Enterprise-grade SFTP reliability improvements

- Remove all unwrap() calls from SftpAttrs::serialize() and from_metadata()
- Add extension advertisement in SSH_FXP_VERSION (10 extensions declared)
- Map std::io::ErrorKind to proper SSH_FX_* status codes (NotFound→FX_NO_SUCH_FILE etc.)
- Add restrict_absolute flag for chroot-like path confinement mode
- Add MAX_HANDLES limit (4096) to prevent handle exhaustion
- Add MAX_XFER_SIZE (1MB) and MAX_HASH_SIZE (256MB) OOM protection
- Fix test compilation errors (SftpHandler::new signature)
- Add build_status_from_io_error() helper for consistent error mapping

markbase-core/src/ssh_server/sftp_handler.rs

@@ -2,7 +2,7 @@
 // 参考OpenSSH sftp-server.c和draft-ietf-secsh-filexfer-02.txt
 
 use crate::ssh_server::packet::{SshPacket, PacketType};
-use anyhow::{Result, anyhow};
+use anyhow::{Result, anyhow, Context};
 use byteorder::{BigEndian, ReadBytesExt, WriteBytesExt};
 use log::{info, warn, debug};
 use std::path::{Path, PathBuf};
@@ -155,83 +155,89 @@ impl SftpAttrs {
     pub fn from_metadata(metadata: &fs::Metadata) -> Self {
         let mut attrs = Self::new();
         
-        // ⭐⭐⭐⭐⭐ Phase 2.2: 添加 uid/gid 字段（修复 "? 0 0" 权限显示问题）
         attrs.flags = SftpAttrFlags::SSH_FILEXFER_ATTR_SIZE
-            | SftpAttrFlags::SSH_FILEXFER_ATTR_UIDGID        // ⭐⭐⭐⭐⭐ 添加 UIDGID flag
+            | SftpAttrFlags::SSH_FILEXFER_ATTR_UIDGID
             | SftpAttrFlags::SSH_FILEXFER_ATTR_PERMISSIONS
             | SftpAttrFlags::SSH_FILEXFER_ATTR_ACMODTIME;
         
         attrs.size = Some(metadata.len());
         attrs.permissions = Some(metadata.permissions().mode());
-        
-        // ⭐⭐⭐⭐⭐ Phase 2.2: 获取 uid/gid（参考OpenSSH sftp-server.c: stat_to_attrib）
         attrs.uid = Some(metadata.uid());
         attrs.gid = Some(metadata.gid());
         
         if let Ok(atime) = metadata.accessed() {
-            attrs.atime = Some(atime.duration_since(std::time::UNIX_EPOCH).unwrap().as_secs() as u32);
+            attrs.atime = atime.duration_since(std::time::UNIX_EPOCH)
+                .ok().map(|d| d.as_secs() as u32);
         }
         
         if let Ok(mtime) = metadata.modified() {
-            attrs.mtime = Some(mtime.duration_since(std::time::UNIX_EPOCH).unwrap().as_secs() as u32);
+            attrs.mtime = mtime.duration_since(std::time::UNIX_EPOCH)
+                .ok().map(|d| d.as_secs() as u32);
         }
         
         attrs
     }
 
-    pub fn serialize(&self) -> Vec<u8> {
-        // ⭐⭐⭐⭐⭐ Phase 1.3: 添加 SSH_FXP_ATTRS 详细日志
-        debug!("Serializing SftpAttrs: flags=0x{:08x}, size={}, uid={}, gid={}, permissions=0x{:08x}, atime={}, mtime={}",
-            self.flags,
-            self.size.unwrap_or(0),
-            self.uid.unwrap_or(0),
-            self.gid.unwrap_or(0),
+    pub fn serialize(&self) -> Result<Vec<u8>> {
+        debug!("Serializing SftpAttrs: flags=0x{:08x}, size={:?}, uid={:?}, gid={:?}, permissions=0x{:08x}, atime={:?}, mtime={:?}",
+            self.flags, self.size, self.uid, self.gid,
             self.permissions.unwrap_or(0),
-            self.atime.unwrap_or(0),
-            self.mtime.unwrap_or(0)
+            self.atime, self.mtime,
         );
         
         let mut buffer = Vec::new();
         
-        buffer.write_u32::<BigEndian>(self.flags).unwrap();
+        buffer.write_u32::<BigEndian>(self.flags)
+            .with_context(|| "serialize attrs flags")?;
         
         if self.flags & SftpAttrFlags::SSH_FILEXFER_ATTR_SIZE != 0 {
             if let Some(size) = self.size {
-                buffer.write_u64::<BigEndian>(size).unwrap();
+                buffer.write_u64::<BigEndian>(size)
+                    .with_context(|| "serialize attrs size")?;
             }
         }
         
         if self.flags & SftpAttrFlags::SSH_FILEXFER_ATTR_UIDGID != 0 {
             if let (Some(uid), Some(gid)) = (self.uid, self.gid) {
-                buffer.write_u32::<BigEndian>(uid).unwrap();
-                buffer.write_u32::<BigEndian>(gid).unwrap();
+                buffer.write_u32::<BigEndian>(uid)
+                    .with_context(|| "serialize attrs uid")?;
+                buffer.write_u32::<BigEndian>(gid)
+                    .with_context(|| "serialize attrs gid")?;
             }
         }
         
         if self.flags & SftpAttrFlags::SSH_FILEXFER_ATTR_PERMISSIONS != 0 {
             if let Some(permissions) = self.permissions {
-                buffer.write_u32::<BigEndian>(permissions).unwrap();
+                buffer.write_u32::<BigEndian>(permissions)
+                    .with_context(|| "serialize attrs perms")?;
             }
         }
         
         if self.flags & SftpAttrFlags::SSH_FILEXFER_ATTR_ACMODTIME != 0 {
             if let (Some(atime), Some(mtime)) = (self.atime, self.mtime) {
-                buffer.write_u32::<BigEndian>(atime).unwrap();
-                buffer.write_u32::<BigEndian>(mtime).unwrap();
+                buffer.write_u32::<BigEndian>(atime)
+                    .with_context(|| "serialize attrs atime")?;
+                buffer.write_u32::<BigEndian>(mtime)
+                    .with_context(|| "serialize attrs mtime")?;
             }
         }
         
         if self.flags & SftpAttrFlags::SSH_FILEXFER_ATTR_EXTENDED != 0 {
-            buffer.write_u32::<BigEndian>(self.extended.len() as u32).unwrap();
+            buffer.write_u32::<BigEndian>(self.extended.len() as u32)
+                .with_context(|| "serialize attrs ext count")?;
             for (name, value) in &self.extended {
-                buffer.write_u32::<BigEndian>(name.len() as u32).unwrap();
-                buffer.write_all(name.as_bytes()).unwrap();
-                buffer.write_u32::<BigEndian>(value.len() as u32).unwrap();
-                buffer.write_all(value.as_bytes()).unwrap();
+                buffer.write_u32::<BigEndian>(name.len() as u32)
+                    .with_context(|| "serialize attrs ext name len")?;
+                buffer.write_all(name.as_bytes())
+                    .with_context(|| "serialize attrs ext name")?;
+                buffer.write_u32::<BigEndian>(value.len() as u32)
+                    .with_context(|| "serialize attrs ext value len")?;
+                buffer.write_all(value.as_bytes())
+                    .with_context(|| "serialize attrs ext value")?;
             }
         }
         
-        buffer
+        Ok(buffer)
     }
 }
 
@@ -258,9 +264,18 @@ pub struct SftpHandler {
     handles: std::collections::HashMap<u32, SftpHandle>,
     // ⭐⭐⭐⭐⭐ Phase 4: 添加 client maxpack 限制（参考OpenSSH sftp-server.c）
     maxpacket: u32,  // 来自 SSH_MSG_CHANNEL_OPEN_CONFIRMATION 的 maximum_packet_size
+    /// 限制绝对路径也在 root_dir 之下（chroot 模式）
+    restrict_absolute: bool,
 }
 
 impl SftpHandler {
+    /// 最大并发handle数（防止资源耗尽）
+    const MAX_HANDLES: usize = 4096;
+    /// 单次读写最大字节数（1MB，防止OOM）
+    const MAX_XFER_SIZE: u32 = 1_048_576;
+    /// 单次hash最大字节数（256MB，平衡安全与性能）
+    const MAX_HASH_SIZE: u64 = 268_435_456;
+
     // ⭐⭐⭐⭐⭐ Phase 4: 修改 new() 方法，接受 maxpack 参数
     pub fn new(root_dir: PathBuf, maxpacket: u32) -> Self {
         let canonical_root = root_dir.canonicalize().unwrap_or(root_dir);
@@ -268,10 +283,16 @@ impl SftpHandler {
             root_dir: canonical_root,
             next_handle_id: 0,
             handles: std::collections::HashMap::new(),
-            maxpacket,  // ⭐⭐⭐⭐⭐ Phase 4: client maxpack 限制
+            maxpacket,
+            restrict_absolute: false,  // 默认允许绝对路径
         }
     }
 
+    /// 设置 restrict_absolute 模式（chroot-like）
+    pub fn set_restrict_absolute(&mut self, restrict: bool) {
+        self.restrict_absolute = restrict;
+    }
+
     /// 处理SFTP请求（参考OpenSSH sftp-server.c: process())
     pub fn handle_request(&mut self, data: &[u8]) -> Result<Vec<u8>> {
         if data.is_empty() {
@@ -320,6 +341,21 @@ impl SftpHandler {
         let version = cursor.read_u32::<BigEndian>()?;
         info!("Client SFTP version: {}", version);
         
+        // Read any extension data client sent (SSH_FXP_INIT may contain extensions)
+        let pos = cursor.position() as usize;
+        let inner = cursor.get_ref();
+        if inner.len() > pos && (inner.len() - pos) >= 4 {
+            let ext_count = match cursor.read_u32::<BigEndian>() {
+                Ok(n) => n,
+                Err(_) => 0,
+            };
+            for i in 0..ext_count {
+                let ext_name = read_sftp_string(&mut cursor).unwrap_or_default();
+                let ext_data = read_sftp_string(&mut cursor).unwrap_or_default();
+                debug!("Client extension[{}]: {} = {}", i, ext_name, ext_data);
+            }
+        }
+        
         let response = self.build_version_response(3)?;
         Ok(response)
     }
@@ -364,6 +400,10 @@ impl SftpHandler {
         
         match file {
             Some(file) => {
+                if self.handles.len() >= Self::MAX_HANDLES {
+                    warn!("SSH_FXP_OPEN: handle limit reached ({})", Self::MAX_HANDLES);
+                    return self.build_status_response(id, SftpStatus::SSH_FX_FAILURE, "Handle limit reached");
+                }
                 let handle_id = self.next_handle_id;
                 self.next_handle_id += 1;
                 
@@ -424,10 +464,8 @@ impl SftpHandler {
             if let Some(ref mut file) = handle.file {
                 file.seek(SeekFrom::Start(offset))?;
                 
-                // ⭐⭐⭐⭐⭐ Phase 4: 限制数据大小，不超过 maxpacket - 1024（参考OpenSSH sftp-server.c）
-                // OpenSSH sftp-server.c: process_read() 中限制数据大小为 maxpacket - 1024 bytes
-                // SSH packet overhead: ~1024 bytes (SSH header + SSH_FXP_DATA header)
-                let max_data_size = self.maxpacket - 1024;
+                // ⭐⭐⭐⭐⭐ Phase 4: 限制数据大小，不超过 maxpacket - 1024 和 MAX_XFER_SIZE
+                let max_data_size = std::cmp::min(self.maxpacket.saturating_sub(1024), Self::MAX_XFER_SIZE);
                 let actual_length = std::cmp::min(length, max_data_size);
                 
                 info!("SSH_FXP_READ limited: requested={}, actual={}", length, actual_length);
@@ -442,7 +480,7 @@ impl SftpHandler {
                         self.build_data_response(id, &buffer)
                     }
                     Err(e) => {
-                        self.build_status_response(id, SftpStatus::SSH_FX_FAILURE, &format!("Read error: {}", e))
+                        self.build_status_from_io_error(id, &e)
                     }
                 }
             } else {
@@ -484,7 +522,7 @@ impl SftpHandler {
                         self.build_status_response(id, SftpStatus::SSH_FX_OK, "Write successful")
                     }
                     Err(e) => {
-                        self.build_status_response(id, SftpStatus::SSH_FX_FAILURE, &format!("Write error: {}", e))
+                        self.build_status_from_io_error(id, &e)
                     }
                 }
             } else {
@@ -540,7 +578,7 @@ impl SftpHandler {
                     self.build_attrs_response(id, &attrs)
                 }
                 Err(e) => {
-                    self.build_status_response(id, SftpStatus::SSH_FX_FAILURE, &format!("Fstat error: {}", e))
+                    self.build_status_from_io_error(id, &e)
                 }
             }
         } else {
@@ -564,6 +602,10 @@ impl SftpHandler {
         
         match fs::read_dir(&full_path) {
             Ok(entries) => {
+                if self.handles.len() >= Self::MAX_HANDLES {
+                    warn!("SSH_FXP_OPENDIR: handle limit reached ({})", Self::MAX_HANDLES);
+                    return self.build_status_response(id, SftpStatus::SSH_FX_FAILURE, "Handle limit reached");
+                }
                 let handle_id = self.next_handle_id;
                 self.next_handle_id += 1;
                 
@@ -582,7 +624,7 @@ impl SftpHandler {
                 self.build_handle_response(id, &handle_id.to_be_bytes())
             }
             Err(e) => {
-                self.build_status_response(id, SftpStatus::SSH_FX_FAILURE, &format!("Opendir error: {}", e))
+                self.build_status_from_io_error(id, &e)
             }
         }
     }
@@ -648,7 +690,7 @@ impl SftpHandler {
                 self.build_status_response(id, SftpStatus::SSH_FX_OK, "File removed")
             }
             Err(e) => {
-                self.build_status_response(id, SftpStatus::SSH_FX_FAILURE, &format!("Remove error: {}", e))
+                self.build_status_from_io_error(id, &e)
             }
         }
     }
@@ -673,7 +715,7 @@ impl SftpHandler {
                 self.build_status_response(id, SftpStatus::SSH_FX_OK, "Directory created")
             }
             Err(e) => {
-                self.build_status_response(id, SftpStatus::SSH_FX_FAILURE, &format!("Mkdir error: {}", e))
+                self.build_status_from_io_error(id, &e)
             }
         }
     }
@@ -697,7 +739,7 @@ impl SftpHandler {
                 self.build_status_response(id, SftpStatus::SSH_FX_OK, "Directory removed")
             }
             Err(e) => {
-                self.build_status_response(id, SftpStatus::SSH_FX_FAILURE, &format!("Rmdir error: {}", e))
+                self.build_status_from_io_error(id, &e)
             }
         }
     }
@@ -770,7 +812,7 @@ impl SftpHandler {
                 self.build_status_response(id, SftpStatus::SSH_FX_OK, "Rename successful")
             }
             Err(e) => {
-                self.build_status_response(id, SftpStatus::SSH_FX_FAILURE, &format!("Rename error: {}", e))
+                self.build_status_from_io_error(id, &e)
             }
         }
     }
@@ -864,7 +906,7 @@ impl SftpHandler {
                 self.build_name_response(id, vec![(target, SftpAttrs::default())])
             }
             Err(e) => {
-                self.build_status_response(id, SftpStatus::SSH_FX_FAILURE, &format!("Readlink error: {}", e))
+                self.build_status_from_io_error(id, &e)
             }
         }
     }
@@ -891,7 +933,7 @@ impl SftpHandler {
                 self.build_status_response(id, SftpStatus::SSH_FX_OK, "Symlink created")
             }
             Err(e) => {
-                self.build_status_response(id, SftpStatus::SSH_FX_FAILURE, &format!("Symlink error: {}", e))
+                self.build_status_from_io_error(id, &e)
             }
         }
         
@@ -994,7 +1036,7 @@ impl SftpHandler {
                     self.wrap_sftp_packet(&response)
                 }
                 Err(e) => {
-                    self.build_status_response(id, SftpStatus::SSH_FX_FAILURE, &format!("statvfs error: {}", e))
+                    self.build_status_from_io_error(id, &e)
                 }
             }
         }
@@ -1052,7 +1094,7 @@ impl SftpHandler {
                 self.build_status_response(id, SftpStatus::SSH_FX_OK, "Hardlink created")
             }
             Err(e) => {
-                self.build_status_response(id, SftpStatus::SSH_FX_FAILURE, &format!("Hardlink error: {}", e))
+                self.build_status_from_io_error(id, &e)
             }
         }
         
@@ -1075,7 +1117,7 @@ impl SftpHandler {
                 self.build_status_response(id, SftpStatus::SSH_FX_OK, "Posix rename successful")
             }
             Err(e) => {
-                self.build_status_response(id, SftpStatus::SSH_FX_FAILURE, &format!("Posix rename error: {}", e))
+                self.build_status_from_io_error(id, &e)
             }
         }
     }
@@ -1088,13 +1130,18 @@ impl SftpHandler {
         
         info!("md5-hash: path={}, offset={}, length={}", path, offset, length);
         
+        let actual_length = std::cmp::min(length, Self::MAX_HASH_SIZE);
+        if actual_length < length {
+            warn!("md5-hash: length reduced from {} to {} (MAX_HASH_SIZE)", length, actual_length);
+        }
+        
         let full_path = self.resolve_path(&path)?;
         
         match File::open(&full_path) {
             Ok(mut file) => {
                 file.seek(SeekFrom::Start(offset))?;
                 
-                let mut buffer = vec![0u8; length as usize];
+                let mut buffer = vec![0u8; actual_length as usize];
                 file.read_exact(&mut buffer)?;
                 
                 // 计算MD5哈希
@@ -1117,7 +1164,7 @@ impl SftpHandler {
                 self.wrap_sftp_packet(&response)
             }
             Err(e) => {
-                self.build_status_response(id, SftpStatus::SSH_FX_FAILURE, &format!("MD5 hash error: {}", e))
+                self.build_status_from_io_error(id, &e)
             }
         }
     }
@@ -1130,13 +1177,18 @@ impl SftpHandler {
         
         info!("sha256-hash: path={}, offset={}, length={}", path, offset, length);
         
+        let actual_length = std::cmp::min(length, Self::MAX_HASH_SIZE);
+        if actual_length < length {
+            warn!("sha256-hash: length reduced from {} to {} (MAX_HASH_SIZE)", length, actual_length);
+        }
+        
         let full_path = self.resolve_path(&path)?;
         
         match File::open(&full_path) {
             Ok(mut file) => {
                 file.seek(SeekFrom::Start(offset))?;
                 
-                let mut buffer = vec![0u8; length as usize];
+                let mut buffer = vec![0u8; actual_length as usize];
                 file.read_exact(&mut buffer)?;
                 
                 // 计算SHA256哈希（使用sha2 crate）
@@ -1162,7 +1214,7 @@ impl SftpHandler {
                 self.wrap_sftp_packet(&response)
             }
             Err(e) => {
-                self.build_status_response(id, SftpStatus::SSH_FX_FAILURE, &format!("SHA256 hash error: {}", e))
+                self.build_status_from_io_error(id, &e)
             }
         }
     }
@@ -1175,13 +1227,18 @@ impl SftpHandler {
         
         info!("sha384-hash: path={}, offset={}, length={}", path, offset, length);
         
+        let actual_length = std::cmp::min(length, Self::MAX_HASH_SIZE);
+        if actual_length < length {
+            warn!("sha384-hash: length reduced from {} to {} (MAX_HASH_SIZE)", length, actual_length);
+        }
+        
         let full_path = self.resolve_path(&path)?;
         
         match File::open(&full_path) {
             Ok(mut file) => {
                 file.seek(SeekFrom::Start(offset))?;
                 
-                let mut buffer = vec![0u8; length as usize];
+                let mut buffer = vec![0u8; actual_length as usize];
                 file.read_exact(&mut buffer)?;
                 
                 // 计算SHA384哈希
@@ -1205,7 +1262,7 @@ impl SftpHandler {
                 self.wrap_sftp_packet(&response)
             }
             Err(e) => {
-                self.build_status_response(id, SftpStatus::SSH_FX_FAILURE, &format!("SHA384 hash error: {}", e))
+                self.build_status_from_io_error(id, &e)
             }
         }
     }
@@ -1218,13 +1275,18 @@ impl SftpHandler {
         
         info!("sha512-hash: path={}, offset={}, length={}", path, offset, length);
         
+        let actual_length = std::cmp::min(length, Self::MAX_HASH_SIZE);
+        if actual_length < length {
+            warn!("sha512-hash: length reduced from {} to {} (MAX_HASH_SIZE)", length, actual_length);
+        }
+        
         let full_path = self.resolve_path(&path)?;
         
         match File::open(&full_path) {
             Ok(mut file) => {
                 file.seek(SeekFrom::Start(offset))?;
                 
-                let mut buffer = vec![0u8; length as usize];
+                let mut buffer = vec![0u8; actual_length as usize];
                 file.read_exact(&mut buffer)?;
                 
                 // 计算SHA512哈希
@@ -1248,7 +1310,7 @@ impl SftpHandler {
                 self.wrap_sftp_packet(&response)
             }
             Err(e) => {
-                self.build_status_response(id, SftpStatus::SSH_FX_FAILURE, &format!("SHA512 hash error: {}", e))
+                self.build_status_from_io_error(id, &e)
             }
         }
     }
@@ -1298,6 +1360,11 @@ impl SftpHandler {
               u32::from_be_bytes([write_handle_bytes[0], write_handle_bytes[1], write_handle_bytes[2], write_handle_bytes[3]]),
               write_offset);
         
+        let actual_length = std::cmp::min(read_length, Self::MAX_XFER_SIZE as u64);
+        if actual_length < read_length {
+            warn!("copy-data: length reduced from {} to {} (MAX_XFER_SIZE)", read_length, actual_length);
+        }
+        
         let read_handle_id = u32::from_be_bytes([read_handle_bytes[0], read_handle_bytes[1], read_handle_bytes[2], read_handle_bytes[3]]);
         let write_handle_id = u32::from_be_bytes([write_handle_bytes[0], write_handle_bytes[1], write_handle_bytes[2], write_handle_bytes[3]]);
         
@@ -1319,7 +1386,7 @@ impl SftpHandler {
         match File::open(&read_path) {
             Ok(mut read_file) => {
                 read_file.seek(SeekFrom::Start(read_offset))?;
-                let mut buffer = vec![0u8; read_length as usize];
+                let mut buffer = vec![0u8; actual_length as usize];
                 read_file.read_exact(&mut buffer)?;
                 
                 // 写入到write_path
@@ -1334,72 +1401,112 @@ impl SftpHandler {
                         response.write_u32::<BigEndian>(id)?;
                         
                         // 返回复制的字节数
-                        response.write_u64::<BigEndian>(read_length)?;
+                        response.write_u64::<BigEndian>(actual_length)?;
                         
                         self.wrap_sftp_packet(&response)
                     }
                     Err(e) => {
-                        self.build_status_response(id, SftpStatus::SSH_FX_FAILURE, &format!("Write file error: {}", e))
+                        self.build_status_from_io_error(id, &e)
                     }
                 }
             }
             Err(e) => {
-                self.build_status_response(id, SftpStatus::SSH_FX_FAILURE, &format!("Read file error: {}", e))
+                self.build_status_from_io_error(id, &e)
             }
         }
     }
 
-    /// 解析路径（安全性检查，参考OpenSSH sftp-server.c: path_resolve())
+    /// 解析路径（安全性检查，参考OpenSSH sftp-server.c: path_resolve()）
+    /// 
+    /// 安全策略：
+    /// - 相对路径：始终限制在 root_dir 之下
+    /// - 绝对路径（restrict_absolute=false）：允许（用户依赖文件系统权限）
+    /// - 绝对路径（restrict_absolute=true）：限制在 root_dir 之下（chroot 模式）
     fn resolve_path(&self, path: &str) -> Result<PathBuf> {
         info!("resolve_path: input={}, root_dir={:?}", path, self.root_dir);
         
         let full_path = if path.is_empty() || path == "." {
             self.root_dir.clone()
         } else if path.starts_with('/') {
-            // Absolute path: allow access to any path (like /tmp)
             PathBuf::from(path)
         } else {
-            // Relative path: must be under root_dir
             self.root_dir.join(path)
         };
         
         info!("resolve_path: full_path={:?}", full_path);
         
-        // Security: Only enforce root_dir check for relative paths
-        // Absolute paths are allowed (user can access any path they have filesystem permissions for)
-        if path.starts_with('/') {
-            // Absolute path: no root_dir check, just return canonicalized path if exists
+        let is_absolute = path.starts_with('/');
+        
+        // 检查路径遍历：对相对路径始终执行，对绝对路径仅在 restrict_absolute 模式下执行
+        let need_check = !is_absolute || self.restrict_absolute;
+        
+        if need_check {
+            if full_path.exists() {
+                let canonical = full_path.canonicalize()
+                    .map_err(|e| anyhow!("Path canonicalize error: {}", e))?;
+                if !canonical.starts_with(&self.root_dir) {
+                    return Err(anyhow!("Path traversal: {:?} not under {:?}", canonical, self.root_dir));
+                }
+                Ok(canonical)
+            } else {
+                // Pre-resolve parent directory for non-existent paths
+                if let Some(parent) = full_path.parent() {
+                    if parent.exists() {
+                        let canonical_parent = parent.canonicalize()
+                            .map_err(|e| anyhow!("Parent canonicalize error: {}", e))?;
+                        let resolved = canonical_parent.join(
+                            full_path.file_name().unwrap_or_default()
+                        );
+                        if !resolved.starts_with(&self.root_dir) {
+                            return Err(anyhow!("Path traversal: {:?} not under {:?}", resolved, self.root_dir));
+                        }
+                        return Ok(resolved);
+                    }
+                }
+                if !full_path.starts_with(&self.root_dir) {
+                    return Err(anyhow!("Path traversal: {:?} not under {:?}", full_path, self.root_dir));
+                }
+                Ok(full_path)
+            }
+        } else {
+            // Absolute path, unrestricted: canonicalize if exists, else return as-is
             if full_path.exists() {
                 Ok(full_path.canonicalize()?)
             } else {
                 Ok(full_path)
             }
-        } else {
-            // Relative path: enforce strict root_dir confinement
-            if full_path.exists() {
-                let canonical_path = full_path.canonicalize()?;
-                if !canonical_path.starts_with(&self.root_dir) {
-                    return Err(anyhow!("Path traversal attempt detected: {:?} not under {:?}", canonical_path, self.root_dir));
-                }
-                Ok(canonical_path)
-            } else {
-                if !full_path.starts_with(&self.root_dir) {
-                    return Err(anyhow!("Path traversal attempt detected: {:?} not under {:?}", full_path, self.root_dir));
-                }
-                Ok(full_path)
-            }
         }
     }
 
-/// 构建SSH_FXP_VERSION响应（参考OpenSSH sftp-server.c）
+/// 构建SSH_FXP_VERSION响应，包含扩展声明（参考OpenSSH sftp-server.c）
     fn build_version_response(&self, version: u32) -> Result<Vec<u8>> {
         let mut buffer = Vec::new();
         
-        // SSH_FXP_VERSION packet
         buffer.write_u8(SftpPacketType::SSH_FXP_VERSION as u8)?;
         buffer.write_u32::<BigEndian>(version)?;
         
-        // Phase 7: SFTP packet需要SSH string格式（uint32(length) + packet_type + payload）
+        // 扩展声明（OpenSSH sftp-server.c: process_init() 中声明支持的扩展）
+        let extensions: &[(&str, &str)] = &[
+            ("posix-rename@openssh.com", "1"),
+            ("hardlink@openssh.com", "1"),
+            ("copy-data@openssh.com", "1"),
+            ("check-file@openssh.com", "1"),
+            ("statvfs@openssh.com", "2"),
+            ("fstatvfs@openssh.com", "2"),
+            ("md5-hash@openssh.com", "1"),
+            ("sha256-hash@openssh.com", "1"),
+            ("sha384-hash@openssh.com", "1"),
+            ("sha512-hash@openssh.com", "1"),
+        ];
+        
+        buffer.write_u32::<BigEndian>(extensions.len() as u32)?;
+        for (name, data) in extensions {
+            buffer.write_u32::<BigEndian>(name.len() as u32)?;
+            buffer.write_all(name.as_bytes())?;
+            buffer.write_u32::<BigEndian>(data.len() as u32)?;
+            buffer.write_all(data.as_bytes())?;
+        }
+        
         self.wrap_sftp_packet(&buffer)
     }
     
@@ -1468,7 +1575,7 @@ impl SftpHandler {
             buffer.write_u32::<BigEndian>(long_name.len() as u32)?;
             buffer.write_all(long_name.as_bytes())?;
             
-            buffer.write_all(&attrs.serialize())?;
+            buffer.write_all(&attrs.serialize()?)?;
         }
         
         self.wrap_sftp_packet(&buffer)
@@ -1480,10 +1587,30 @@ impl SftpHandler {
         
         buffer.write_u8(SftpPacketType::SSH_FXP_ATTRS as u8)?;
         buffer.write_u32::<BigEndian>(id)?;
-        buffer.write_all(&attrs.serialize())?;
+        buffer.write_all(&attrs.serialize()?)?;
         
         self.wrap_sftp_packet(&buffer)
     }
+
+    /// 将 std::io::Error 映射为对应的 SSH_FX_* 状态码
+    fn map_io_error_kind(err: &std::io::Error) -> SftpStatus {
+        match err.kind() {
+            std::io::ErrorKind::NotFound => SftpStatus::SSH_FX_NO_SUCH_FILE,
+            std::io::ErrorKind::PermissionDenied => SftpStatus::SSH_FX_PERMISSION_DENIED,
+            std::io::ErrorKind::AlreadyExists => SftpStatus::SSH_FX_FAILURE,
+            std::io::ErrorKind::InvalidInput => SftpStatus::SSH_FX_BAD_MESSAGE,
+            std::io::ErrorKind::WriteZero => SftpStatus::SSH_FX_FAILURE,
+            std::io::ErrorKind::UnexpectedEof => SftpStatus::SSH_FX_EOF,
+            _ => SftpStatus::SSH_FX_FAILURE,
+        }
+    }
+    
+    /// 根据 Error 构建状态响应（自动映射错误类型）
+    fn build_status_from_io_error(&self, id: u32, err: &std::io::Error) -> Result<Vec<u8>> {
+        let status = Self::map_io_error_kind(err);
+        let msg = format!("{}", err);
+        self.build_status_response(id, status, &msg)
+    }
 }
 
 /// 读取SFTP字符串（参考draft-ietf-secsh-filexfer-02.txt）
@@ -1553,7 +1680,7 @@ mod tests {
     #[test]
     fn test_sftp_handler_creation() {
         let temp_dir = TempDir::new().unwrap();
-        let handler = SftpHandler::new(temp_dir.path().to_path_buf());
+        let handler = SftpHandler::new(temp_dir.path().to_path_buf(), 32768);
         assert_eq!(handler.next_handle_id, 0);
     }
     
@@ -1573,7 +1700,7 @@ mod tests {
     #[test]
     fn test_sftp_handle_init() {
         let temp_dir = TempDir::new().unwrap();
-        let mut handler = SftpHandler::new(temp_dir.path().to_path_buf());
+        let mut handler = SftpHandler::new(temp_dir.path().to_path_buf(), 32768);
         
         let init_packet = vec![1, 0, 0, 0, 3];
         let response = handler.handle_request(&init_packet).unwrap();