Work on POSIX-like read() and lseek() support.

VirtualFileSystem/FileHandle.cpp

@@ -11,7 +11,61 @@ FileHandle::~FileHandle()
 {
 }
 
-ByteBuffer FileHandle::read()
+bool additionWouldOverflow(FileOffset a, FileOffset b)
+{
+    ASSERT(a > 0);
+    uint64_t ua = a;
+    return (ua + b) > maxFileOffset;
+}
+
+FileOffset FileHandle::lseek(FileOffset offset, SeekType seekType)
+{
+    if (!m_vnode)
+        return -EBADF;
+
+    // FIXME: The file type should be cached on the vnode.
+    //        It's silly that we have to do a full metadata lookup here.
+    auto metadata = m_vnode->inode.metadata();
+    if (metadata.isSocket() || metadata.isFIFO())
+        return -ESPIPE;
+
+    FileOffset newOffset;
+
+    switch (seekType) {
+    case SeekType::Absolute:
+        newOffset = offset;
+        break;
+    case SeekType::RelativeToCurrent:
+        newOffset = m_currentOffset + offset;
+        if (additionWouldOverflow(m_currentOffset, offset))
+            return -EOVERFLOW;
+        if (newOffset < 0)
+            return -EINVAL;
+        break;
+    case SeekType::RelativeToEnd:
+        // FIXME: Implement!
+        notImplemented();
+        break;
+    default:
+        return -EINVAL;
+    }
+
+    m_currentOffset = newOffset;
+    return m_currentOffset;
+}
+
+ssize_t FileHandle::read(byte* buffer, size_t count)
+{
+    if (m_vnode->isCharacterDevice()) {
+        // FIXME: What should happen to m_currentOffset?
+        return m_vnode->characterDevice()->read(buffer, count);
+    }
+    ssize_t nread = m_vnode->fileSystem()->readInodeBytes(m_vnode->inode, m_currentOffset, count, buffer);
+    m_currentOffset += nread;
+    return nread;
+}
+
+ByteBuffer FileHandle::readEntireFile()
 {
     if (m_vnode->isCharacterDevice()) {
         auto buffer = ByteBuffer::createUninitialized(1024);