From c1678823d808932c065942e9aa0f7d7fe0246b8a Mon Sep 17 00:00:00 2001
From: Felix Morgner <felix.morgner@ost.ch>
Date: Sat, 22 Aug 2026 14:30:33 +0200
Subject: kernel/fs: add backing inode to mount

---
 kernel/kernel/filesystem/ext2/inode.cpp | 41 +++++++++++++++++++++++++++++----
 kernel/kernel/filesystem/ext2/inode.hpp |  4 ++++
 kernel/kernel/filesystem/mount.cpp      | 10 +++++++-
 kernel/kernel/filesystem/mount.hpp      |  4 ++++
 4 files changed, 53 insertions(+), 6 deletions(-)

diff --git a/kernel/kernel/filesystem/ext2/inode.cpp b/kernel/kernel/filesystem/ext2/inode.cpp
index 5e80f40b..7059527a 100644
--- a/kernel/kernel/filesystem/ext2/inode.cpp
+++ b/kernel/kernel/filesystem/ext2/inode.cpp
@@ -67,10 +67,16 @@ namespace kernel::filesystem::ext2
       }
       else
       {
+        auto inode = backing_inode();
+        if (!inode)
+        {
+          return kstd::failure(inode.error());
+        }
+
         auto const block_start_offset = block_number.value() * (*filesystem)->block_size();
         auto const read_offset = block_start_offset + in_block_offset;
-        auto const read_result =
-            (*filesystem)->backing_inode()->read(buffer.subspan(bytes_read.value, bytes_to_read.value), read_offset);
+
+        auto const read_result = (*inode)->read(buffer.subspan(bytes_read.value, bytes_to_read.value), read_offset);
         if (!read_result)
         {
           return kstd::failure(read_result.error());
@@ -140,9 +146,14 @@ namespace kernel::filesystem::ext2
       auto const block_start_offset = block_number.value() * (*filesystem)->block_size();
       auto const write_offset = block_start_offset + in_block_offset;
 
-      auto const write_result = (*filesystem)
-                                    ->backing_inode()
-                                    ->write(buffer.subspan(bytes_written.value, bytes_to_write.value), write_offset);
+      auto inode = backing_inode();
+      if (!inode)
+      {
+        return kstd::failure(inode.error());
+      }
+
+      auto const write_result =
+          (*inode)->write(buffer.subspan(bytes_written.value, bytes_to_write.value), write_offset);
       if (!write_result)
       {
         return kstd::failure(write_result.error());
@@ -318,4 +329,24 @@ namespace kernel::filesystem::ext2
     return kstd::failure(vfs_errc::not_mounted);
   }
 
+  auto inode::backing_inode() -> kstd::result<kstd::shared_ptr<kernel::filesystem::inode>>
+  {
+    if (auto mount = owning_mount().lock())
+    {
+      return mount->backing_inode();
+    }
+
+    return kstd::failure(vfs_errc::not_mounted);
+  }
+
+  auto inode::backing_inode() const -> kstd::result<kstd::shared_ptr<kernel::filesystem::inode const>>
+  {
+    if (auto mount = owning_mount().lock())
+    {
+      return mount->backing_inode();
+    }
+
+    return kstd::failure(vfs_errc::not_mounted);
+  }
+
 }  // namespace kernel::filesystem::ext2
diff --git a/kernel/kernel/filesystem/ext2/inode.hpp b/kernel/kernel/filesystem/ext2/inode.hpp
index 41231cab..cae1d801 100644
--- a/kernel/kernel/filesystem/ext2/inode.hpp
+++ b/kernel/kernel/filesystem/ext2/inode.hpp
@@ -105,6 +105,10 @@ namespace kernel::filesystem::ext2
 
     [[nodiscard]] auto get_filesystem() const -> kstd::result<kstd::shared_ptr<filesystem const>>;
 
+    [[nodiscard]] auto backing_inode() -> kstd::result<kstd::shared_ptr<kernel::filesystem::inode>>;
+
+    [[nodiscard]] auto backing_inode() const -> kstd::result<kstd::shared_ptr<kernel::filesystem::inode const>>;
+
     //! The inode number on disk.
     uint32_t m_inode_number{};
     //! The inode data.
diff --git a/kernel/kernel/filesystem/mount.cpp b/kernel/kernel/filesystem/mount.cpp
index b1335735..995c12c8 100644
--- a/kernel/kernel/filesystem/mount.cpp
+++ b/kernel/kernel/filesystem/mount.cpp
@@ -62,7 +62,10 @@ namespace kernel::filesystem
       return kstd::failure(root_inode.error());
     }
 
-    return attach(mount_dentry, fs, *root_inode, parent_mount, source_mount);
+    auto instance = attach(mount_dentry, fs, *root_inode, parent_mount, source_mount);
+    instance->m_backing_inode = backing_inode;
+
+    return instance;
   }
 
   auto mount::mount_dentry() const -> kstd::shared_ptr<dentry> const &
@@ -75,6 +78,11 @@ namespace kernel::filesystem
     return m_filesystem;
   }
 
+  auto mount::backing_inode() const -> kstd::shared_ptr<inode> const &
+  {
+    return m_backing_inode;
+  }
+
   auto mount::root_dentry() const -> kstd::shared_ptr<dentry> const &
   {
     return m_root_dentry;
diff --git a/kernel/kernel/filesystem/mount.hpp b/kernel/kernel/filesystem/mount.hpp
index c07de071..d8d9c625 100644
--- a/kernel/kernel/filesystem/mount.hpp
+++ b/kernel/kernel/filesystem/mount.hpp
@@ -49,6 +49,9 @@ namespace kernel::filesystem
     //! Get the filesystem instance being mounted.
     [[nodiscard]] auto get_filesystem() const -> kstd::shared_ptr<filesystem> const &;
 
+    //! Get the backing inode, if any, of this mount.
+    [[nodiscard]] auto backing_inode() const -> kstd::shared_ptr<inode> const &;
+
     //! Get the path at which the filesystem is mounted.
     [[nodiscard]] auto mount_path() const -> kstd::string;
 
@@ -91,6 +94,7 @@ namespace kernel::filesystem
     kstd::shared_ptr<dentry> m_mount_dentry{};
     kstd::shared_ptr<dentry> m_root_dentry{};
     kstd::shared_ptr<filesystem> m_filesystem{};
+    kstd::shared_ptr<inode> m_backing_inode{};
     kstd::shared_ptr<mount> m_parent_mount{};
     kstd::weak_ptr<mount> m_source_mount{};
     std::atomic_size_t m_ref_count{0};
-- 
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