diff --git a/fs/fs.c b/fs/fs.c
index 8417067..3b5f284 100644
--- a/fs/fs.c
+++ b/fs/fs.c
@@ -229,6 +229,16 @@
 	files[f->no].in_use = 0;
 }
 
+static int check_fd(int fd)
+{
+	if (fd < 0 || fd >= MAX_FILES || !files[fd].in_use) {
+		errno = -EBADF;
+		return errno;
+	}
+
+	return 0;
+}
+
 static struct device_d *get_fs_device_by_path(char **path)
 {
 	struct device_d *dev;
@@ -457,6 +467,9 @@
 	struct fs_driver_d *fsdrv;
 	FILE *f = &files[fd];
 
+	if (check_fd(fd))
+		return errno;
+
 	dev = f->dev;
 
 	fsdrv = (struct fs_driver_d *)dev->driver->type_data;
@@ -474,6 +487,9 @@
 	struct fs_driver_d *fsdrv;
 	FILE *f = &files[fd];
 
+	if (check_fd(fd))
+		return errno;
+
 	dev = f->dev;
 
 	fsdrv = (struct fs_driver_d *)dev->driver->type_data;
@@ -494,6 +510,9 @@
 	struct fs_driver_d *fsdrv;
 	FILE *f = &files[fd];
 
+	if (check_fd(fd))
+		return errno;
+
 	dev = f->dev;
 
 	fsdrv = (struct fs_driver_d *)dev->driver->type_data;
@@ -524,6 +543,9 @@
 	FILE *f = &files[fildes];
 	off_t pos;
 
+	if (check_fd(fildes))
+		return -1;
+
 	errno = 0;
 
 	dev = f->dev;
@@ -567,6 +589,9 @@
 	struct fs_driver_d *fsdrv;
 	FILE *f = &files[fd];
 
+	if (check_fd(fd))
+		return errno;
+
 	dev = f->dev;
 
 	fsdrv = (struct fs_driver_d *)dev->driver->type_data;
@@ -589,6 +614,9 @@
 	struct fs_driver_d *fsdrv;
 	FILE *f = &files[fd];
 
+	if (check_fd(fd))
+		return errno;
+
 	dev = f->dev;
 
 	fsdrv = (struct fs_driver_d *)dev->driver->type_data;
@@ -627,6 +655,9 @@
 	FILE *f = &files[fd];
 	void *ret = (void *)-1;
 
+	if (check_fd(fd))
+		return ret;
+
 	dev = f->dev;
 
 	fsdrv = (struct fs_driver_d *)dev->driver->type_data;
@@ -646,6 +677,9 @@
 	struct fs_driver_d *fsdrv;
 	FILE *f = &files[fd];
 
+	if (check_fd(fd))
+		return errno;
+
 	dev = f->dev;
 
 	fsdrv = (struct fs_driver_d *)dev->driver->type_data;