {
             return m_handles;

kernel/src/fs/ext2/FileSystem.cpp

@@ -1,4 +1,23 @@
 #include "fs/ext2/FileSystem.h"
+#include "fs/ext2/Inode.h"
+#include <luna/Alignment.h>
+
+static VFS::InodeType vfs_type_from_ext2_type(mode_t mode)
+{
+    auto type = mode & 0xf000;
+
+    switch (type)
+    {
+    case EXT2_FIFO: return VFS::InodeType::FIFO;
+    case EXT2_CHR: return VFS::InodeType::CharacterDevice;
+    case EXT2_DIR: return VFS::InodeType::Directory;
+    case EXT2_BLK: return VFS::InodeType::BlockDevice;
+    case EXT2_REG: return VFS::InodeType::RegularFile;
+    case EXT2_LNK: return VFS::InodeType::Symlink;
+    case EXT2_SOCK: [[fallthrough]]; // TODO: Sockets not supported on Luna at the moment.
+    default: fail("ext2: Unknown or unsupported inode type");
+    }
+}
 
 namespace Ext2
 {
@@ -6,15 +25,54 @@ namespace Ext2
     {
     }
 
-    Result<SharedPtr<VFS::Inode>> FileSystem::find_inode_by_number(ino_t inode)
+    Result<SharedPtr<VFS::Inode>> FileSystem::find_inode_by_number(ino_t inum)
     {
-        auto maybe_inode = m_inode_cache.try_get(inode);
+        check(inum < m_superblock.nr_inodes);
+
+        auto maybe_inode = m_inode_cache.try_get(inum);
         if (maybe_inode.has_value()) return maybe_inode.value();
 
+        const u32 block_group = (u32)((inum - 1) / m_superblock.inodes_per_block_group);
+
+        const auto* block_group_descriptor = TRY(find_block_group_descriptor(block_group));
+        check(block_group_descriptor);
+
+        // FIXME: This is determined by a field in the Superblock if the Ext2 revision >= 1.0.
+        static constexpr usize INODE_SIZE = 128;
+
+        const u64 index = (inum - 1) % m_superblock.inodes_per_block_group;
+
+        const u64 inode_address = (block_group_descriptor->inode_table_start * m_block_size) + (index * INODE_SIZE);
+
+        auto inode = TRY(adopt_shared_if_nonnull(new (std::nothrow) Ext2::Inode({}, this)));
+        TRY(m_host_device->read((u8*)&inode->m_raw_inode, inode_address, INODE_SIZE));
+        inode->m_type = vfs_type_from_ext2_type(inode->m_raw_inode.mode);
+        inode->m_inum = inum;
+
+        kdbgln("ext2: Read inode %lu with mode %#x (%#x + %#o), size %lu", inum, inode->m_raw_inode.mode,
+               inode->m_raw_inode.mode & 0xf000, inode->mode(), inode->size());
+
         // TODO: Locate the inode's block group descriptor and find it in the block group's inode table.
         return err(ENOENT);
     }
 
+    Result<const BlockGroupDescriptor*> FileSystem::find_block_group_descriptor(u32 index)
+    {
+        check(index < m_block_groups);
+
+        auto maybe_desc = m_block_group_descriptor_cache.try_get_ref(index);
+        if (maybe_desc) return maybe_desc;
+
+        const u64 address = (m_superblock.first_data_block + 1) * m_block_size + (index * sizeof(BlockGroupDescriptor));
+
+        BlockGroupDescriptor descriptor;
+        TRY(m_host_device->read((u8*)&descriptor, address, sizeof(descriptor)));
+
+        check(TRY(m_block_group_descriptor_cache.try_set(index, descriptor)));
+
+        return m_block_group_descriptor_cache.try_get_ref(index);
+    }
+
     Result<SharedPtr<VFS::FileSystem>> FileSystem::create(SharedPtr<Device> host_device)
     {
         SharedPtr<FileSystem> fs = TRY(adopt_shared_if_nonnull(new (std::nothrow) FileSystem()));
@@ -22,6 +80,19 @@ namespace Ext2
         if (nread != 1024) return err(EINVAL);                            // Source had an invalid superblock.
         if (fs->m_superblock.signature != EXT2_MAGIC) return err(EINVAL); // Source had an invalid superblock.
 
+        fs->m_host_device = host_device;
+
+        fs->m_block_size = 1024 << fs->m_superblock.log_block_size;
+        fs->m_block_groups = get_blocks_from_size(fs->m_superblock.nr_blocks, fs->m_superblock.blocks_per_block_group);
+
+        kdbgln("ext2: Mounting new Ext2 file system, block size=%lu, blocks=%u, inodes=%u, block group=(%u blocks, %u "
+               "inodes), %lu block groups",
+               fs->m_block_size, fs->m_superblock.nr_blocks, fs->m_superblock.nr_inodes,
+               fs->m_superblock.blocks_per_block_group, fs->m_superblock.inodes_per_block_group, fs->m_block_groups);
+
+        // Lookup the root inode.
+        fs->find_inode_by_number(2);
+
         // TODO: Implement basic Ext2 reading, enough to be able to mount a volume.
         return err(ENOTSUP);
     }

kernel/src/fs/ext2/FileSystem.h

@@ -11,10 +11,10 @@ namespace Ext2
     {
         u32 nr_inodes;
         u32 nr_blocks;
-        u32 nr_reserved;
-        u32 nr_free_inodes;
+        u32 nr_reserved_blocks;
         u32 nr_free_blocks;
-        u32 superblock_block;
+        u32 nr_free_inodes;
+        u32 first_data_block;
         u32 log_block_size;
         u32 log_fragment_size;
         u32 blocks_per_block_group;
@@ -22,23 +22,64 @@ namespace Ext2
         u32 inodes_per_block_group;
         u32 last_mount_time;
         u32 last_write_time;
+
         u16 mounts_since_last_fsck;
         u16 mounts_allowed_before_fsck;
         u16 signature;
         u16 fs_state;
         u16 error_action;
         u16 minor_version;
+
         u32 last_fsck_time;
         u32 fsck_time_interval;
         u32 os_id;
         u32 major_version;
+
         u16 reserved_block_uid;
         u16 reserved_block_gid;
         // TODO: Add extended superblock fields.
         u8 padding[1024 - 84];
     };
 
+    struct [[gnu::packed]] BlockGroupDescriptor
+    {
+        u32 block_usage_addr;
+        u32 inode_usage_addr;
+        u32 inode_table_start;
+        u16 nr_free_blocks;
+        u16 nr_free_inodes;
+        u16 nr_directories;
+        u8 padding[32 - 18];
+    };
+
+    struct [[gnu::packed]] RawInode
+    {
+        u16 mode;
+        u16 uid;
+        u32 size_low;
+        u32 atime;
+        u32 create_time;
+        u32 mtime;
+        u32 delete_time;
+        u16 gid;
+        u16 nlinks;
+        u32 sectors_used;
+        u32 flags;
+        u32 os_specific_1;
+        u32 direct_pointers[12];
+        u32 singly_indirect_ptr;
+        u32 doubly_indirect_ptr;
+        u32 triply_indirect_ptr;
+        u32 gen_number;
+        u32 extended_attrs;
+        u32 size_high;
+        u32 frag_addr;
+        u32 os_specific_2[3];
+    };
+
     static_assert(sizeof(Superblock) == 1024);
+    static_assert(sizeof(BlockGroupDescriptor) == 32);
+    static_assert(sizeof(RawInode) == 128);
 
     class FileSystem : public VFS::FileSystem
     {
@@ -73,6 +114,11 @@ namespace Ext2
             return {};
         }
 
+        bool is_readonly() const override
+        {
+            return true;
+        }
+
         static Result<SharedPtr<VFS::FileSystem>> create(SharedPtr<Device> host_device);
 
         dev_t host_device_id() const override
@@ -81,6 +127,7 @@ namespace Ext2
         }
 
         Result<SharedPtr<VFS::Inode>> find_inode_by_number(ino_t inode);
+        Result<const BlockGroupDescriptor*> find_block_group_descriptor(u32 index);
 
         virtual ~FileSystem() = default;
 
@@ -94,8 +141,13 @@ namespace Ext2
 
         Superblock m_superblock;
 
+        u64 m_block_size;
+        u64 m_block_groups;
+
         // FIXME: This inode cache will keep all inodes in it alive despite having no other references to it, but we're
         // not worrying about that as for now the filesystem implementation is read-only.
         HashMap<ino_t, SharedPtr<VFS::Inode>> m_inode_cache;
+
+        HashMap<u32, BlockGroupDescriptor> m_block_group_descriptor_cache;
     };
 }

kernel/src/fs/ext2/Inode.cpp

@@ -0,0 +1,13 @@
+#include "fs/ext2/Inode.h"
+
+namespace Ext2
+{
+    Inode::Inode(Badge<FileSystem>, FileSystem* fs) : m_fs(fs)
+    {
+    }
+
+    Result<usize> Inode::read(u8* /*buf*/, usize /*offset*/, usize /*length*/) const
+    {
+        fail("FIXME: Ext2::Inode::read()");
+    }
+}

kernel/src/fs/ext2/Inode.h

@@ -0,0 +1,99 @@
+#pragma once
+#include "fs/ext2/FileSystem.h"
+
+#define EXT2_FIFO 0x1000
+#define EXT2_CHR 0x2000
+#define EXT2_DIR 0x4000
+#define EXT2_BLK 0x6000
+#define EXT2_REG 0x8000
+#define EXT2_LNK 0xA000
+#define EXT2_SOCK 0xC000
+
+namespace Ext2
+{
+    class Inode : public VFS::FileInode
+    {
+      public:
+        VFS::InodeType type() const override
+        {
+            return m_type;
+        }
+
+        usize size() const override
+        {
+            // FIXME: If EXT2_REVISION >= 1.0, use size_high as well.
+            return m_raw_inode.size_low;
+        }
+
+        mode_t mode() const override
+        {
+            return m_raw_inode.mode & 07777;
+        }
+
+        nlink_t nlinks() const override
+        {
+            return m_raw_inode.nlinks;
+        }
+
+        u32 uid() const override
+        {
+            return m_raw_inode.uid;
+        }
+
+        u32 gid() const override
+        {
+            return m_raw_inode.gid;
+        }
+
+        usize inode_number() const override
+        {
+            return m_inum;
+        }
+
+        VFS::FileSystem* fs() const override
+        {
+            return m_fs;
+        }
+
+        void did_link() override
+        {
+        }
+
+        void did_unlink() override
+        {
+        }
+
+        Result<void> chmod(mode_t) override
+        {
+            return err(EROFS);
+        }
+
+        Result<void> chown(u32, u32) override
+        {
+            return err(EROFS);
+        }
+
+        Result<usize> read(u8* buf, usize offset, usize length) const override;
+
+        Result<usize> write(const u8*, usize, usize) override
+        {
+            return err(EROFS);
+        }
+
+        Result<void> truncate(usize) override
+        {
+            return err(EROFS);
+        }
+
+        Inode(Badge<FileSystem>, FileSystem* fs);
+        virtual ~Inode() = default;
+
+      private:
+        VFS::InodeType m_type;
+        RawInode m_raw_inode;
+        FileSystem* m_fs;
+        ino_t m_inum;
+
+        friend class FileSystem;
+    };
+}

kernel/src/sys/open.cpp

@@ -63,6 +63,8 @@ Result<u64> sys_openat(Registers*, SyscallArgs args)
         inode->chown(current->auth.euid, current->auth.egid);
     }
 
+    if ((flags & O_WRONLY) && inode->fs() && inode->fs()->is_readonly()) return err(EROFS);
+
     if (inode->type() != VFS::InodeType::Directory && (flags & O_DIRECTORY)) return err(ENOTDIR);
 
     if (inode->type() == VFS::InodeType::Directory)

libluna/include/luna/HashMap.h

@@ -32,6 +32,13 @@ template <typename K, typename V> struct HashMap
         return p->value;
     }
 
+    V* try_get_ref(const K& key)
+    {
+        auto* p = m_table.try_find(HashPair<K, V> { key, {} });
+        if (!p) return nullptr;
+        return p->value.value_ptr();
+    }
+
     bool try_remove(const K& key)
     {
         return m_table.try_remove(HashPair<K, V> { key, {} });