lib: fs: implement memory-mapped file i/o

lib/libfs/interface/write.c

@@ -0,0 +1,29 @@
+#include <errno.h>
+#include <fs/context.h>
+#include <fs/file.h>
+#include <fs/status.h>
+
+extern kern_status_t fs_msg_write(
+	xpc_context_t *xpc,
+	const xpc_endpoint_t *sender,
+	const xpc_buffer_t *data,
+	int *out_err,
+	size_t *out_nr_written,
+	void *arg)
+{
+	struct fs_context *ctx = arg;
+	struct fs_file *f = fs_context_get_file(ctx, sender->e_port);
+	if (!f) {
+		*out_err = EBADF;
+		return KERN_OK;
+	}
+
+	size_t start = fs_file_get_cursor(f);
+	enum fs_status status = fs_file_write(f, data, data->buf_len);
+	size_t end = fs_file_get_cursor(f);
+
+	*out_err = fs_status_to_errno(status);
+	*out_nr_written = end - start;
+
+	return KERN_OK;
+}