diff --git a/src/sfs.rs b/src/sfs.rs
index 41bdcf4..92c25be 100644
--- a/src/sfs.rs
+++ b/src/sfs.rs
@@ -6,7 +6,7 @@ use core::fmt::{Debug, Formatter, Error};
 use core::any::Any;
 use dirty::Dirty;
 use structs::*;
-use vfs::{self, Device, INode, FileSystem};
+use vfs::{self, Device, INode, FileSystem, FsError};
 use util::*;
 use spin::{Mutex, RwLock};
 
@@ -249,11 +249,15 @@ impl INodeImpl {
 
 impl vfs::INode for INodeImpl {
     fn read_at(&self, offset: usize, buf: &mut [u8]) -> vfs::Result<usize> {
-        assert_eq!(self.disk_inode.read().type_, FileType::File, "read_at is only available on file");
+        if(self.disk_inode.read().type_!=FileType::File){
+            return Err(FsError::NotFile);
+        }
         self._read_at(offset, buf)
     }
     fn write_at(&self, offset: usize, buf: &[u8]) -> vfs::Result<usize> {
-        assert_eq!(self.disk_inode.read().type_, FileType::File, "write_at is only available on file");
+        if(self.disk_inode.read().type_!=FileType::File){
+            return Err(FsError::NotFile);
+        }
         self._write_at(offset, buf)
     }
     /// the size returned here is logical size(entry num for directory), not the disk space used.
@@ -280,16 +284,22 @@ impl vfs::INode for INodeImpl {
         Ok(())
     }
     fn resize(&self, len: usize) -> vfs::Result<()> {
-        assert_eq!(self.disk_inode.read().type_, FileType::File, "resize is only available on file");
+        if(self.disk_inode.read().type_!=FileType::File){
+            return Err(FsError::NotFile);
+        }
         self._resize(len)
     }
     fn create(&self, name: &str, type_: vfs::FileType) -> vfs::Result<Arc<vfs::INode>> {
         let info = self.info()?;
-        assert_eq!(info.type_, vfs::FileType::Dir);
+        if(info.type_!=vfs::FileType::Dir){
+            return Err(FsError::NotDir);
+        }
         assert!(info.nlinks > 0);
 
         // Ensure the name is not exist
-        assert!(self.get_file_inode_id(name).is_none(), "file name exist");
+        if(!self.get_file_inode_id(name).is_none()){
+            return Err(FsError::EntryExist);
+        }
 
         // Create new INode
         let inode = match type_ {
@@ -314,12 +324,14 @@ impl vfs::INode for INodeImpl {
         Ok(inode)
     }
     fn unlink(&self, name: &str) -> vfs::Result<()> {
+        let info = self.info()?;
+        if(info.type_!=vfs::FileType::Dir){
+            return Err(FsError::NotDir)
+        }
         assert!(name != ".");
         assert!(name != "..");
-        let info = self.info()?;
-        assert_eq!(info.type_, vfs::FileType::Dir);
 
-        let (inode_id, entry_id) = self.get_file_inode_and_entry_id(name).ok_or(())?;
+        let (inode_id, entry_id) = self.get_file_inode_and_entry_id(name).ok_or(FsError::EntryNotFound)?;
         let inode = self.fs.get_inode(inode_id);
 
         let type_ = inode.disk_inode.read().type_;
@@ -338,12 +350,20 @@ impl vfs::INode for INodeImpl {
     }
     fn link(&self, name: &str, other: &Arc<INode>) -> vfs::Result<()> {
         let info = self.info()?;
-        assert_eq!(info.type_, vfs::FileType::Dir);
+        if(info.type_!=vfs::FileType::Dir){
+            return Err(FsError::NotDir)
+        }
         assert!(info.nlinks > 0);
-        assert!(self.get_file_inode_id(name).is_none(), "file name exist");
-        let child = other.downcast_ref::<INodeImpl>().unwrap();
-        assert!(Arc::ptr_eq(&self.fs, &child.fs));
-        assert!(child.info()?.type_ != vfs::FileType::Dir);
+        if(!self.get_file_inode_id(name).is_none()){
+            return Err(FsError::EntryExist);
+        }
+        let child = other.downcast_ref::<INodeImpl>().ok_or(FsError::NotSameFs)?;
+        if(!Arc::ptr_eq(&self.fs, &child.fs)){
+            return Err(FsError::NotSameFs);
+        }
+        if(child.info()?.type_ == vfs::FileType::Dir){
+            return Err(FsError::IsDir);
+        }
         let entry = DiskEntry {
             id: child.id as u32,
             name: Str256::from(name),
@@ -356,12 +376,16 @@ impl vfs::INode for INodeImpl {
     }
     fn rename(&self, old_name: &str, new_name: &str) -> vfs::Result<()> {
         let info = self.info()?;
-        assert_eq!(info.type_, vfs::FileType::Dir);
+        if(info.type_!=vfs::FileType::Dir){
+            return Err(FsError::NotDir)
+        }
         assert!(info.nlinks > 0);
 
-        assert!(self.get_file_inode_id(new_name).is_none(), "file name exist");
+        if(!self.get_file_inode_id(new_name).is_none()){
+            return Err(FsError::EntryExist);
+        }
 
-        let (_, entry_id) = self.get_file_inode_and_entry_id(old_name).ok_or(())?;
+        let (_, entry_id) = self.get_file_inode_and_entry_id(old_name).ok_or(FsError::EntryNotFound)?;
 
         // in place modify name
         let mut entry: DiskEntry = unsafe { uninitialized() };
@@ -374,16 +398,24 @@ impl vfs::INode for INodeImpl {
     }
     fn move_(&self, old_name: &str, target: &Arc<INode>, new_name: &str) -> vfs::Result<()> {
         let info = self.info()?;
-        assert_eq!(info.type_, vfs::FileType::Dir);
+        if(info.type_!=vfs::FileType::Dir){
+            return Err(FsError::NotDir)
+        }
         assert!(info.nlinks > 0);
-        let dest = target.downcast_ref::<INodeImpl>().unwrap();
-        assert!(Arc::ptr_eq(&self.fs, &dest.fs));
-        assert!(dest.info()?.type_ == vfs::FileType::Dir);
+        let dest = target.downcast_ref::<INodeImpl>().ok_or(FsError::NotSameFs)?;
+        if(!Arc::ptr_eq(&self.fs, &dest.fs)){
+            return Err(FsError::NotSameFs);
+        }
+        if(dest.info()?.type_ != vfs::FileType::Dir){
+            return Err(FsError::NotDir)
+        }
         assert!(dest.info()?.nlinks > 0);
 
-        assert!(dest.get_file_inode_id(new_name).is_none(), "file name exist");
+        if(!self.get_file_inode_id(new_name).is_none()){
+            return Err(FsError::EntryExist);
+        }
 
-        let (inode_id, entry_id) = self.get_file_inode_and_entry_id(old_name).ok_or(())?;
+        let (inode_id, entry_id) = self.get_file_inode_and_entry_id(old_name).ok_or(FsError::EntryNotFound)?;
         let inode = self.fs.get_inode(inode_id);
 
         let entry = DiskEntry {
@@ -405,12 +437,16 @@ impl vfs::INode for INodeImpl {
     }
     fn find(&self, name: &str) -> vfs::Result<Arc<vfs::INode>> {
         let info = self.info()?;
-        assert_eq!(info.type_, vfs::FileType::Dir);
-        let inode_id = self.get_file_inode_id(name).ok_or(())?;
+        if(info.type_!=vfs::FileType::Dir){
+            return Err(FsError::NotDir)
+        }
+        let inode_id = self.get_file_inode_id(name).ok_or(FsError::EntryNotFound)?;
         Ok(self.fs.get_inode(inode_id))
     }
     fn get_entry(&self, id: usize) -> vfs::Result<String> {
-        assert_eq!(self.disk_inode.read().type_, FileType::Dir);
+        if(self.disk_inode.read().type_!=FileType::Dir){
+            return Err(FsError::NotDir)
+        }
         assert!(id < self.disk_inode.read().blocks as usize);
         let mut entry: DiskEntry = unsafe { uninitialized() };
         self._read_at(id as usize * BLKSIZE, entry.as_buf_mut())?;
diff --git a/src/vfs.rs b/src/vfs.rs
index d7965a5..9d3624a 100644
--- a/src/vfs.rs
+++ b/src/vfs.rs
@@ -1,6 +1,7 @@
 use alloc::{vec::Vec, string::String, sync::Arc};
 use core::fmt::Debug;
 use core::any::Any;
+use core::result;
 
 /// Interface for FS to read & write
 ///     TODO: use std::io::{Read, Write}
@@ -40,18 +41,24 @@ impl INode {
         self.as_any_ref().downcast_ref::<T>()
     }
     pub fn list(&self) -> Result<Vec<String>> {
-        let info = self.info().unwrap();
-        assert_eq!(info.type_, FileType::Dir);
+        let info = self.info()?;
+        if(info.type_ != FileType::Dir){
+            return Err(FsError::NotDir);
+        }
         Ok((0..info.size).map(|i| {
             self.get_entry(i).unwrap()
         }).collect())
     }
     pub fn lookup(&self, path: &str) -> Result<Arc<INode>> {
-        assert_eq!(self.info().unwrap().type_, FileType::Dir);
+        if(self.info()?.type_ != FileType::Dir){
+            return Err(FsError::NotDir);
+        }
         let mut result = self.find(".")?;
         let mut rest_path = path;
         while rest_path != "" {
-            assert_eq!(result.info()?.type_, FileType::Dir);
+            if(result.info()?.type_!= FileType::Dir){
+                return Err(FsError::NotDir);
+            }
             let name;
             match rest_path.find('/') {
                 None => {
@@ -64,7 +71,7 @@ impl INode {
                 }
             };
             match result.find(name) {
-                Err(_) => return Err(()),
+                Err(error) => return Err(error),
                 Ok(inode) => result = inode,
             };
         }
@@ -96,7 +103,21 @@ pub struct FsInfo {
     pub max_file_size: usize,
 }
 
-pub type Result<T> = core::result::Result<T, ()>;
+
+#[derive(Debug)]
+pub enum FsError {
+    NotSupported,//E_UNIMP
+    NotFile,//E_ISDIR
+    IsDir,//E_ISDIR, used only in link
+    NotDir,//E_NOTDIR
+    EntryNotFound,//E_NOENT
+    EntryExist,//E_EXIST
+    NotSameFs,//E_XDEV
+    //and something else
+
+}
+
+pub type Result<T> = result::Result<T,FsError>;
 
 /// Abstract filesystem
 pub trait FileSystem: Sync {