14 files changed, 554 insertions, 130 deletions

diff --git a/src/core/CMakeLists.txt b/src/core/CMakeLists.txt
index 23f717472..24dcc405f 100644
--- a/src/core/CMakeLists.txt
+++ b/src/core/CMakeLists.txt
@@ -59,8 +59,12 @@ add_library(core STATIC
     file_sys/fs_path.h
     file_sys/fs_path_utility.h
     file_sys/fs_string_util.h
+    file_sys/fsa/fs_i_directory.h
+    file_sys/fsa/fs_i_file.h
+    file_sys/fsa/fs_i_filesystem.h
     file_sys/fsmitm_romfsbuild.cpp
     file_sys/fsmitm_romfsbuild.h
+    file_sys/fssrv/fssrv_sf_path.h
     file_sys/fssystem/fs_i_storage.h
     file_sys/fssystem/fs_types.h
     file_sys/fssystem/fssystem_aes_ctr_counter_extended_storage.cpp
diff --git a/src/core/file_sys/fs_filesystem.h b/src/core/file_sys/fs_filesystem.h
index 7f237b7fa..7065ef6a6 100644
--- a/src/core/file_sys/fs_filesystem.h
+++ b/src/core/file_sys/fs_filesystem.h
@@ -23,6 +23,8 @@ enum class OpenDirectoryMode : u64 {
     File = (1 << 1),
 
     All = (Directory | File),
+
+    NotRequireFileSize = (1 << 31),
 };
 DECLARE_ENUM_FLAG_OPERATORS(OpenDirectoryMode)
 
diff --git a/src/core/file_sys/fs_memory_management.h b/src/core/file_sys/fs_memory_management.h
index f03c6354b..080017c5d 100644
--- a/src/core/file_sys/fs_memory_management.h
+++ b/src/core/file_sys/fs_memory_management.h
@@ -10,7 +10,7 @@ namespace FileSys {
 
 constexpr size_t RequiredAlignment = alignof(u64);
 
-void* AllocateUnsafe(size_t size) {
+inline void* AllocateUnsafe(size_t size) {
     // Allocate
     void* const ptr = ::operator new(size, std::align_val_t{RequiredAlignment});
 
@@ -21,16 +21,16 @@ void* AllocateUnsafe(size_t size) {
     return ptr;
 }
 
-void DeallocateUnsafe(void* ptr, size_t size) {
+inline void DeallocateUnsafe(void* ptr, size_t size) {
     // Deallocate the pointer
     ::operator delete(ptr, std::align_val_t{RequiredAlignment});
 }
 
-void* Allocate(size_t size) {
+inline void* Allocate(size_t size) {
     return AllocateUnsafe(size);
 }
 
-void Deallocate(void* ptr, size_t size) {
+inline void Deallocate(void* ptr, size_t size) {
     // If the pointer is non-null, deallocate it
     if (ptr != nullptr) {
         DeallocateUnsafe(ptr, size);
diff --git a/src/core/file_sys/fs_path.h b/src/core/file_sys/fs_path.h
index 56ba08a6a..1566e82b9 100644
--- a/src/core/file_sys/fs_path.h
+++ b/src/core/file_sys/fs_path.h
@@ -381,7 +381,7 @@ public:
 
         // Check that it's possible for us to remove a child
         auto* p = m_write_buffer.Get();
-        s32 len = std::strlen(p);
+        s32 len = static_cast<s32>(std::strlen(p));
         R_UNLESS(len != 1 || (p[0] != '/' && p[0] != '.'), ResultNotImplemented);
 
         // Handle a trailing separator
diff --git a/src/core/file_sys/fs_string_util.h b/src/core/file_sys/fs_string_util.h
index 874e09054..c751a8f1a 100644
--- a/src/core/file_sys/fs_string_util.h
+++ b/src/core/file_sys/fs_string_util.h
@@ -20,6 +20,11 @@ constexpr int Strlen(const T* str) {
 }
 
 template <typename T>
+constexpr int Strnlen(const T* str, std::size_t count) {
+    return Strnlen(str, static_cast<int>(count));
+}
+
+template <typename T>
 constexpr int Strnlen(const T* str, int count) {
     ASSERT(str != nullptr);
     ASSERT(count >= 0);
@@ -33,6 +38,11 @@ constexpr int Strnlen(const T* str, int count) {
 }
 
 template <typename T>
+constexpr int Strncmp(const T* lhs, const T* rhs, std::size_t count) {
+    return Strncmp(lhs, rhs, static_cast<int>(count));
+}
+
+template <typename T>
 constexpr int Strncmp(const T* lhs, const T* rhs, int count) {
     ASSERT(lhs != nullptr);
     ASSERT(rhs != nullptr);
@@ -52,6 +62,11 @@ constexpr int Strncmp(const T* lhs, const T* rhs, int count) {
 }
 
 template <typename T>
+static constexpr int Strlcpy(T* dst, const T* src, std::size_t count) {
+    return Strlcpy<T>(dst, src, static_cast<int>(count));
+}
+
+template <typename T>
 static constexpr int Strlcpy(T* dst, const T* src, int count) {
     ASSERT(dst != nullptr);
     ASSERT(src != nullptr);
diff --git a/src/core/file_sys/fsa/fs_i_directory.h b/src/core/file_sys/fsa/fs_i_directory.h
new file mode 100644
index 000000000..a4135efec
--- /dev/null
+++ b/src/core/file_sys/fsa/fs_i_directory.h
@@ -0,0 +1,89 @@
+// SPDX-FileCopyrightText: Copyright 2024 yuzu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+#include "common/common_types.h"
+#include "core/file_sys/errors.h"
+#include "core/file_sys/fs_directory.h"
+#include "core/file_sys/fs_file.h"
+#include "core/file_sys/fs_filesystem.h"
+#include "core/file_sys/savedata_factory.h"
+#include "core/file_sys/vfs/vfs.h"
+#include "core/hle/result.h"
+
+namespace FileSys::Fsa {
+
+class IDirectory {
+public:
+    IDirectory(VirtualDir backend_, OpenDirectoryMode mode) : backend(std::move(backend_)) {
+        // TODO(DarkLordZach): Verify that this is the correct behavior.
+        // Build entry index now to save time later.
+        if (True(mode & OpenDirectoryMode::Directory)) {
+            BuildEntryIndex(entries, backend->GetSubdirectories(), DirectoryEntryType::Directory);
+        }
+        if (True(mode & OpenDirectoryMode::File)) {
+            BuildEntryIndex(entries, backend->GetFiles(), DirectoryEntryType::File);
+        }
+    }
+    virtual ~IDirectory() {}
+
+    Result Read(s64* out_count, DirectoryEntry* out_entries, s64 max_entries) {
+        R_UNLESS(out_count != nullptr, ResultNullptrArgument);
+        if (max_entries == 0) {
+            *out_count = 0;
+            R_SUCCEED();
+        }
+        R_UNLESS(out_entries != nullptr, ResultNullptrArgument);
+        R_UNLESS(max_entries > 0, ResultInvalidArgument);
+        R_RETURN(this->DoRead(out_count, out_entries, max_entries));
+    }
+
+    Result GetEntryCount(s64* out) {
+        R_UNLESS(out != nullptr, ResultNullptrArgument);
+        R_RETURN(this->DoGetEntryCount(out));
+    }
+
+private:
+    virtual Result DoRead(s64* out_count, DirectoryEntry* out_entries, s64 max_entries) {
+        const u64 actual_entries =
+            std::min(static_cast<u64>(max_entries), entries.size() - next_entry_index);
+        auto* begin = reinterpret_cast<u8*>(entries.data() + next_entry_index);
+
+        next_entry_index += actual_entries;
+        *out_count = actual_entries;
+
+        out_entries = reinterpret_cast<DirectoryEntry*>(begin);
+
+        R_SUCCEED();
+    }
+
+    virtual Result DoGetEntryCount(s64* out) {
+        *out = entries.size() - next_entry_index;
+        R_SUCCEED();
+    }
+
+    // TODO: Remove this when VFS is gone
+    template <typename T>
+    void BuildEntryIndex(std::vector<DirectoryEntry>& entries, const std::vector<T>& new_data,
+                         DirectoryEntryType type) {
+        entries.reserve(entries.size() + new_data.size());
+
+        for (const auto& new_entry : new_data) {
+            auto name = new_entry->GetName();
+
+            if (type == DirectoryEntryType::File && name == GetSaveDataSizeFileName()) {
+                continue;
+            }
+
+            entries.emplace_back(name, static_cast<s8>(type),
+                                 type == DirectoryEntryType::Directory ? 0 : new_entry->GetSize());
+        }
+    }
+
+    VirtualDir backend;
+    std::vector<DirectoryEntry> entries;
+    u64 next_entry_index = 0;
+};
+
+} // namespace FileSys::Fsa
diff --git a/src/core/file_sys/fsa/fs_i_file.h b/src/core/file_sys/fsa/fs_i_file.h
new file mode 100644
index 000000000..6dd0f6439
--- /dev/null
+++ b/src/core/file_sys/fsa/fs_i_file.h
@@ -0,0 +1,167 @@
+// SPDX-FileCopyrightText: Copyright 2024 yuzu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+#include "common/overflow.h"
+#include "core/file_sys/errors.h"
+#include "core/file_sys/fs_file.h"
+#include "core/file_sys/fs_filesystem.h"
+#include "core/file_sys/fs_operate_range.h"
+#include "core/file_sys/vfs/vfs.h"
+#include "core/file_sys/vfs/vfs_types.h"
+#include "core/hle/result.h"
+
+namespace FileSys::Fsa {
+
+class IFile {
+public:
+    IFile(VirtualFile backend_) : backend(std::move(backend_)) {}
+    virtual ~IFile() {}
+
+    Result Read(size_t* out, s64 offset, void* buffer, size_t size, const ReadOption& option) {
+        // Check that we have an output pointer
+        R_UNLESS(out != nullptr, ResultNullptrArgument);
+
+        // If we have nothing to read, just succeed
+        if (size == 0) {
+            *out = 0;
+            R_SUCCEED();
+        }
+
+        // Check that the read is valid
+        R_UNLESS(buffer != nullptr, ResultNullptrArgument);
+        R_UNLESS(offset >= 0, ResultOutOfRange);
+        R_UNLESS(Common::CanAddWithoutOverflow<s64>(offset, size), ResultOutOfRange);
+
+        // Do the read
+        R_RETURN(this->DoRead(out, offset, buffer, size, option));
+    }
+
+    Result Read(size_t* out, s64 offset, void* buffer, size_t size) {
+        R_RETURN(this->Read(out, offset, buffer, size, ReadOption::None));
+    }
+
+    Result GetSize(s64* out) {
+        R_UNLESS(out != nullptr, ResultNullptrArgument);
+        R_RETURN(this->DoGetSize(out));
+    }
+
+    Result Flush() {
+        R_RETURN(this->DoFlush());
+    }
+
+    Result Write(s64 offset, const void* buffer, size_t size, const WriteOption& option) {
+        // Handle the zero-size case
+        if (size == 0) {
+            if (option.HasFlushFlag()) {
+                R_TRY(this->Flush());
+            }
+            R_SUCCEED();
+        }
+
+        // Check the write is valid
+        R_UNLESS(buffer != nullptr, ResultNullptrArgument);
+        R_UNLESS(offset >= 0, ResultOutOfRange);
+        R_UNLESS(Common::CanAddWithoutOverflow<s64>(offset, size), ResultOutOfRange);
+
+        R_RETURN(this->DoWrite(offset, buffer, size, option));
+    }
+
+    Result SetSize(s64 size) {
+        R_UNLESS(size >= 0, ResultOutOfRange);
+        R_RETURN(this->DoSetSize(size));
+    }
+
+    Result OperateRange(void* dst, size_t dst_size, OperationId op_id, s64 offset, s64 size,
+                        const void* src, size_t src_size) {
+        R_RETURN(this->DoOperateRange(dst, dst_size, op_id, offset, size, src, src_size));
+    }
+
+    Result OperateRange(OperationId op_id, s64 offset, s64 size) {
+        R_RETURN(this->DoOperateRange(nullptr, 0, op_id, offset, size, nullptr, 0));
+    }
+
+protected:
+    Result DryRead(size_t* out, s64 offset, size_t size, const ReadOption& option,
+                   OpenMode open_mode) {
+        // Check that we can read
+        R_UNLESS(static_cast<u32>(open_mode & OpenMode::Read) != 0, ResultReadNotPermitted);
+
+        // Get the file size, and validate our offset
+        s64 file_size = 0;
+        R_TRY(this->DoGetSize(std::addressof(file_size)));
+        R_UNLESS(offset <= file_size, ResultOutOfRange);
+
+        *out = static_cast<size_t>(std::min(file_size - offset, static_cast<s64>(size)));
+        R_SUCCEED();
+    }
+
+    Result DrySetSize(s64 size, OpenMode open_mode) {
+        // Check that we can write
+        R_UNLESS(static_cast<u32>(open_mode & OpenMode::Write) != 0, ResultWriteNotPermitted);
+        R_SUCCEED();
+    }
+
+    Result DryWrite(bool* out_append, s64 offset, size_t size, const WriteOption& option,
+                    OpenMode open_mode) {
+        // Check that we can write
+        R_UNLESS(static_cast<u32>(open_mode & OpenMode::Write) != 0, ResultWriteNotPermitted);
+
+        // Get the file size
+        s64 file_size = 0;
+        R_TRY(this->DoGetSize(&file_size));
+
+        // Determine if we need to append
+        *out_append = false;
+        if (file_size < offset + static_cast<s64>(size)) {
+            R_UNLESS(static_cast<u32>(open_mode & OpenMode::AllowAppend) != 0,
+                     ResultFileExtensionWithoutOpenModeAllowAppend);
+            *out_append = true;
+        }
+
+        R_SUCCEED();
+    }
+
+private:
+    Result DoRead(size_t* out, s64 offset, void* buffer, size_t size, const ReadOption& option) {
+        std::vector<u8> output = backend->ReadBytes(size, offset);
+        *out = output.size();
+        buffer = output.data();
+        R_SUCCEED();
+    }
+
+    Result DoGetSize(s64* out) {
+        *out = backend->GetSize();
+        R_SUCCEED();
+    }
+
+    Result DoFlush() {
+        // Exists for SDK compatibiltity -- No need to flush file.
+        R_SUCCEED();
+    }
+
+    Result DoWrite(s64 offset, const void* buffer, size_t size, const WriteOption& option) {
+        const std::size_t written = backend->Write(static_cast<const u8*>(buffer), size, offset);
+
+        ASSERT_MSG(written == size,
+                   "Could not write all bytes to file (requested={:016X}, actual={:016X}).", size,
+                   written);
+
+        R_SUCCEED();
+    }
+
+    Result DoSetSize(s64 size) {
+        backend->Resize(size);
+        R_SUCCEED();
+    }
+
+    Result DoOperateRange(void* dst, size_t dst_size, OperationId op_id, s64 offset, s64 size,
+                          const void* src, size_t src_size) {
+        R_THROW(ResultNotImplemented);
+    }
+
+    VirtualFile backend;
+};
+
+} // namespace FileSys::Fsa
diff --git a/src/core/file_sys/fsa/fs_i_filesystem.h b/src/core/file_sys/fsa/fs_i_filesystem.h
new file mode 100644
index 000000000..e92284459
--- /dev/null
+++ b/src/core/file_sys/fsa/fs_i_filesystem.h
@@ -0,0 +1,207 @@
+// SPDX-FileCopyrightText: Copyright 2024 yuzu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+#include "core/file_sys/errors.h"
+#include "core/file_sys/fs_filesystem.h"
+#include "core/file_sys/fs_path.h"
+#include "core/file_sys/vfs/vfs_types.h"
+#include "core/hle/result.h"
+#include "core/hle/service/filesystem/filesystem.h"
+
+namespace FileSys::Fsa {
+
+class IFile;
+class IDirectory;
+
+enum class QueryId {
+    SetConcatenationFileAttribute = 0,
+    UpdateMac = 1,
+    IsSignedSystemPartitionOnSdCardValid = 2,
+    QueryUnpreparedFileInformation = 3,
+};
+
+class IFileSystem {
+public:
+    IFileSystem(VirtualDir backend_) : backend{std::move(backend_)} {}
+    virtual ~IFileSystem() {}
+
+    Result CreateFile(const Path& path, s64 size, CreateOption option) {
+        R_UNLESS(size >= 0, ResultOutOfRange);
+        R_RETURN(this->DoCreateFile(path, size, static_cast<int>(option)));
+    }
+
+    Result CreateFile(const Path& path, s64 size) {
+        R_RETURN(this->CreateFile(path, size, CreateOption::None));
+    }
+
+    Result DeleteFile(const Path& path) {
+        R_RETURN(this->DoDeleteFile(path));
+    }
+
+    Result CreateDirectory(const Path& path) {
+        R_RETURN(this->DoCreateDirectory(path));
+    }
+
+    Result DeleteDirectory(const Path& path) {
+        R_RETURN(this->DoDeleteDirectory(path));
+    }
+
+    Result DeleteDirectoryRecursively(const Path& path) {
+        R_RETURN(this->DoDeleteDirectoryRecursively(path));
+    }
+
+    Result RenameFile(const Path& old_path, const Path& new_path) {
+        R_RETURN(this->DoRenameFile(old_path, new_path));
+    }
+
+    Result RenameDirectory(const Path& old_path, const Path& new_path) {
+        R_RETURN(this->DoRenameDirectory(old_path, new_path));
+    }
+
+    Result GetEntryType(DirectoryEntryType* out, const Path& path) {
+        R_RETURN(this->DoGetEntryType(out, path));
+    }
+
+    Result OpenFile(VirtualFile* out_file, const Path& path, OpenMode mode) {
+        R_UNLESS(out_file != nullptr, ResultNullptrArgument);
+        R_UNLESS(static_cast<u32>(mode & OpenMode::ReadWrite) != 0, ResultInvalidOpenMode);
+        R_UNLESS(static_cast<u32>(mode & ~OpenMode::All) == 0, ResultInvalidOpenMode);
+        R_RETURN(this->DoOpenFile(out_file, path, mode));
+    }
+
+    Result OpenDirectory(VirtualDir* out_dir, const Path& path, OpenDirectoryMode mode) {
+        R_UNLESS(out_dir != nullptr, ResultNullptrArgument);
+        R_UNLESS(static_cast<u64>(mode & OpenDirectoryMode::All) != 0, ResultInvalidOpenMode);
+        R_UNLESS(static_cast<u64>(
+                     mode & ~(OpenDirectoryMode::All | OpenDirectoryMode::NotRequireFileSize)) == 0,
+                 ResultInvalidOpenMode);
+        R_RETURN(this->DoOpenDirectory(out_dir, path, mode));
+    }
+
+    Result Commit() {
+        R_RETURN(this->DoCommit());
+    }
+
+    Result GetFreeSpaceSize(s64* out, const Path& path) {
+        R_UNLESS(out != nullptr, ResultNullptrArgument);
+        R_RETURN(this->DoGetFreeSpaceSize(out, path));
+    }
+
+    Result GetTotalSpaceSize(s64* out, const Path& path) {
+        R_UNLESS(out != nullptr, ResultNullptrArgument);
+        R_RETURN(this->DoGetTotalSpaceSize(out, path));
+    }
+
+    Result CleanDirectoryRecursively(const Path& path) {
+        R_RETURN(this->DoCleanDirectoryRecursively(path));
+    }
+
+    Result GetFileTimeStampRaw(FileTimeStampRaw* out, const Path& path) {
+        R_UNLESS(out != nullptr, ResultNullptrArgument);
+        R_RETURN(this->DoGetFileTimeStampRaw(out, path));
+    }
+
+    Result QueryEntry(char* dst, size_t dst_size, const char* src, size_t src_size, QueryId query,
+                      const Path& path) {
+        R_RETURN(this->DoQueryEntry(dst, dst_size, src, src_size, query, path));
+    }
+
+    // These aren't accessible as commands
+    Result CommitProvisionally(s64 counter) {
+        R_RETURN(this->DoCommitProvisionally(counter));
+    }
+
+    Result Rollback() {
+        R_RETURN(this->DoRollback());
+    }
+
+    Result Flush() {
+        R_RETURN(this->DoFlush());
+    }
+
+private:
+    Result DoCreateFile(const Path& path, s64 size, int flags) {
+        R_RETURN(backend.CreateFile(path.GetString(), size));
+    }
+
+    Result DoDeleteFile(const Path& path) {
+        R_RETURN(backend.DeleteFile(path.GetString()));
+    }
+
+    Result DoCreateDirectory(const Path& path) {
+        R_RETURN(backend.CreateDirectory(path.GetString()));
+    }
+
+    Result DoDeleteDirectory(const Path& path) {
+        R_RETURN(backend.DeleteDirectory(path.GetString()));
+    }
+
+    Result DoDeleteDirectoryRecursively(const Path& path) {
+        R_RETURN(backend.DeleteDirectoryRecursively(path.GetString()));
+    }
+
+    Result DoRenameFile(const Path& old_path, const Path& new_path) {
+        R_RETURN(backend.RenameFile(old_path.GetString(), new_path.GetString()));
+    }
+
+    Result DoRenameDirectory(const Path& old_path, const Path& new_path) {
+        R_RETURN(backend.RenameDirectory(old_path.GetString(), new_path.GetString()));
+    }
+
+    Result DoGetEntryType(DirectoryEntryType* out, const Path& path) {
+        R_RETURN(backend.GetEntryType(out, path.GetString()));
+    }
+
+    Result DoOpenFile(VirtualFile* out_file, const Path& path, OpenMode mode) {
+        R_RETURN(backend.OpenFile(out_file, path.GetString(), mode));
+    }
+
+    Result DoOpenDirectory(VirtualDir* out_directory, const Path& path,
+                           OpenDirectoryMode mode) {
+        R_RETURN(backend.OpenDirectory(out_directory, path.GetString()));
+    }
+
+    Result DoCommit() {
+        R_THROW(ResultNotImplemented);
+    }
+
+    Result DoGetFreeSpaceSize(s64* out, const Path& path) {
+        R_THROW(ResultNotImplemented);
+    }
+
+    Result DoGetTotalSpaceSize(s64* out, const Path& path) {
+        R_THROW(ResultNotImplemented);
+    }
+
+    Result DoCleanDirectoryRecursively(const Path& path) {
+        R_RETURN(backend.CleanDirectoryRecursively(path.GetString()));
+    }
+
+    Result DoGetFileTimeStampRaw(FileTimeStampRaw* out, const Path& path) {
+        R_RETURN(backend.GetFileTimeStampRaw(out, path.GetString()));
+    }
+
+    Result DoQueryEntry(char* dst, size_t dst_size, const char* src, size_t src_size, QueryId query,
+                        const Path& path) {
+        R_THROW(ResultNotImplemented);
+    }
+
+    // These aren't accessible as commands
+    Result DoCommitProvisionally(s64 counter) {
+        R_THROW(ResultNotImplemented);
+    }
+
+    Result DoRollback() {
+        R_THROW(ResultNotImplemented);
+    }
+
+    Result DoFlush() {
+        R_THROW(ResultNotImplemented);
+    }
+
+    Service::FileSystem::VfsDirectoryServiceWrapper backend;
+};
+
+} // namespace FileSys::Fsa
diff --git a/src/core/hle/service/filesystem/fsp/fs_i_directory.cpp b/src/core/hle/service/filesystem/fsp/fs_i_directory.cpp
index 39690018b..661da5326 100644
--- a/src/core/hle/service/filesystem/fsp/fs_i_directory.cpp
+++ b/src/core/hle/service/filesystem/fsp/fs_i_directory.cpp
@@ -8,43 +8,15 @@
 
 namespace Service::FileSystem {
 
-template <typename T>
-static void BuildEntryIndex(std::vector<FileSys::DirectoryEntry>& entries,
-                            const std::vector<T>& new_data, FileSys::DirectoryEntryType type) {
-    entries.reserve(entries.size() + new_data.size());
-
-    for (const auto& new_entry : new_data) {
-        auto name = new_entry->GetName();
-
-        if (type == FileSys::DirectoryEntryType::File &&
-            name == FileSys::GetSaveDataSizeFileName()) {
-            continue;
-        }
-
-        entries.emplace_back(name, static_cast<s8>(type),
-                             type == FileSys::DirectoryEntryType::Directory ? 0
-                                                                            : new_entry->GetSize());
-    }
-}
-
-IDirectory::IDirectory(Core::System& system_, FileSys::VirtualDir backend_,
+IDirectory::IDirectory(Core::System& system_, FileSys::VirtualDir directory_,
                        FileSys::OpenDirectoryMode mode)
-    : ServiceFramework{system_, "IDirectory"}, backend(std::move(backend_)) {
+    : ServiceFramework{system_, "IDirectory"},
+      backend(std::make_unique<FileSys::Fsa::IDirectory>(directory_, mode)) {
     static const FunctionInfo functions[] = {
         {0, &IDirectory::Read, "Read"},
         {1, &IDirectory::GetEntryCount, "GetEntryCount"},
     };
     RegisterHandlers(functions);
-
-    // TODO(DarkLordZach): Verify that this is the correct behavior.
-    // Build entry index now to save time later.
-    if (True(mode & FileSys::OpenDirectoryMode::Directory)) {
-        BuildEntryIndex(entries, backend->GetSubdirectories(),
-                        FileSys::DirectoryEntryType::Directory);
-    }
-    if (True(mode & FileSys::OpenDirectoryMode::File)) {
-        BuildEntryIndex(entries, backend->GetFiles(), FileSys::DirectoryEntryType::File);
-    }
 }
 
 void IDirectory::Read(HLERequestContext& ctx) {
@@ -53,32 +25,26 @@ void IDirectory::Read(HLERequestContext& ctx) {
     // Calculate how many entries we can fit in the output buffer
     const u64 count_entries = ctx.GetWriteBufferNumElements<FileSys::DirectoryEntry>();
 
-    // Cap at total number of entries.
-    const u64 actual_entries = std::min(count_entries, entries.size() - next_entry_index);
-
-    // Determine data start and end
-    const auto* begin = reinterpret_cast<u8*>(entries.data() + next_entry_index);
-    const auto* end = reinterpret_cast<u8*>(entries.data() + next_entry_index + actual_entries);
-    const auto range_size = static_cast<std::size_t>(std::distance(begin, end));
-
-    next_entry_index += actual_entries;
+    s64 out_count{};
+    FileSys::DirectoryEntry* out_entries = nullptr;
+    const auto result = backend->Read(&out_count, out_entries, count_entries);
 
     // Write the data to memory
-    ctx.WriteBuffer(begin, range_size);
+    ctx.WriteBuffer(out_entries, out_count);
 
     IPC::ResponseBuilder rb{ctx, 4};
-    rb.Push(ResultSuccess);
-    rb.Push(actual_entries);
+    rb.Push(result);
+    rb.Push(out_count);
 }
 
 void IDirectory::GetEntryCount(HLERequestContext& ctx) {
     LOG_DEBUG(Service_FS, "called");
 
-    u64 count = entries.size() - next_entry_index;
+    s64 out_count{};
 
     IPC::ResponseBuilder rb{ctx, 4};
-    rb.Push(ResultSuccess);
-    rb.Push(count);
+    rb.Push(backend->GetEntryCount(&out_count));
+    rb.Push(out_count);
 }
 
 } // namespace Service::FileSystem
diff --git a/src/core/hle/service/filesystem/fsp/fs_i_directory.h b/src/core/hle/service/filesystem/fsp/fs_i_directory.h
index 793ecfcd7..0dec4367b 100644
--- a/src/core/hle/service/filesystem/fsp/fs_i_directory.h
+++ b/src/core/hle/service/filesystem/fsp/fs_i_directory.h
@@ -3,6 +3,7 @@
 
 #pragma once
 
+#include "core/file_sys/fsa/fs_i_directory.h"
 #include "core/file_sys/vfs/vfs.h"
 #include "core/hle/service/filesystem/filesystem.h"
 #include "core/hle/service/service.h"
@@ -15,13 +16,11 @@ namespace Service::FileSystem {
 
 class IDirectory final : public ServiceFramework<IDirectory> {
 public:
-    explicit IDirectory(Core::System& system_, FileSys::VirtualDir backend_,
+    explicit IDirectory(Core::System& system_, FileSys::VirtualDir directory_,
                         FileSys::OpenDirectoryMode mode);
 
 private:
-    FileSys::VirtualDir backend;
-    std::vector<FileSys::DirectoryEntry> entries;
-    u64 next_entry_index = 0;
+    std::unique_ptr<FileSys::Fsa::IDirectory> backend;
 
     void Read(HLERequestContext& ctx);
     void GetEntryCount(HLERequestContext& ctx);
diff --git a/src/core/hle/service/filesystem/fsp/fs_i_file.cpp b/src/core/hle/service/filesystem/fsp/fs_i_file.cpp
index 9a18f6ec5..8fb8620de 100644
--- a/src/core/hle/service/filesystem/fsp/fs_i_file.cpp
+++ b/src/core/hle/service/filesystem/fsp/fs_i_file.cpp
@@ -7,8 +7,8 @@
 
 namespace Service::FileSystem {
 
-IFile::IFile(Core::System& system_, FileSys::VirtualFile backend_)
-    : ServiceFramework{system_, "IFile"}, backend(std::move(backend_)) {
+IFile::IFile(Core::System& system_, FileSys::VirtualFile file_)
+    : ServiceFramework{system_, "IFile"}, backend{std::make_unique<FileSys::Fsa::IFile>(file_)} {
     static const FunctionInfo functions[] = {
         {0, &IFile::Read, "Read"},
         {1, &IFile::Write, "Write"},
@@ -29,79 +29,40 @@ void IFile::Read(HLERequestContext& ctx) {
 
     LOG_DEBUG(Service_FS, "called, option={}, offset=0x{:X}, length={}", option, offset, length);
 
-    // Error checking
-    if (length < 0) {
-        LOG_ERROR(Service_FS, "Length is less than 0, length={}", length);
-        IPC::ResponseBuilder rb{ctx, 2};
-        rb.Push(FileSys::ResultInvalidSize);
-        return;
-    }
-    if (offset < 0) {
-        LOG_ERROR(Service_FS, "Offset is less than 0, offset={}", offset);
-        IPC::ResponseBuilder rb{ctx, 2};
-        rb.Push(FileSys::ResultInvalidOffset);
-        return;
-    }
-
     // Read the data from the Storage backend
-    std::vector<u8> output = backend->ReadBytes(length, offset);
+    std::vector<u8> output(length);
+    std::size_t bytes_read;
+    const auto result = backend->Read(&bytes_read, offset, output.data(), length);
 
     // Write the data to memory
     ctx.WriteBuffer(output);
 
     IPC::ResponseBuilder rb{ctx, 4};
-    rb.Push(ResultSuccess);
-    rb.Push(static_cast<u64>(output.size()));
+    rb.Push(result);
+    rb.Push(static_cast<u64>(bytes_read));
 }
 
 void IFile::Write(HLERequestContext& ctx) {
     IPC::RequestParser rp{ctx};
-    const u64 option = rp.Pop<u64>();
+    const auto option = rp.PopRaw<FileSys::WriteOption>();
+    [[maybe_unused]] const u32 unused = rp.Pop<u32>();
     const s64 offset = rp.Pop<s64>();
     const s64 length = rp.Pop<s64>();
 
-    LOG_DEBUG(Service_FS, "called, option={}, offset=0x{:X}, length={}", option, offset, length);
-
-    // Error checking
-    if (length < 0) {
-        LOG_ERROR(Service_FS, "Length is less than 0, length={}", length);
-        IPC::ResponseBuilder rb{ctx, 2};
-        rb.Push(FileSys::ResultInvalidSize);
-        return;
-    }
-    if (offset < 0) {
-        LOG_ERROR(Service_FS, "Offset is less than 0, offset={}", offset);
-        IPC::ResponseBuilder rb{ctx, 2};
-        rb.Push(FileSys::ResultInvalidOffset);
-        return;
-    }
+    LOG_DEBUG(Service_FS, "called, option={}, offset=0x{:X}, length={}", option.value, offset,
+              length);
 
     const auto data = ctx.ReadBuffer();
 
-    ASSERT_MSG(static_cast<s64>(data.size()) <= length,
-               "Attempting to write more data than requested (requested={:016X}, actual={:016X}).",
-               length, data.size());
-
-    // Write the data to the Storage backend
-    const auto write_size =
-        static_cast<std::size_t>(std::distance(data.begin(), data.begin() + length));
-    const std::size_t written = backend->Write(data.data(), write_size, offset);
-
-    ASSERT_MSG(static_cast<s64>(written) == length,
-               "Could not write all bytes to file (requested={:016X}, actual={:016X}).", length,
-               written);
-
     IPC::ResponseBuilder rb{ctx, 2};
-    rb.Push(ResultSuccess);
+    rb.Push(backend->Write(offset, data.data(), length, option));
 }
 
 void IFile::Flush(HLERequestContext& ctx) {
     LOG_DEBUG(Service_FS, "called");
 
-    // Exists for SDK compatibiltity -- No need to flush file.
-
     IPC::ResponseBuilder rb{ctx, 2};
-    rb.Push(ResultSuccess);
+    rb.Push(backend->Flush());
 }
 
 void IFile::SetSize(HLERequestContext& ctx) {
@@ -109,18 +70,17 @@ void IFile::SetSize(HLERequestContext& ctx) {
     const u64 size = rp.Pop<u64>();
     LOG_DEBUG(Service_FS, "called, size={}", size);
 
-    backend->Resize(size);
-
     IPC::ResponseBuilder rb{ctx, 2};
-    rb.Push(ResultSuccess);
+    rb.Push(backend->SetSize(size));
 }
 
 void IFile::GetSize(HLERequestContext& ctx) {
-    const u64 size = backend->GetSize();
+    s64 size;
+    const auto result = backend->GetSize(&size);
     LOG_DEBUG(Service_FS, "called, size={}", size);
 
     IPC::ResponseBuilder rb{ctx, 4};
-    rb.Push(ResultSuccess);
+    rb.Push(result);
     rb.Push<u64>(size);
 }
 
diff --git a/src/core/hle/service/filesystem/fsp/fs_i_file.h b/src/core/hle/service/filesystem/fsp/fs_i_file.h
index 5e5430c67..887fd3ba2 100644
--- a/src/core/hle/service/filesystem/fsp/fs_i_file.h
+++ b/src/core/hle/service/filesystem/fsp/fs_i_file.h
@@ -3,6 +3,7 @@
 
 #pragma once
 
+#include "core/file_sys/fsa/fs_i_file.h"
 #include "core/hle/service/filesystem/filesystem.h"
 #include "core/hle/service/service.h"
 
@@ -10,10 +11,10 @@ namespace Service::FileSystem {
 
 class IFile final : public ServiceFramework<IFile> {
 public:
-    explicit IFile(Core::System& system_, FileSys::VirtualFile backend_);
+    explicit IFile(Core::System& system_, FileSys::VirtualFile file_);
 
 private:
-    FileSys::VirtualFile backend;
+    std::unique_ptr<FileSys::Fsa::IFile> backend;
 
     void Read(HLERequestContext& ctx);
     void Write(HLERequestContext& ctx);
diff --git a/src/core/hle/service/filesystem/fsp/fs_i_filesystem.cpp b/src/core/hle/service/filesystem/fsp/fs_i_filesystem.cpp
index efa394dd1..1e69d22b8 100644
--- a/src/core/hle/service/filesystem/fsp/fs_i_filesystem.cpp
+++ b/src/core/hle/service/filesystem/fsp/fs_i_filesystem.cpp
@@ -9,9 +9,9 @@
 
 namespace Service::FileSystem {
 
-IFileSystem::IFileSystem(Core::System& system_, FileSys::VirtualDir backend_, SizeGetter size_)
-    : ServiceFramework{system_, "IFileSystem"}, backend{std::move(backend_)}, size{std::move(
-                                                                                  size_)} {
+IFileSystem::IFileSystem(Core::System& system_, FileSys::VirtualDir dir_, SizeGetter size_)
+    : ServiceFramework{system_, "IFileSystem"},
+      backend{std::make_unique<FileSys::Fsa::IFileSystem>(dir_)}, size{std::move(size_)} {
     static const FunctionInfo functions[] = {
         {0, &IFileSystem::CreateFile, "CreateFile"},
         {1, &IFileSystem::DeleteFile, "DeleteFile"},
@@ -39,6 +39,7 @@ void IFileSystem::CreateFile(HLERequestContext& ctx) {
 
     const auto file_buffer = ctx.ReadBuffer();
     const std::string name = Common::StringFromBuffer(file_buffer);
+    const auto path = FileSys::Path(name.c_str());
 
     const u64 file_mode = rp.Pop<u64>();
     const u32 file_size = rp.Pop<u32>();
@@ -47,67 +48,75 @@ void IFileSystem::CreateFile(HLERequestContext& ctx) {
               file_size);
 
     IPC::ResponseBuilder rb{ctx, 2};
-    rb.Push(backend.CreateFile(name, file_size));
+    rb.Push(backend->CreateFile(path, file_size));
 }
 
 void IFileSystem::DeleteFile(HLERequestContext& ctx) {
     const auto file_buffer = ctx.ReadBuffer();
     const std::string name = Common::StringFromBuffer(file_buffer);
+    const auto path = FileSys::Path(name.c_str());
 
     LOG_DEBUG(Service_FS, "called. file={}", name);
 
     IPC::ResponseBuilder rb{ctx, 2};
-    rb.Push(backend.DeleteFile(name));
+    rb.Push(backend->DeleteFile(path));
 }
 
 void IFileSystem::CreateDirectory(HLERequestContext& ctx) {
     const auto file_buffer = ctx.ReadBuffer();
     const std::string name = Common::StringFromBuffer(file_buffer);
+    const auto path = FileSys::Path(name.c_str());
 
     LOG_DEBUG(Service_FS, "called. directory={}", name);
 
     IPC::ResponseBuilder rb{ctx, 2};
-    rb.Push(backend.CreateDirectory(name));
+    rb.Push(backend->CreateDirectory(path));
 }
 
 void IFileSystem::DeleteDirectory(HLERequestContext& ctx) {
     const auto file_buffer = ctx.ReadBuffer();
     const std::string name = Common::StringFromBuffer(file_buffer);
+    const auto path = FileSys::Path(name.c_str());
 
     LOG_DEBUG(Service_FS, "called. directory={}", name);
 
     IPC::ResponseBuilder rb{ctx, 2};
-    rb.Push(backend.DeleteDirectory(name));
+    rb.Push(backend->DeleteDirectory(path));
 }
 
 void IFileSystem::DeleteDirectoryRecursively(HLERequestContext& ctx) {
     const auto file_buffer = ctx.ReadBuffer();
     const std::string name = Common::StringFromBuffer(file_buffer);
+    const auto path = FileSys::Path(name.c_str());
 
     LOG_DEBUG(Service_FS, "called. directory={}", name);
 
     IPC::ResponseBuilder rb{ctx, 2};
-    rb.Push(backend.DeleteDirectoryRecursively(name));
+    rb.Push(backend->DeleteDirectoryRecursively(path));
 }
 
 void IFileSystem::CleanDirectoryRecursively(HLERequestContext& ctx) {
     const auto file_buffer = ctx.ReadBuffer();
     const std::string name = Common::StringFromBuffer(file_buffer);
+    const auto path = FileSys::Path(name.c_str());
 
     LOG_DEBUG(Service_FS, "called. Directory: {}", name);
 
     IPC::ResponseBuilder rb{ctx, 2};
-    rb.Push(backend.CleanDirectoryRecursively(name));
+    rb.Push(backend->CleanDirectoryRecursively(path));
 }
 
 void IFileSystem::RenameFile(HLERequestContext& ctx) {
     const std::string src_name = Common::StringFromBuffer(ctx.ReadBuffer(0));
     const std::string dst_name = Common::StringFromBuffer(ctx.ReadBuffer(1));
 
+    const auto src_path = FileSys::Path(src_name.c_str());
+    const auto dst_path = FileSys::Path(dst_name.c_str());
+
     LOG_DEBUG(Service_FS, "called. file '{}' to file '{}'", src_name, dst_name);
 
     IPC::ResponseBuilder rb{ctx, 2};
-    rb.Push(backend.RenameFile(src_name, dst_name));
+    rb.Push(backend->RenameFile(src_path, dst_path));
 }
 
 void IFileSystem::OpenFile(HLERequestContext& ctx) {
@@ -115,13 +124,14 @@ void IFileSystem::OpenFile(HLERequestContext& ctx) {
 
     const auto file_buffer = ctx.ReadBuffer();
     const std::string name = Common::StringFromBuffer(file_buffer);
+    const auto path = FileSys::Path(name.c_str());
 
     const auto mode = static_cast<FileSys::OpenMode>(rp.Pop<u32>());
 
     LOG_DEBUG(Service_FS, "called. file={}, mode={}", name, mode);
 
     FileSys::VirtualFile vfs_file{};
-    auto result = backend.OpenFile(&vfs_file, name, mode);
+    auto result = backend->OpenFile(&vfs_file, path, mode);
     if (result != ResultSuccess) {
         IPC::ResponseBuilder rb{ctx, 2};
         rb.Push(result);
@@ -140,12 +150,13 @@ void IFileSystem::OpenDirectory(HLERequestContext& ctx) {
 
     const auto file_buffer = ctx.ReadBuffer();
     const std::string name = Common::StringFromBuffer(file_buffer);
+    const auto path = FileSys::Path(name.c_str());
     const auto mode = rp.PopRaw<FileSys::OpenDirectoryMode>();
 
     LOG_DEBUG(Service_FS, "called. directory={}, mode={}", name, mode);
 
     FileSys::VirtualDir vfs_dir{};
-    auto result = backend.OpenDirectory(&vfs_dir, name);
+    auto result = backend->OpenDirectory(&vfs_dir, path, mode);
     if (result != ResultSuccess) {
         IPC::ResponseBuilder rb{ctx, 2};
         rb.Push(result);
@@ -162,11 +173,12 @@ void IFileSystem::OpenDirectory(HLERequestContext& ctx) {
 void IFileSystem::GetEntryType(HLERequestContext& ctx) {
     const auto file_buffer = ctx.ReadBuffer();
     const std::string name = Common::StringFromBuffer(file_buffer);
+    const auto path = FileSys::Path(name.c_str());
 
     LOG_DEBUG(Service_FS, "called. file={}", name);
 
     FileSys::DirectoryEntryType vfs_entry_type{};
-    auto result = backend.GetEntryType(&vfs_entry_type, name);
+    auto result = backend->GetEntryType(&vfs_entry_type, path);
     if (result != ResultSuccess) {
         IPC::ResponseBuilder rb{ctx, 2};
         rb.Push(result);
@@ -204,11 +216,12 @@ void IFileSystem::GetTotalSpaceSize(HLERequestContext& ctx) {
 void IFileSystem::GetFileTimeStampRaw(HLERequestContext& ctx) {
     const auto file_buffer = ctx.ReadBuffer();
     const std::string name = Common::StringFromBuffer(file_buffer);
+    const auto path = FileSys::Path(name.c_str());
 
     LOG_WARNING(Service_FS, "(Partial Implementation) called. file={}", name);
 
     FileSys::FileTimeStampRaw vfs_timestamp{};
-    auto result = backend.GetFileTimeStampRaw(&vfs_timestamp, name);
+    auto result = backend->GetFileTimeStampRaw(&vfs_timestamp, path);
     if (result != ResultSuccess) {
         IPC::ResponseBuilder rb{ctx, 2};
         rb.Push(result);
diff --git a/src/core/hle/service/filesystem/fsp/fs_i_filesystem.h b/src/core/hle/service/filesystem/fsp/fs_i_filesystem.h
index b06b3ef0e..d500be725 100644
--- a/src/core/hle/service/filesystem/fsp/fs_i_filesystem.h
+++ b/src/core/hle/service/filesystem/fsp/fs_i_filesystem.h
@@ -3,6 +3,7 @@
 
 #pragma once
 
+#include "core/file_sys/fsa/fs_i_filesystem.h"
 #include "core/file_sys/vfs/vfs.h"
 #include "core/hle/service/filesystem/filesystem.h"
 #include "core/hle/service/filesystem/fsp/fsp_util.h"
@@ -12,7 +13,7 @@ namespace Service::FileSystem {
 
 class IFileSystem final : public ServiceFramework<IFileSystem> {
 public:
-    explicit IFileSystem(Core::System& system_, FileSys::VirtualDir backend_, SizeGetter size_);
+    explicit IFileSystem(Core::System& system_, FileSys::VirtualDir dir_, SizeGetter size_);
 
     void CreateFile(HLERequestContext& ctx);
     void DeleteFile(HLERequestContext& ctx);
@@ -31,7 +32,7 @@ public:
     void GetFileSystemAttribute(HLERequestContext& ctx);
 
 private:
-    VfsDirectoryServiceWrapper backend;
+    std::unique_ptr<FileSys::Fsa::IFileSystem> backend;
     SizeGetter size;
 };