Kernel/SysFS: Add Symbolic link functionality to the filesystem

This will be used later on to help connecting a node at /sys/dev/block/
that represents a Storage device to a directory in /sys/devices/storage/
with details on that device in that directory.

Kernel/FileSystem/SysFS.cpp

@@ -131,6 +131,31 @@ ErrorOr<void> SysFSInode::truncate(u64 size)
     return m_associated_component->truncate(size);
 }
 
+ErrorOr<NonnullRefPtr<SysFSLinkInode>> SysFSLinkInode::try_create(SysFS const& sysfs, SysFSComponent const& component)
+{
+    return adopt_nonnull_ref_or_enomem(new (nothrow) SysFSLinkInode(sysfs, component));
+}
+
+SysFSLinkInode::SysFSLinkInode(SysFS const& fs, SysFSComponent const& component)
+    : SysFSInode(fs, component)
+{
+}
+
+SysFSLinkInode::~SysFSLinkInode() = default;
+
+InodeMetadata SysFSLinkInode::metadata() const
+{
+    // NOTE: No locking required as m_associated_component or its component index will never change during our lifetime.
+    InodeMetadata metadata;
+    metadata.inode = { fsid(), m_associated_component->component_index() };
+    metadata.mode = S_IFLNK | S_IRUSR | S_IRGRP | S_IROTH | S_IXOTH;
+    metadata.uid = 0;
+    metadata.gid = 0;
+    metadata.size = 0;
+    metadata.mtime = TimeManagement::boot_time();
+    return metadata;
+}
+
 ErrorOr<NonnullRefPtr<SysFSDirectoryInode>> SysFSDirectoryInode::try_create(SysFS const& sysfs, SysFSComponent const& component)
 {
     return adopt_nonnull_ref_or_enomem(new (nothrow) SysFSDirectoryInode(sysfs, component));

Kernel/FileSystem/SysFS.h

@@ -62,6 +62,19 @@ protected:
     NonnullRefPtr<SysFSComponent> m_associated_component;
 };
 
+class SysFSLinkInode : public SysFSInode {
+    friend class SysFS;
+
+public:
+    static ErrorOr<NonnullRefPtr<SysFSLinkInode>> try_create(SysFS const&, SysFSComponent const&);
+    virtual ~SysFSLinkInode() override;
+
+protected:
+    SysFSLinkInode(SysFS const&, SysFSComponent const&);
+    // ^Inode
+    virtual InodeMetadata metadata() const override;
+};
+
 class SysFSDirectoryInode : public SysFSInode {
     friend class SysFS;
 

Kernel/FileSystem/SysFS/Component.cpp

@@ -57,6 +57,53 @@ mode_t SysFSComponent::permissions() const
     return S_IRUSR | S_IRGRP | S_IROTH;
 }
 
+ErrorOr<size_t> SysFSSymbolicLink::read_bytes(off_t offset, size_t count, UserOrKernelBuffer& buffer, OpenFileDescription*) const
+{
+    auto blob = TRY(try_to_generate_buffer());
+
+    if ((size_t)offset >= blob->size())
+        return 0;
+
+    ssize_t nread = min(static_cast<off_t>(blob->size() - offset), static_cast<off_t>(count));
+    TRY(buffer.write(blob->data() + offset, nread));
+    return nread;
+}
+
+ErrorOr<NonnullOwnPtr<KBuffer>> SysFSSymbolicLink::try_to_generate_buffer() const
+{
+    auto return_path_to_mount_point = TRY(try_generate_return_path_to_mount_point());
+    if (!m_pointed_component)
+        return Error::from_errno(EIO);
+    auto pointed_component_base_name = MUST(KString::try_create(m_pointed_component->name()));
+    auto pointed_component_relative_path = MUST(m_pointed_component->relative_path(move(pointed_component_base_name), 0));
+    auto full_return_and_target_path = TRY(KString::formatted("{}{}", return_path_to_mount_point->view(), pointed_component_relative_path->view()));
+    return KBuffer::try_create_with_bytes("SysFSSymbolicLink"sv, full_return_and_target_path->view().bytes());
+}
+
+static ErrorOr<NonnullOwnPtr<KString>> generate_return_path_to_mount_point(NonnullOwnPtr<KString> current_path, size_t remaining_hop)
+{
+    if (remaining_hop == 0)
+        return current_path;
+    auto new_path = TRY(KString::formatted("../{}"sv, current_path->view()));
+    remaining_hop--;
+    return generate_return_path_to_mount_point(move(new_path), remaining_hop);
+}
+
+ErrorOr<NonnullOwnPtr<KString>> SysFSSymbolicLink::try_generate_return_path_to_mount_point() const
+{
+    auto hops_from_mountpoint = TRY(relative_path_hops_count_from_mountpoint());
+    if (hops_from_mountpoint == 0)
+        return KString::try_create("./"sv);
+    auto start_return_path = TRY(KString::try_create("./"sv));
+    return generate_return_path_to_mount_point(move(start_return_path), hops_from_mountpoint);
+}
+
+SysFSSymbolicLink::SysFSSymbolicLink(SysFSDirectory const& parent_directory, SysFSComponent const& pointed_component)
+    : SysFSComponent(parent_directory)
+    , m_pointed_component(pointed_component)
+{
+}
+
 ErrorOr<void> SysFSDirectory::traverse_as_directory(FileSystemID fsid, Function<ErrorOr<void>(FileSystem::DirectoryEntryView const&)> callback) const
 {
     MutexLocker locker(SysFSComponentRegistry::the().get_lock());
@@ -91,6 +138,11 @@ ErrorOr<NonnullRefPtr<SysFSInode>> SysFSDirectory::to_inode(SysFS const& sysfs_i
     return TRY(SysFSDirectoryInode::try_create(sysfs_instance, *this));
 }
 
+ErrorOr<NonnullRefPtr<SysFSInode>> SysFSSymbolicLink::to_inode(SysFS const& sysfs_instance) const
+{
+    return TRY(SysFSLinkInode::try_create(sysfs_instance, *this));
+}
+
 ErrorOr<NonnullRefPtr<SysFSInode>> SysFSComponent::to_inode(SysFS const& sysfs_instance) const
 {
     return SysFSInode::try_create(sysfs_instance, *this);

Kernel/FileSystem/SysFS/Component.h

@@ -56,6 +56,20 @@ private:
     InodeIndex m_component_index {};
 };
 
+class SysFSSymbolicLink : public SysFSComponent {
+public:
+    virtual ErrorOr<size_t> read_bytes(off_t, size_t, UserOrKernelBuffer&, OpenFileDescription*) const override final;
+    virtual ErrorOr<NonnullRefPtr<SysFSInode>> to_inode(SysFS const& sysfs_instance) const override final;
+
+protected:
+    ErrorOr<NonnullOwnPtr<KString>> try_generate_return_path_to_mount_point() const;
+    ErrorOr<NonnullOwnPtr<KBuffer>> try_to_generate_buffer() const;
+
+    explicit SysFSSymbolicLink(SysFSDirectory const& parent_directory, SysFSComponent const& pointed_component);
+
+    RefPtr<SysFSComponent> m_pointed_component;
+};
+
 class SysFSDirectory : public SysFSComponent {
 public:
     virtual ErrorOr<void> traverse_as_directory(FileSystemID, Function<ErrorOr<void>(FileSystem::DirectoryEntryView const&)>) const override;