diff --git a/PREUPLOAD.cfg b/PREUPLOAD.cfg
index a6b1f95..00f34b9 100644
--- a/PREUPLOAD.cfg
+++ b/PREUPLOAD.cfg
@@ -4,6 +4,7 @@
 bpfmt = true
 clang_format = true
 jsonlint = true
+google_java_format = true
 pylint3 = true
 rustfmt = true
 xmllint = true
diff --git a/authfs/aidl/com/android/virt/fs/IVirtFdService.aidl b/authfs/aidl/com/android/virt/fs/IVirtFdService.aidl
index 628ee3c..189f43a 100644
--- a/authfs/aidl/com/android/virt/fs/IVirtFdService.aidl
+++ b/authfs/aidl/com/android/virt/fs/IVirtFdService.aidl
@@ -45,4 +45,10 @@
 
     /** Returns the fs-verity signature of the given file ID. */
     byte[] readFsveritySignature(int id);
+
+    /**
+     * Writes the buffer to the given file ID from the file's offset. Returns the number of bytes
+     * written.
+     */
+    int writeFile(int id, in byte[] buf, long offset);
 }
diff --git a/authfs/fd_server/Android.bp b/authfs/fd_server/Android.bp
index 6f010ce..9c810a8 100644
--- a/authfs/fd_server/Android.bp
+++ b/authfs/fd_server/Android.bp
@@ -7,6 +7,7 @@
     srcs: ["src/main.rs"],
     rustlibs: [
         "authfs_aidl_interface-rust",
+        "libandroid_logger",
         "libanyhow",
         "libbinder_rs",
         "libclap",
diff --git a/authfs/fd_server/src/main.rs b/authfs/fd_server/src/main.rs
index cbd7712..9395afc 100644
--- a/authfs/fd_server/src/main.rs
+++ b/authfs/fd_server/src/main.rs
@@ -42,7 +42,8 @@
     BnVirtFdService, IVirtFdService, ERROR_IO, ERROR_UNKNOWN_FD, MAX_REQUESTING_DATA,
 };
 use authfs_aidl_interface::binder::{
-    add_service, ExceptionCode, Interface, ProcessState, Result as BinderResult, Status, Strong,
+    add_service, ExceptionCode, Interface, ProcessState, Result as BinderResult, Status,
+    StatusCode, Strong,
 };
 
 const SERVICE_NAME: &str = "authfs_fd_server";
@@ -70,38 +71,41 @@
     }
 }
 
-/// Configuration of a read-only file to serve by this server. The file is supposed to be verifiable
-/// with the associated fs-verity metadata.
-struct ReadonlyFdConfig {
-    /// The file to read from. fs-verity metadata can be retrieved from this file's FD.
-    file: File,
+/// Configuration of a file descriptor to be served/exposed/shared.
+enum FdConfig {
+    /// A read-only file to serve by this server. The file is supposed to be verifiable with the
+    /// associated fs-verity metadata.
+    Readonly {
+        /// The file to read from. fs-verity metadata can be retrieved from this file's FD.
+        file: File,
 
-    /// Alternative Merkle tree stored in another file.
-    alt_merkle_file: Option<File>,
+        /// Alternative Merkle tree stored in another file.
+        alt_merkle_tree: Option<File>,
 
-    /// Alternative signature stored in another file.
-    alt_signature_file: Option<File>,
+        /// Alternative signature stored in another file.
+        alt_signature: Option<File>,
+    },
+
+    /// A readable/writable file to serve by this server. This backing file should just be a
+    /// regular file and does not have any specific property.
+    ReadWrite(File),
 }
 
 struct FdService {
-    /// A pool of read-only files
-    fd_pool: BTreeMap<i32, ReadonlyFdConfig>,
+    /// A pool of opened files, may be readonly or read-writable.
+    fd_pool: BTreeMap<i32, FdConfig>,
 }
 
 impl FdService {
-    pub fn new_binder(fd_pool: BTreeMap<i32, ReadonlyFdConfig>) -> Strong<dyn IVirtFdService> {
+    pub fn new_binder(fd_pool: BTreeMap<i32, FdConfig>) -> Strong<dyn IVirtFdService> {
         let result = BnVirtFdService::new_binder(FdService { fd_pool });
         result.as_binder().set_requesting_sid(false);
         result
     }
 
-    fn get_file_config(&self, id: i32) -> BinderResult<&ReadonlyFdConfig> {
+    fn get_file_config(&self, id: i32) -> BinderResult<&FdConfig> {
         self.fd_pool.get(&id).ok_or_else(|| Status::from(ERROR_UNKNOWN_FD))
     }
-
-    fn get_file(&self, id: i32) -> BinderResult<&File> {
-        Ok(&self.get_file_config(id)?.file)
-    }
 }
 
 impl Interface for FdService {}
@@ -111,38 +115,88 @@
         let size: usize = validate_and_cast_size(size)?;
         let offset: u64 = validate_and_cast_offset(offset)?;
 
-        read_into_buf(self.get_file(id)?, size, offset).map_err(|e| {
-            error!("readFile: read error: {}", e);
-            Status::from(ERROR_IO)
-        })
+        match self.get_file_config(id)? {
+            FdConfig::Readonly { file, .. } | FdConfig::ReadWrite(file) => {
+                read_into_buf(&file, size, offset).map_err(|e| {
+                    error!("readFile: read error: {}", e);
+                    Status::from(ERROR_IO)
+                })
+            }
+        }
     }
 
     fn readFsverityMerkleTree(&self, id: i32, offset: i64, size: i32) -> BinderResult<Vec<u8>> {
         let size: usize = validate_and_cast_size(size)?;
         let offset: u64 = validate_and_cast_offset(offset)?;
 
-        if let Some(file) = &self.get_file_config(id)?.alt_merkle_file {
-            read_into_buf(&file, size, offset).map_err(|e| {
-                error!("readFsverityMerkleTree: read error: {}", e);
-                Status::from(ERROR_IO)
-            })
-        } else {
-            // TODO(victorhsieh) retrieve from the fd when the new ioctl is ready
-            Err(new_binder_exception(ExceptionCode::UNSUPPORTED_OPERATION, "Not implemented yet"))
+        match &self.get_file_config(id)? {
+            FdConfig::Readonly { alt_merkle_tree, .. } => {
+                if let Some(file) = &alt_merkle_tree {
+                    read_into_buf(&file, size, offset).map_err(|e| {
+                        error!("readFsverityMerkleTree: read error: {}", e);
+                        Status::from(ERROR_IO)
+                    })
+                } else {
+                    // TODO(victorhsieh) retrieve from the fd when the new ioctl is ready
+                    Err(new_binder_exception(
+                        ExceptionCode::UNSUPPORTED_OPERATION,
+                        "Not implemented yet",
+                    ))
+                }
+            }
+            FdConfig::ReadWrite(_file) => {
+                // For a writable file, Merkle tree is not expected to be served since Auth FS
+                // doesn't trust it anyway. Auth FS may keep the Merkle tree privately for its own
+                // use.
+                Err(new_binder_exception(ExceptionCode::UNSUPPORTED_OPERATION, "Unsupported"))
+            }
         }
     }
 
     fn readFsveritySignature(&self, id: i32) -> BinderResult<Vec<u8>> {
-        if let Some(file) = &self.get_file_config(id)?.alt_signature_file {
-            // Supposedly big enough buffer size to store signature.
-            let size = MAX_REQUESTING_DATA as usize;
-            read_into_buf(&file, size, 0).map_err(|e| {
-                error!("readFsveritySignature: read error: {}", e);
-                Status::from(ERROR_IO)
-            })
-        } else {
-            // TODO(victorhsieh) retrieve from the fd when the new ioctl is ready
-            Err(new_binder_exception(ExceptionCode::UNSUPPORTED_OPERATION, "Not implemented yet"))
+        match &self.get_file_config(id)? {
+            FdConfig::Readonly { alt_signature, .. } => {
+                if let Some(file) = &alt_signature {
+                    // Supposedly big enough buffer size to store signature.
+                    let size = MAX_REQUESTING_DATA as usize;
+                    read_into_buf(&file, size, 0).map_err(|e| {
+                        error!("readFsveritySignature: read error: {}", e);
+                        Status::from(ERROR_IO)
+                    })
+                } else {
+                    // TODO(victorhsieh) retrieve from the fd when the new ioctl is ready
+                    Err(new_binder_exception(
+                        ExceptionCode::UNSUPPORTED_OPERATION,
+                        "Not implemented yet",
+                    ))
+                }
+            }
+            FdConfig::ReadWrite(_file) => {
+                // There is no signature for a writable file.
+                Err(new_binder_exception(ExceptionCode::UNSUPPORTED_OPERATION, "Unsupported"))
+            }
+        }
+    }
+
+    fn writeFile(&self, id: i32, buf: &[u8], offset: i64) -> BinderResult<i32> {
+        match &self.get_file_config(id)? {
+            FdConfig::Readonly { .. } => Err(StatusCode::INVALID_OPERATION.into()),
+            FdConfig::ReadWrite(file) => {
+                let offset: u64 = offset.try_into().map_err(|_| {
+                    new_binder_exception(ExceptionCode::ILLEGAL_ARGUMENT, "Invalid offset")
+                })?;
+                // Check buffer size just to make `as i32` safe below.
+                if buf.len() > i32::MAX as usize {
+                    return Err(new_binder_exception(
+                        ExceptionCode::ILLEGAL_ARGUMENT,
+                        "Buffer size is too big",
+                    ));
+                }
+                Ok(file.write_at(buf, offset).map_err(|e| {
+                    error!("writeFile: write error: {}", e);
+                    Status::from(ERROR_IO)
+                })? as i32)
+            }
         }
     }
 }
@@ -169,29 +223,42 @@
     Ok(unsafe { File::from_raw_fd(fd) })
 }
 
-fn parse_arg_ro_fds(arg: &str) -> Result<(i32, ReadonlyFdConfig)> {
+fn parse_arg_ro_fds(arg: &str) -> Result<(i32, FdConfig)> {
     let result: Result<Vec<i32>, _> = arg.split(':').map(|x| x.parse::<i32>()).collect();
     let fds = result?;
     if fds.len() > 3 {
         bail!("Too many options: {}", arg);
     }
-
     Ok((
         fds[0],
-        ReadonlyFdConfig {
+        FdConfig::Readonly {
             file: fd_to_file(fds[0])?,
-            alt_merkle_file: fds.get(1).map(|fd| fd_to_file(*fd)).transpose()?,
-            alt_signature_file: fds.get(2).map(|fd| fd_to_file(*fd)).transpose()?,
+            // Alternative Merkle tree, if provided
+            alt_merkle_tree: fds.get(1).map(|fd| fd_to_file(*fd)).transpose()?,
+            // Alternative signature, if provided
+            alt_signature: fds.get(2).map(|fd| fd_to_file(*fd)).transpose()?,
         },
     ))
 }
 
-fn parse_args() -> Result<BTreeMap<i32, ReadonlyFdConfig>> {
+fn parse_arg_rw_fds(arg: &str) -> Result<(i32, FdConfig)> {
+    let fd = arg.parse::<i32>()?;
+    let file = fd_to_file(fd)?;
+    if file.metadata()?.len() > 0 {
+        bail!("File is expected to be empty");
+    }
+    Ok((fd, FdConfig::ReadWrite(file)))
+}
+
+fn parse_args() -> Result<BTreeMap<i32, FdConfig>> {
     #[rustfmt::skip]
     let matches = clap::App::new("fd_server")
         .arg(clap::Arg::with_name("ro-fds")
              .long("ro-fds")
-             .required(true)
+             .multiple(true)
+             .number_of_values(1))
+        .arg(clap::Arg::with_name("rw-fds")
+             .long("rw-fds")
              .multiple(true)
              .number_of_values(1))
         .get_matches();
@@ -203,10 +270,20 @@
             fd_pool.insert(fd, config);
         }
     }
+    if let Some(args) = matches.values_of("rw-fds") {
+        for arg in args {
+            let (fd, config) = parse_arg_rw_fds(arg)?;
+            fd_pool.insert(fd, config);
+        }
+    }
     Ok(fd_pool)
 }
 
 fn main() -> Result<()> {
+    android_logger::init_once(
+        android_logger::Config::default().with_tag("fd_server").with_min_level(log::Level::Debug),
+    );
+
     let fd_pool = parse_args()?;
 
     ProcessState::start_thread_pool();
diff --git a/authfs/src/common.rs b/authfs/src/common.rs
index 522397f..6556fde 100644
--- a/authfs/src/common.rs
+++ b/authfs/src/common.rs
@@ -20,3 +20,59 @@
 pub fn divide_roundup(dividend: u64, divisor: u64) -> u64 {
     (dividend + divisor - 1) / divisor
 }
+
+/// Given `offset` and `length`, generates (offset, size) tuples that together form the same length,
+/// and aligned to `alignment`.
+pub struct ChunkedSizeIter {
+    remaining: usize,
+    offset: u64,
+    alignment: usize,
+}
+
+impl ChunkedSizeIter {
+    pub fn new(remaining: usize, offset: u64, alignment: usize) -> Self {
+        ChunkedSizeIter { remaining, offset, alignment }
+    }
+}
+
+impl Iterator for ChunkedSizeIter {
+    type Item = (u64, usize);
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.remaining == 0 {
+            return None;
+        }
+        let chunk_data_size = std::cmp::min(
+            self.remaining,
+            self.alignment - (self.offset % self.alignment as u64) as usize,
+        );
+        let retval = (self.offset, chunk_data_size);
+        self.offset += chunk_data_size as u64;
+        self.remaining = self.remaining.saturating_sub(chunk_data_size);
+        Some(retval)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn collect_chunk_read_iter(remaining: usize, offset: u64) -> Vec<(u64, usize)> {
+        ChunkedSizeIter::new(remaining, offset, 4096).collect::<Vec<_>>()
+    }
+
+    #[test]
+    fn test_chunk_read_iter() {
+        assert_eq!(collect_chunk_read_iter(4096, 0), [(0, 4096)]);
+        assert_eq!(collect_chunk_read_iter(8192, 0), [(0, 4096), (4096, 4096)]);
+        assert_eq!(collect_chunk_read_iter(8192, 4096), [(4096, 4096), (8192, 4096)]);
+
+        assert_eq!(
+            collect_chunk_read_iter(16384, 1),
+            [(1, 4095), (4096, 4096), (8192, 4096), (12288, 4096), (16384, 1)]
+        );
+
+        assert_eq!(collect_chunk_read_iter(0, 0), []);
+        assert_eq!(collect_chunk_read_iter(0, 100), []);
+    }
+}
diff --git a/authfs/src/file.rs b/authfs/src/file.rs
new file mode 100644
index 0000000..89fbd9d
--- /dev/null
+++ b/authfs/src/file.rs
@@ -0,0 +1,49 @@
+mod local_file;
+mod remote_file;
+
+pub use local_file::LocalFileReader;
+pub use remote_file::{RemoteFileEditor, RemoteFileReader, RemoteMerkleTreeReader};
+
+use std::io;
+
+use authfs_aidl_interface::aidl::com::android::virt::fs::IVirtFdService;
+use authfs_aidl_interface::binder::{get_interface, Strong};
+
+// TODO(victorhsieh): use remote binder.
+pub fn get_local_binder() -> Strong<dyn IVirtFdService::IVirtFdService> {
+    let service_name = "authfs_fd_server";
+    get_interface(&service_name).expect("Cannot reach authfs_fd_server binder service")
+}
+
+/// A trait for reading data by chunks. The data is assumed readonly and has fixed length. Chunks
+/// can be read by specifying the chunk index. Only the last chunk may have incomplete chunk size.
+pub trait ReadOnlyDataByChunk {
+    /// Read the `chunk_index`-th chunk to `buf`. Each slice/chunk has size `CHUNK_SIZE` except for
+    /// the last one, which can be an incomplete chunk. `buf` is currently required to be large
+    /// enough to hold a full chunk of data. Reading beyond the file size (including empty file)
+    /// will crash.
+    fn read_chunk(&self, chunk_index: u64, buf: &mut [u8]) -> io::Result<usize>;
+}
+
+/// A trait to write a buffer to the destination at a given offset. The implementation does not
+/// necessarily own or maintain the destination state.
+///
+/// NB: The trait is required in a member of `fusefs::AuthFs`, which is required to be Sync and
+/// immutable (this the member).
+pub trait RandomWrite {
+    /// Writes `buf` to the destination at `offset`. Returns the written size, which may not be the
+    /// full buffer.
+    fn write_at(&self, buf: &[u8], offset: u64) -> io::Result<usize>;
+
+    /// Writes the full `buf` to the destination at `offset`.
+    fn write_all_at(&self, buf: &[u8], offset: u64) -> io::Result<()> {
+        let mut input_offset = 0;
+        let mut output_offset = offset;
+        while input_offset < buf.len() {
+            let size = self.write_at(&buf[input_offset..], output_offset)?;
+            input_offset += size;
+            output_offset += size as u64;
+        }
+        Ok(())
+    }
+}
diff --git a/authfs/src/reader.rs b/authfs/src/file/local_file.rs
similarity index 64%
rename from authfs/src/reader.rs
rename to authfs/src/file/local_file.rs
index 0242afa..0692767 100644
--- a/authfs/src/reader.rs
+++ b/authfs/src/file/local_file.rs
@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2020 The Android Open Source Project
+ * Copyright (C) 2021 The Android Open Source Project
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -14,24 +14,13 @@
  * limitations under the License.
  */
 
-//! A module for reading data by chunks.
-
 use std::fs::File;
 use std::io::Result;
 use std::os::unix::fs::FileExt;
 
+use super::ReadOnlyDataByChunk;
 use crate::common::CHUNK_SIZE;
 
-/// A trait for reading data by chunks. The data is assumed readonly and has fixed length. Chunks
-/// can be read by specifying the chunk index. Only the last chunk may have incomplete chunk size.
-pub trait ReadOnlyDataByChunk {
-    /// Read the `chunk_index`-th chunk to `buf`. Each slice/chunk has size `CHUNK_SIZE` except for
-    /// the last one, which can be an incomplete chunk. `buf` is currently required to be large
-    /// enough to hold a full chunk of data. Reading beyond the file size (including empty file)
-    /// will crash.
-    fn read_chunk(&self, chunk_index: u64, buf: &mut [u8]) -> Result<usize>;
-}
-
 fn chunk_index_to_range(size: u64, chunk_index: u64) -> Result<(u64, u64)> {
     let start = chunk_index * CHUNK_SIZE;
     assert!(start < size);
@@ -40,16 +29,16 @@
 }
 
 /// A read-only file that can be read by chunks.
-pub struct ChunkedFileReader {
+pub struct LocalFileReader {
     file: File,
     size: u64,
 }
 
-impl ChunkedFileReader {
-    /// Creates a `ChunkedFileReader` to read from for the specified `path`.
-    pub fn new(file: File) -> Result<ChunkedFileReader> {
+impl LocalFileReader {
+    /// Creates a `LocalFileReader` to read from for the specified `path`.
+    pub fn new(file: File) -> Result<LocalFileReader> {
         let size = file.metadata()?.len();
-        Ok(ChunkedFileReader { file, size })
+        Ok(LocalFileReader { file, size })
     }
 
     pub fn len(&self) -> u64 {
@@ -57,7 +46,7 @@
     }
 }
 
-impl ReadOnlyDataByChunk for ChunkedFileReader {
+impl ReadOnlyDataByChunk for LocalFileReader {
     fn read_chunk(&self, chunk_index: u64, buf: &mut [u8]) -> Result<usize> {
         debug_assert!(buf.len() as u64 >= CHUNK_SIZE);
         let (start, end) = chunk_index_to_range(self.size, chunk_index)?;
@@ -73,7 +62,7 @@
 
     #[test]
     fn test_read_4k_file() -> Result<()> {
-        let file_reader = ChunkedFileReader::new(File::open("testdata/input.4k")?)?;
+        let file_reader = LocalFileReader::new(File::open("testdata/input.4k")?)?;
         let mut buf = [0u8; 4096];
         let size = file_reader.read_chunk(0, &mut buf)?;
         assert_eq!(size, buf.len());
@@ -82,7 +71,7 @@
 
     #[test]
     fn test_read_4k1_file() -> Result<()> {
-        let file_reader = ChunkedFileReader::new(File::open("testdata/input.4k1")?)?;
+        let file_reader = LocalFileReader::new(File::open("testdata/input.4k1")?)?;
         let mut buf = [0u8; 4096];
         let size = file_reader.read_chunk(0, &mut buf)?;
         assert_eq!(size, buf.len());
@@ -93,7 +82,7 @@
 
     #[test]
     fn test_read_4m_file() -> Result<()> {
-        let file_reader = ChunkedFileReader::new(File::open("testdata/input.4m")?)?;
+        let file_reader = LocalFileReader::new(File::open("testdata/input.4m")?)?;
         for index in 0..file_reader.len() / 4096 {
             let mut buf = [0u8; 4096];
             let size = file_reader.read_chunk(index, &mut buf)?;
@@ -105,7 +94,7 @@
     #[test]
     #[should_panic]
     fn test_read_beyond_file_size() {
-        let file_reader = ChunkedFileReader::new(File::open("testdata/input.4k").unwrap()).unwrap();
+        let file_reader = LocalFileReader::new(File::open("testdata/input.4k").unwrap()).unwrap();
         let mut buf = [0u8; 4096];
         let _ = file_reader.read_chunk(1u64, &mut buf); // should panic
     }
@@ -115,7 +104,7 @@
     fn test_read_empty_file() {
         let mut temp_file = temp_dir();
         temp_file.push("authfs_test_empty_file");
-        let file_reader = ChunkedFileReader::new(File::create(temp_file).unwrap()).unwrap();
+        let file_reader = LocalFileReader::new(File::create(temp_file).unwrap()).unwrap();
         let mut buf = [0u8; 4096];
         let _ = file_reader.read_chunk(0, &mut buf); // should panic
     }
diff --git a/authfs/src/file/remote_file.rs b/authfs/src/file/remote_file.rs
new file mode 100644
index 0000000..dbf6bd9
--- /dev/null
+++ b/authfs/src/file/remote_file.rs
@@ -0,0 +1,123 @@
+/*
+ * Copyright (C) 2021 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+use std::convert::TryFrom;
+use std::io;
+use std::io::Write;
+use std::sync::{Arc, Mutex};
+
+use super::{RandomWrite, ReadOnlyDataByChunk};
+use crate::common::CHUNK_SIZE;
+
+use authfs_aidl_interface::aidl::com::android::virt::fs::IVirtFdService;
+use authfs_aidl_interface::binder::Strong;
+
+type VirtFdService = Strong<dyn IVirtFdService::IVirtFdService>;
+
+fn remote_read_chunk(
+    service: &Arc<Mutex<VirtFdService>>,
+    remote_fd: i32,
+    chunk_index: u64,
+    mut buf: &mut [u8],
+) -> io::Result<usize> {
+    let offset = i64::try_from(chunk_index * CHUNK_SIZE)
+        .map_err(|_| io::Error::from_raw_os_error(libc::EOVERFLOW))?;
+
+    let chunk = service
+        .lock()
+        .unwrap()
+        .readFile(remote_fd, offset, buf.len() as i32)
+        .map_err(|e| io::Error::new(io::ErrorKind::Other, e.get_description()))?;
+    buf.write(&chunk)
+}
+
+pub struct RemoteFileReader {
+    // This needs to have Sync trait to be used in fuse::worker::start_message_loop.
+    service: Arc<Mutex<VirtFdService>>,
+    file_fd: i32,
+}
+
+impl RemoteFileReader {
+    pub fn new(service: Arc<Mutex<VirtFdService>>, file_fd: i32) -> Self {
+        RemoteFileReader { service, file_fd }
+    }
+}
+
+impl ReadOnlyDataByChunk for RemoteFileReader {
+    fn read_chunk(&self, chunk_index: u64, buf: &mut [u8]) -> io::Result<usize> {
+        remote_read_chunk(&self.service, self.file_fd, chunk_index, buf)
+    }
+}
+
+pub struct RemoteMerkleTreeReader {
+    // This needs to be a Sync to be used in fuse::worker::start_message_loop.
+    // TODO(victorhsieh): change to Strong<> once binder supports it.
+    service: Arc<Mutex<VirtFdService>>,
+    file_fd: i32,
+}
+
+impl RemoteMerkleTreeReader {
+    pub fn new(service: Arc<Mutex<VirtFdService>>, file_fd: i32) -> Self {
+        RemoteMerkleTreeReader { service, file_fd }
+    }
+}
+
+impl ReadOnlyDataByChunk for RemoteMerkleTreeReader {
+    fn read_chunk(&self, chunk_index: u64, mut buf: &mut [u8]) -> io::Result<usize> {
+        let offset = i64::try_from(chunk_index * CHUNK_SIZE)
+            .map_err(|_| io::Error::from_raw_os_error(libc::EOVERFLOW))?;
+
+        let chunk = self
+            .service
+            .lock()
+            .unwrap()
+            .readFsverityMerkleTree(self.file_fd, offset, buf.len() as i32)
+            .map_err(|e| io::Error::new(io::ErrorKind::Other, e.get_description()))?;
+        buf.write(&chunk)
+    }
+}
+
+pub struct RemoteFileEditor {
+    // This needs to have Sync trait to be used in fuse::worker::start_message_loop.
+    service: Arc<Mutex<VirtFdService>>,
+    file_fd: i32,
+}
+
+impl RemoteFileEditor {
+    pub fn new(service: Arc<Mutex<VirtFdService>>, file_fd: i32) -> Self {
+        RemoteFileEditor { service, file_fd }
+    }
+}
+
+impl RandomWrite for RemoteFileEditor {
+    fn write_at(&self, buf: &[u8], offset: u64) -> io::Result<usize> {
+        let offset =
+            i64::try_from(offset).map_err(|_| io::Error::from_raw_os_error(libc::EOVERFLOW))?;
+        let size = self
+            .service
+            .lock()
+            .unwrap()
+            .writeFile(self.file_fd, &buf, offset)
+            .map_err(|e| io::Error::new(io::ErrorKind::Other, e.get_description()))?;
+        Ok(size as usize) // within range because size is supposed to <= buf.len(), which is a usize
+    }
+}
+
+impl ReadOnlyDataByChunk for RemoteFileEditor {
+    fn read_chunk(&self, chunk_index: u64, buf: &mut [u8]) -> io::Result<usize> {
+        remote_read_chunk(&self.service, self.file_fd, chunk_index, buf)
+    }
+}
diff --git a/authfs/src/fsverity.rs b/authfs/src/fsverity.rs
index 37d96c1..1515574 100644
--- a/authfs/src/fsverity.rs
+++ b/authfs/src/fsverity.rs
@@ -16,8 +16,9 @@
 
 mod builder;
 mod common;
+mod editor;
 mod sys;
 mod verifier;
 
-pub use self::builder::MerkleLeaves;
-pub use self::verifier::FsverityChunkedFileReader;
+pub use editor::VerifiedFileEditor;
+pub use verifier::VerifiedFileReader;
diff --git a/authfs/src/fsverity/builder.rs b/authfs/src/fsverity/builder.rs
index 607d3a7..94b9718 100644
--- a/authfs/src/fsverity/builder.rs
+++ b/authfs/src/fsverity/builder.rs
@@ -47,13 +47,17 @@
         .collect()
 }
 
-#[allow(dead_code)]
 impl MerkleLeaves {
     /// Creates a `MerkleLeaves` instance with empty data.
     pub fn new() -> Self {
         Self { leaves: Vec::new(), file_size: 0 }
     }
 
+    /// Gets size of the file represented by `MerkleLeaves`.
+    pub fn file_size(&self) -> u64 {
+        self.file_size
+    }
+
     /// Updates the hash of the `index`-th leaf, and increase the size to `size_at_least` if the
     /// current size is smaller.
     pub fn update_hash(&mut self, index: usize, hash: &Sha256Hash, size_at_least: u64) {
diff --git a/authfs/src/fsverity/editor.rs b/authfs/src/fsverity/editor.rs
new file mode 100644
index 0000000..543e9ac
--- /dev/null
+++ b/authfs/src/fsverity/editor.rs
@@ -0,0 +1,482 @@
+/*
+ * Copyright (C) 2021 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+//! A module for writing to a file from a trusted world to an untrusted storage.
+//!
+//! Architectural Model:
+//!  * Trusted world: the writer, a signing secret, has some memory, but NO persistent storage.
+//!  * Untrusted world: persistent storage, assuming untrusted.
+//!  * IPC mechanism between trusted and untrusted world
+//!
+//! Use cases:
+//!  * In the trusted world, we want to generate a large file, sign it, and share the signature for
+//!    a third party to verify the file.
+//!  * In the trusted world, we want to read a previously signed file back with signature check
+//!    without having to touch the whole file.
+//!
+//! Requirements:
+//!  * Communication between trusted and untrusted world is not cheap, and files can be large.
+//!  * A file write pattern may not be sequential, neither does read.
+//!
+//! Considering the above, a technique similar to fs-verity is used. fs-verity uses an alternative
+//! hash function, a Merkle tree, to calculate the hash of file content. A file update at any
+//! location will propagate the hash update from the leaf to the root node. Unlike fs-verity, which
+//! assumes static files, to support write operation, we need to allow the file (thus tree) to
+//! update.
+//!
+//! For the trusted world to generate a large file with random write and hash it, the writer needs
+//! to hold some private information and update the Merkle tree during a file write (or even when
+//! the Merkle tree needs to be stashed to the untrusted storage).
+//!
+//! A write to a file must update the root hash. In order for the root hash to update, a tree
+//! walk to update from the write location to the root node is necessary. Importantly, in case when
+//! (part of) the Merkle tree needs to be read from the untrusted storage (e.g. not yet verified in
+//! cache), the original path must be verified by the trusted signature before the update to happen.
+//!
+//! Denial-of-service is a known weakness if the untrusted storage decides to simply remove the
+//! file. But there is nothing we can do in this architecture.
+//!
+//! Rollback attack is another possible attack, but can be addressed with a rollback counter when
+//! possible.
+
+use std::io;
+use std::sync::{Arc, RwLock};
+
+use super::builder::MerkleLeaves;
+use crate::common::{ChunkedSizeIter, CHUNK_SIZE};
+use crate::crypto::{CryptoError, Sha256Hash, Sha256Hasher};
+use crate::file::{RandomWrite, ReadOnlyDataByChunk};
+
+// Implement the conversion from `CryptoError` to `io::Error` just to avoid manual error type
+// mapping below.
+impl From<CryptoError> for io::Error {
+    fn from(error: CryptoError) -> Self {
+        io::Error::new(io::ErrorKind::Other, error)
+    }
+}
+
+/// VerifiedFileEditor provides an integrity layer to an underlying read-writable file, which may
+/// not be stored in a trusted environment. Only new, empty files are currently supported.
+pub struct VerifiedFileEditor<F: ReadOnlyDataByChunk + RandomWrite> {
+    file: F,
+    merkle_tree: Arc<RwLock<MerkleLeaves>>,
+}
+
+impl<F: ReadOnlyDataByChunk + RandomWrite> VerifiedFileEditor<F> {
+    /// Wraps a supposedly new file for integrity protection.
+    pub fn new(file: F) -> Self {
+        Self { file, merkle_tree: Arc::new(RwLock::new(MerkleLeaves::new())) }
+    }
+
+    /// Calculates the fs-verity digest of the current file.
+    #[allow(dead_code)]
+    pub fn calculate_fsverity_digest(&self) -> io::Result<Sha256Hash> {
+        let merkle_tree = self.merkle_tree.read().unwrap();
+        merkle_tree.calculate_fsverity_digest().map_err(|e| io::Error::new(io::ErrorKind::Other, e))
+    }
+
+    fn new_hash_for_incomplete_write(
+        &self,
+        source: &[u8],
+        offset_from_alignment: usize,
+        output_chunk_index: usize,
+        merkle_tree: &mut MerkleLeaves,
+    ) -> io::Result<Sha256Hash> {
+        // The buffer is initialized to 0 purposely. To calculate the block hash, the data is
+        // 0-padded to the block size. When a chunk read is less than a chunk, the initial value
+        // conveniently serves the padding purpose.
+        let mut orig_data = [0u8; CHUNK_SIZE as usize];
+
+        // If previous data exists, read back and verify against the known hash (since the
+        // storage / remote server is not trusted).
+        if merkle_tree.is_index_valid(output_chunk_index) {
+            self.read_chunk(output_chunk_index as u64, &mut orig_data)?;
+
+            // Verify original content
+            let hash = Sha256Hasher::new()?.update(&orig_data)?.finalize()?;
+            if !merkle_tree.is_consistent(output_chunk_index, &hash) {
+                return Err(io::Error::new(io::ErrorKind::InvalidData, "Inconsistent hash"));
+            }
+        }
+
+        Ok(Sha256Hasher::new()?
+            .update(&orig_data[..offset_from_alignment])?
+            .update(source)?
+            .update(&orig_data[offset_from_alignment + source.len()..])?
+            .finalize()?)
+    }
+
+    fn new_chunk_hash(
+        &self,
+        source: &[u8],
+        offset_from_alignment: usize,
+        current_size: usize,
+        output_chunk_index: usize,
+        merkle_tree: &mut MerkleLeaves,
+    ) -> io::Result<Sha256Hash> {
+        if current_size as u64 == CHUNK_SIZE {
+            // Case 1: If the chunk is a complete one, just calculate the hash, regardless of
+            // write location.
+            Ok(Sha256Hasher::new()?.update(source)?.finalize()?)
+        } else {
+            // Case 2: For an incomplete write, calculate the hash based on previous data (if
+            // any).
+            self.new_hash_for_incomplete_write(
+                source,
+                offset_from_alignment,
+                output_chunk_index,
+                merkle_tree,
+            )
+        }
+    }
+
+    pub fn size(&self) -> u64 {
+        self.merkle_tree.read().unwrap().file_size()
+    }
+}
+
+impl<F: ReadOnlyDataByChunk + RandomWrite> RandomWrite for VerifiedFileEditor<F> {
+    fn write_at(&self, buf: &[u8], offset: u64) -> io::Result<usize> {
+        // Since we don't need to support 32-bit CPU, make an assert to make conversion between
+        // u64 and usize easy below. Otherwise, we need to check `divide_roundup(offset + buf.len()
+        // <= usize::MAX` or handle `TryInto` errors.
+        debug_assert!(usize::MAX as u64 == u64::MAX, "Only 64-bit arch is supported");
+
+        // The write range may not be well-aligned with the chunk boundary. There are various cases
+        // to deal with:
+        //  1. A write of a full 4K chunk.
+        //  2. A write of an incomplete chunk, possibly beyond the original EOF.
+        //
+        // Note that a write beyond EOF can create a hole. But we don't need to handle it here
+        // because holes are zeros, and leaves in MerkleLeaves are hashes of 4096-zeros by
+        // default.
+
+        // Now iterate on the input data, considering the alignment at the destination.
+        for (output_offset, current_size) in
+            ChunkedSizeIter::new(buf.len(), offset, CHUNK_SIZE as usize)
+        {
+            // Lock the tree for the whole write for now. There may be room to improve to increase
+            // throughput.
+            let mut merkle_tree = self.merkle_tree.write().unwrap();
+
+            let offset_in_buf = (output_offset - offset) as usize;
+            let source = &buf[offset_in_buf as usize..offset_in_buf as usize + current_size];
+            let output_chunk_index = (output_offset / CHUNK_SIZE) as usize;
+            let offset_from_alignment = (output_offset % CHUNK_SIZE) as usize;
+
+            let new_hash = match self.new_chunk_hash(
+                source,
+                offset_from_alignment,
+                current_size,
+                output_chunk_index,
+                &mut merkle_tree,
+            ) {
+                Ok(hash) => hash,
+                Err(e) => {
+                    // Return early when any error happens before the right. Even if the hash is not
+                    // consistent for the current chunk, we can still consider the earlier writes
+                    // successful. Note that nothing persistent has been done in this iteration.
+                    let written = output_offset - offset;
+                    if written > 0 {
+                        return Ok(written as usize);
+                    }
+                    return Err(e);
+                }
+            };
+
+            // A failed, partial write here will make the backing file inconsistent to the (old)
+            // hash. Nothing can be done within this writer, but at least it still maintains the
+            // (original) integrity for the file. To matches what write(2) describes for an error
+            // case (though it's about direct I/O), "Partial data may be written ... should be
+            // considered inconsistent", an error below is propagated.
+            self.file.write_all_at(&source, output_offset)?;
+
+            // Update the hash only after the write succeeds. Note that this only attempts to keep
+            // the tree consistent to what has been written regardless the actual state beyond the
+            // writer.
+            let size_at_least = offset.saturating_add(buf.len() as u64);
+            merkle_tree.update_hash(output_chunk_index, &new_hash, size_at_least);
+        }
+        Ok(buf.len())
+    }
+}
+
+impl<F: ReadOnlyDataByChunk + RandomWrite> ReadOnlyDataByChunk for VerifiedFileEditor<F> {
+    fn read_chunk(&self, chunk_index: u64, buf: &mut [u8]) -> io::Result<usize> {
+        self.file.read_chunk(chunk_index, buf)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    // Test data below can be generated by:
+    //  $ perl -e 'print "\x{00}" x 6000' > foo
+    //  $ perl -e 'print "\x{01}" x 5000' >> foo
+    //  $ fsverity digest foo
+    use super::*;
+    use anyhow::Result;
+    use std::cell::RefCell;
+    use std::convert::TryInto;
+
+    struct InMemoryEditor {
+        data: RefCell<Vec<u8>>,
+        fail_read: bool,
+    }
+
+    impl InMemoryEditor {
+        pub fn new() -> InMemoryEditor {
+            InMemoryEditor { data: RefCell::new(Vec::new()), fail_read: false }
+        }
+    }
+
+    impl RandomWrite for InMemoryEditor {
+        fn write_at(&self, buf: &[u8], offset: u64) -> io::Result<usize> {
+            let begin: usize =
+                offset.try_into().map_err(|e| io::Error::new(io::ErrorKind::Other, e))?;
+            let end = begin + buf.len();
+            if end > self.data.borrow().len() {
+                self.data.borrow_mut().resize(end, 0);
+            }
+            self.data.borrow_mut().as_mut_slice()[begin..end].copy_from_slice(&buf);
+            Ok(buf.len())
+        }
+    }
+
+    impl ReadOnlyDataByChunk for InMemoryEditor {
+        fn read_chunk(&self, chunk_index: u64, buf: &mut [u8]) -> io::Result<usize> {
+            debug_assert!(buf.len() as u64 >= CHUNK_SIZE);
+
+            if self.fail_read {
+                return Err(io::Error::new(io::ErrorKind::Other, "test!"));
+            }
+
+            let borrowed = self.data.borrow();
+            let chunk = &borrowed
+                .chunks(CHUNK_SIZE as usize)
+                .nth(chunk_index as usize)
+                .ok_or_else(|| {
+                    io::Error::new(
+                        io::ErrorKind::InvalidInput,
+                        format!("read_chunk out of bound: index {}", chunk_index),
+                    )
+                })?;
+            buf[..chunk.len()].copy_from_slice(&chunk);
+            Ok(chunk.len())
+        }
+    }
+
+    #[test]
+    fn test_writer() -> Result<()> {
+        let writer = InMemoryEditor::new();
+        let buf = [1; 4096];
+        assert_eq!(writer.data.borrow().len(), 0);
+
+        assert_eq!(writer.write_at(&buf, 16384)?, 4096);
+        assert_eq!(writer.data.borrow()[16384..16384 + 4096], buf);
+
+        assert_eq!(writer.write_at(&buf, 2048)?, 4096);
+        assert_eq!(writer.data.borrow()[2048..2048 + 4096], buf);
+
+        assert_eq!(writer.data.borrow().len(), 16384 + 4096);
+        Ok(())
+    }
+
+    #[test]
+    fn test_verified_writer_no_write() -> Result<()> {
+        // Verify fs-verity hash without any write.
+        let file = VerifiedFileEditor::new(InMemoryEditor::new());
+        assert_eq!(
+            file.calculate_fsverity_digest()?,
+            to_u8_vec("3d248ca542a24fc62d1c43b916eae5016878e2533c88238480b26128a1f1af95")
+                .as_slice()
+        );
+        Ok(())
+    }
+
+    #[test]
+    fn test_verified_writer_from_zero() -> Result<()> {
+        // Verify a write of a full chunk.
+        let file = VerifiedFileEditor::new(InMemoryEditor::new());
+        assert_eq!(file.write_at(&[1; 4096], 0)?, 4096);
+        assert_eq!(
+            file.calculate_fsverity_digest()?,
+            to_u8_vec("cd0875ca59c7d37e962c5e8f5acd3770750ac80225e2df652ce5672fd34500af")
+                .as_slice()
+        );
+
+        // Verify a write of across multiple chunks.
+        let file = VerifiedFileEditor::new(InMemoryEditor::new());
+        assert_eq!(file.write_at(&[1; 4097], 0)?, 4097);
+        assert_eq!(
+            file.calculate_fsverity_digest()?,
+            to_u8_vec("2901b849fda2d91e3929524561c4a47e77bb64734319759507b2029f18b9cc52")
+                .as_slice()
+        );
+
+        // Verify another write of across multiple chunks.
+        let file = VerifiedFileEditor::new(InMemoryEditor::new());
+        assert_eq!(file.write_at(&[1; 10000], 0)?, 10000);
+        assert_eq!(
+            file.calculate_fsverity_digest()?,
+            to_u8_vec("7545409b556071554d18973a29b96409588c7cda4edd00d5586b27a11e1a523b")
+                .as_slice()
+        );
+        Ok(())
+    }
+
+    #[test]
+    fn test_verified_writer_unaligned() -> Result<()> {
+        // Verify small, unaligned write beyond EOF.
+        let file = VerifiedFileEditor::new(InMemoryEditor::new());
+        assert_eq!(file.write_at(&[1; 5], 3)?, 5);
+        assert_eq!(
+            file.calculate_fsverity_digest()?,
+            to_u8_vec("a23fc5130d3d7b3323fc4b4a5e79d5d3e9ddf3a3f5872639e867713512c6702f")
+                .as_slice()
+        );
+
+        // Verify bigger, unaligned write beyond EOF.
+        let file = VerifiedFileEditor::new(InMemoryEditor::new());
+        assert_eq!(file.write_at(&[1; 6000], 4000)?, 6000);
+        assert_eq!(
+            file.calculate_fsverity_digest()?,
+            to_u8_vec("d16d4c1c186d757e646f76208b21254f50d7f07ea07b1505ff48b2a6f603f989")
+                .as_slice()
+        );
+        Ok(())
+    }
+
+    #[test]
+    fn test_verified_writer_with_hole() -> Result<()> {
+        // Verify an aligned write beyond EOF with holes.
+        let file = VerifiedFileEditor::new(InMemoryEditor::new());
+        assert_eq!(file.write_at(&[1; 4096], 4096)?, 4096);
+        assert_eq!(
+            file.calculate_fsverity_digest()?,
+            to_u8_vec("4df2aefd8c2a9101d1d8770dca3ede418232eabce766bb8e020395eae2e97103")
+                .as_slice()
+        );
+
+        // Verify an unaligned write beyond EOF with holes.
+        let file = VerifiedFileEditor::new(InMemoryEditor::new());
+        assert_eq!(file.write_at(&[1; 5000], 6000)?, 5000);
+        assert_eq!(
+            file.calculate_fsverity_digest()?,
+            to_u8_vec("47d5da26f6934484e260630a69eb2eebb21b48f69bc8fbf8486d1694b7dba94f")
+                .as_slice()
+        );
+
+        // Just another example with a small write.
+        let file = VerifiedFileEditor::new(InMemoryEditor::new());
+        assert_eq!(file.write_at(&[1; 5], 16381)?, 5);
+        assert_eq!(
+            file.calculate_fsverity_digest()?,
+            to_u8_vec("8bd118821fb4aff26bb4b51d485cc481a093c68131b7f4f112e9546198449752")
+                .as_slice()
+        );
+        Ok(())
+    }
+
+    #[test]
+    fn test_verified_writer_various_writes() -> Result<()> {
+        let file = VerifiedFileEditor::new(InMemoryEditor::new());
+        assert_eq!(file.write_at(&[1; 2048], 0)?, 2048);
+        assert_eq!(file.write_at(&[1; 2048], 4096 + 2048)?, 2048);
+        assert_eq!(
+            file.calculate_fsverity_digest()?,
+            to_u8_vec("4c433d8640c888b629dc673d318cbb8d93b1eebcc784d9353e07f09f0dcfe707")
+                .as_slice()
+        );
+        assert_eq!(file.write_at(&[1; 2048], 2048)?, 2048);
+        assert_eq!(file.write_at(&[1; 2048], 4096)?, 2048);
+        assert_eq!(
+            file.calculate_fsverity_digest()?,
+            to_u8_vec("2a476d58eb80394052a3a783111e1458ac3ecf68a7878183fed86ca0ff47ec0d")
+                .as_slice()
+        );
+        assert_eq!(file.write_at(&[0; 2048], 2048)?, 2048);
+        assert_eq!(file.write_at(&[0; 2048], 4096)?, 2048);
+        assert_eq!(
+            file.calculate_fsverity_digest()?,
+            to_u8_vec("4c433d8640c888b629dc673d318cbb8d93b1eebcc784d9353e07f09f0dcfe707")
+                .as_slice()
+        );
+        assert_eq!(file.write_at(&[1; 4096], 2048)?, 4096);
+        assert_eq!(
+            file.calculate_fsverity_digest()?,
+            to_u8_vec("2a476d58eb80394052a3a783111e1458ac3ecf68a7878183fed86ca0ff47ec0d")
+                .as_slice()
+        );
+        assert_eq!(file.write_at(&[1; 2048], 8192)?, 2048);
+        assert_eq!(file.write_at(&[1; 2048], 8192 + 2048)?, 2048);
+        assert_eq!(
+            file.calculate_fsverity_digest()?,
+            to_u8_vec("23cbac08371e6ee838ebcc7ae6512b939d2226e802337be7b383c3e046047d24")
+                .as_slice()
+        );
+        Ok(())
+    }
+
+    #[test]
+    fn test_verified_writer_inconsistent_read() -> Result<()> {
+        let file = VerifiedFileEditor::new(InMemoryEditor::new());
+        assert_eq!(file.write_at(&[1; 8192], 0)?, 8192);
+
+        // Replace the expected hash of the first/0-th chunk. An incomplete write will fail when it
+        // detects the inconsistent read.
+        {
+            let mut merkle_tree = file.merkle_tree.write().unwrap();
+            let overriding_hash = [42; Sha256Hasher::HASH_SIZE];
+            merkle_tree.update_hash(0, &overriding_hash, 8192);
+        }
+        assert!(file.write_at(&[1; 1], 2048).is_err());
+
+        // A write of full chunk can still succeed. Also fixed the inconsistency.
+        assert_eq!(file.write_at(&[1; 4096], 4096)?, 4096);
+
+        // Replace the expected hash of the second/1-th chunk. A write range from previous chunk can
+        // still succeed, but returns early due to an inconsistent read but still successfully. A
+        // resumed write will fail since no bytes can be written due to the same inconsistency.
+        {
+            let mut merkle_tree = file.merkle_tree.write().unwrap();
+            let overriding_hash = [42; Sha256Hasher::HASH_SIZE];
+            merkle_tree.update_hash(1, &overriding_hash, 8192);
+        }
+        assert_eq!(file.write_at(&[10; 8000], 0)?, 4096);
+        assert!(file.write_at(&[10; 8000 - 4096], 4096).is_err());
+        Ok(())
+    }
+
+    #[test]
+    fn test_verified_writer_failed_read_back() -> Result<()> {
+        let mut writer = InMemoryEditor::new();
+        writer.fail_read = true;
+        let file = VerifiedFileEditor::new(writer);
+        assert_eq!(file.write_at(&[1; 8192], 0)?, 8192);
+
+        // When a read back is needed, a read failure will fail to write.
+        assert!(file.write_at(&[1; 1], 2048).is_err());
+        Ok(())
+    }
+
+    fn to_u8_vec(hex_str: &str) -> Vec<u8> {
+        assert!(hex_str.len() % 2 == 0);
+        (0..hex_str.len())
+            .step_by(2)
+            .map(|i| u8::from_str_radix(&hex_str[i..i + 2], 16).unwrap())
+            .collect()
+    }
+}
diff --git a/authfs/src/fsverity/verifier.rs b/authfs/src/fsverity/verifier.rs
index fd108f5..4021ce1 100644
--- a/authfs/src/fsverity/verifier.rs
+++ b/authfs/src/fsverity/verifier.rs
@@ -22,7 +22,7 @@
 use crate::auth::Authenticator;
 use crate::common::{divide_roundup, CHUNK_SIZE};
 use crate::crypto::{CryptoError, Sha256Hasher};
-use crate::reader::ReadOnlyDataByChunk;
+use crate::file::ReadOnlyDataByChunk;
 
 const ZEROS: [u8; CHUNK_SIZE as usize] = [0u8; CHUNK_SIZE as usize];
 
@@ -125,21 +125,21 @@
     Ok(formatted_digest)
 }
 
-pub struct FsverityChunkedFileReader<F: ReadOnlyDataByChunk, M: ReadOnlyDataByChunk> {
+pub struct VerifiedFileReader<F: ReadOnlyDataByChunk, M: ReadOnlyDataByChunk> {
     chunked_file: F,
     file_size: u64,
     merkle_tree: M,
     root_hash: HashBuffer,
 }
 
-impl<F: ReadOnlyDataByChunk, M: ReadOnlyDataByChunk> FsverityChunkedFileReader<F, M> {
+impl<F: ReadOnlyDataByChunk, M: ReadOnlyDataByChunk> VerifiedFileReader<F, M> {
     pub fn new<A: Authenticator>(
         authenticator: &A,
         chunked_file: F,
         file_size: u64,
         sig: Vec<u8>,
         merkle_tree: M,
-    ) -> Result<FsverityChunkedFileReader<F, M>, FsverityError> {
+    ) -> Result<VerifiedFileReader<F, M>, FsverityError> {
         let mut buf = [0u8; CHUNK_SIZE as usize];
         let size = merkle_tree.read_chunk(0, &mut buf)?;
         if buf.len() != size {
@@ -149,7 +149,7 @@
         let formatted_digest = build_fsverity_formatted_digest(&root_hash, file_size)?;
         let valid = authenticator.verify(&sig, &formatted_digest)?;
         if valid {
-            Ok(FsverityChunkedFileReader { chunked_file, file_size, merkle_tree, root_hash })
+            Ok(VerifiedFileReader { chunked_file, file_size, merkle_tree, root_hash })
         } else {
             Err(FsverityError::BadSignature)
         }
@@ -157,7 +157,7 @@
 }
 
 impl<F: ReadOnlyDataByChunk, M: ReadOnlyDataByChunk> ReadOnlyDataByChunk
-    for FsverityChunkedFileReader<F, M>
+    for VerifiedFileReader<F, M>
 {
     fn read_chunk(&self, chunk_index: u64, buf: &mut [u8]) -> io::Result<usize> {
         debug_assert!(buf.len() as u64 >= CHUNK_SIZE);
@@ -176,13 +176,12 @@
 mod tests {
     use super::*;
     use crate::auth::FakeAuthenticator;
-    use crate::reader::{ChunkedFileReader, ReadOnlyDataByChunk};
+    use crate::file::{LocalFileReader, ReadOnlyDataByChunk};
     use anyhow::Result;
     use std::fs::File;
     use std::io::Read;
 
-    type LocalFsverityChunkedFileReader =
-        FsverityChunkedFileReader<ChunkedFileReader, ChunkedFileReader>;
+    type LocalVerifiedFileReader = VerifiedFileReader<LocalFileReader, LocalFileReader>;
 
     fn total_chunk_number(file_size: u64) -> u64 {
         (file_size + 4095) / 4096
@@ -193,21 +192,15 @@
         content_path: &str,
         merkle_tree_path: &str,
         signature_path: &str,
-    ) -> Result<(LocalFsverityChunkedFileReader, u64)> {
-        let file_reader = ChunkedFileReader::new(File::open(content_path)?)?;
+    ) -> Result<(LocalVerifiedFileReader, u64)> {
+        let file_reader = LocalFileReader::new(File::open(content_path)?)?;
         let file_size = file_reader.len();
-        let merkle_tree = ChunkedFileReader::new(File::open(merkle_tree_path)?)?;
+        let merkle_tree = LocalFileReader::new(File::open(merkle_tree_path)?)?;
         let mut sig = Vec::new();
         let _ = File::open(signature_path)?.read_to_end(&mut sig)?;
         let authenticator = FakeAuthenticator::always_succeed();
         Ok((
-            FsverityChunkedFileReader::new(
-                &authenticator,
-                file_reader,
-                file_size,
-                sig,
-                merkle_tree,
-            )?,
+            VerifiedFileReader::new(&authenticator, file_reader, file_size, sig, merkle_tree)?,
             file_size,
         ))
     }
@@ -280,18 +273,12 @@
     #[test]
     fn invalid_signature() -> Result<()> {
         let authenticator = FakeAuthenticator::always_fail();
-        let file_reader = ChunkedFileReader::new(File::open("testdata/input.4m")?)?;
+        let file_reader = LocalFileReader::new(File::open("testdata/input.4m")?)?;
         let file_size = file_reader.len();
-        let merkle_tree = ChunkedFileReader::new(File::open("testdata/input.4m.merkle_dump")?)?;
+        let merkle_tree = LocalFileReader::new(File::open("testdata/input.4m.merkle_dump")?)?;
         let sig = include_bytes!("../../testdata/input.4m.fsv_sig").to_vec();
-        assert!(FsverityChunkedFileReader::new(
-            &authenticator,
-            file_reader,
-            file_size,
-            sig,
-            merkle_tree
-        )
-        .is_err());
+        assert!(VerifiedFileReader::new(&authenticator, file_reader, file_size, sig, merkle_tree)
+            .is_err());
         Ok(())
     }
 }
diff --git a/authfs/src/fusefs.rs b/authfs/src/fusefs.rs
index f5dd6ec..13ec87d 100644
--- a/authfs/src/fusefs.rs
+++ b/authfs/src/fusefs.rs
@@ -26,31 +26,30 @@
 use std::path::Path;
 use std::time::Duration;
 
-use fuse::filesystem::{Context, DirEntry, DirectoryIterator, Entry, FileSystem, ZeroCopyWriter};
+use fuse::filesystem::{
+    Context, DirEntry, DirectoryIterator, Entry, FileSystem, FsOptions, ZeroCopyReader,
+    ZeroCopyWriter,
+};
 use fuse::mount::MountOption;
 
-use crate::common::{divide_roundup, CHUNK_SIZE};
-use crate::fsverity::FsverityChunkedFileReader;
-use crate::reader::{ChunkedFileReader, ReadOnlyDataByChunk};
-use crate::remote_file::{RemoteChunkedFileReader, RemoteFsverityMerkleTreeReader};
+use crate::common::{divide_roundup, ChunkedSizeIter, CHUNK_SIZE};
+use crate::file::{
+    LocalFileReader, RandomWrite, ReadOnlyDataByChunk, RemoteFileEditor, RemoteFileReader,
+    RemoteMerkleTreeReader,
+};
+use crate::fsverity::{VerifiedFileEditor, VerifiedFileReader};
 
 const DEFAULT_METADATA_TIMEOUT: std::time::Duration = Duration::from_secs(5);
 
 pub type Inode = u64;
 type Handle = u64;
 
-type RemoteFsverityChunkedFileReader =
-    FsverityChunkedFileReader<RemoteChunkedFileReader, RemoteFsverityMerkleTreeReader>;
-
-// A debug only type where everything are stored as local files.
-type FileBackedFsverityChunkedFileReader =
-    FsverityChunkedFileReader<ChunkedFileReader, ChunkedFileReader>;
-
 pub enum FileConfig {
-    LocalVerifiedFile(FileBackedFsverityChunkedFileReader, u64),
-    LocalUnverifiedFile(ChunkedFileReader, u64),
-    RemoteVerifiedFile(RemoteFsverityChunkedFileReader, u64),
-    RemoteUnverifiedFile(RemoteChunkedFileReader, u64),
+    LocalVerifiedReadonlyFile(VerifiedFileReader<LocalFileReader, LocalFileReader>, u64),
+    LocalUnverifiedReadonlyFile(LocalFileReader, u64),
+    RemoteVerifiedReadonlyFile(VerifiedFileReader<RemoteFileReader, RemoteMerkleTreeReader>, u64),
+    RemoteUnverifiedReadonlyFile(RemoteFileReader, u64),
+    RemoteVerifiedNewFile(VerifiedFileEditor<RemoteFileEditor>),
 }
 
 struct AuthFs {
@@ -92,11 +91,20 @@
     }
 }
 
-fn create_stat(ino: libc::ino_t, file_size: u64) -> io::Result<libc::stat64> {
+enum FileMode {
+    ReadOnly,
+    ReadWrite,
+}
+
+fn create_stat(ino: libc::ino_t, file_size: u64, file_mode: FileMode) -> io::Result<libc::stat64> {
     let mut st = unsafe { MaybeUninit::<libc::stat64>::zeroed().assume_init() };
 
     st.st_ino = ino;
-    st.st_mode = libc::S_IFREG | libc::S_IRUSR | libc::S_IRGRP | libc::S_IROTH;
+    st.st_mode = match file_mode {
+        // Until needed, let's just grant the owner access.
+        FileMode::ReadOnly => libc::S_IFREG | libc::S_IRUSR,
+        FileMode::ReadWrite => libc::S_IFREG | libc::S_IRUSR | libc::S_IWUSR,
+    };
     st.st_dev = 0;
     st.st_nlink = 1;
     st.st_uid = 0;
@@ -111,35 +119,6 @@
     Ok(st)
 }
 
-/// An iterator that generates (offset, size) for a chunked read operation, where offset is the
-/// global file offset, and size is the amount of read from the offset.
-struct ChunkReadIter {
-    remaining: usize,
-    offset: u64,
-}
-
-impl ChunkReadIter {
-    pub fn new(remaining: usize, offset: u64) -> Self {
-        ChunkReadIter { remaining, offset }
-    }
-}
-
-impl Iterator for ChunkReadIter {
-    type Item = (u64, usize);
-
-    fn next(&mut self) -> Option<Self::Item> {
-        if self.remaining == 0 {
-            return None;
-        }
-        let chunk_data_size =
-            std::cmp::min(self.remaining, (CHUNK_SIZE - self.offset % CHUNK_SIZE) as usize);
-        let retval = (self.offset, chunk_data_size);
-        self.offset += chunk_data_size as u64;
-        self.remaining = self.remaining.saturating_sub(chunk_data_size);
-        Some(retval)
-    }
-}
-
 fn offset_to_chunk_index(offset: u64) -> u64 {
     offset / CHUNK_SIZE
 }
@@ -153,7 +132,7 @@
 ) -> io::Result<usize> {
     let remaining = file_size.saturating_sub(offset);
     let size_to_read = std::cmp::min(size as usize, remaining as usize);
-    let total = ChunkReadIter::new(size_to_read, offset).try_fold(
+    let total = ChunkedSizeIter::new(size_to_read, offset, CHUNK_SIZE as usize).try_fold(
         0,
         |total, (current_offset, planned_data_size)| {
             // TODO(victorhsieh): There might be a non-trivial way to avoid this copy. For example,
@@ -197,6 +176,12 @@
         self.max_write
     }
 
+    fn init(&self, _capable: FsOptions) -> io::Result<FsOptions> {
+        // Enable writeback cache for better performance especially since our bandwidth to the
+        // backend service is limited.
+        Ok(FsOptions::WRITEBACK_CACHE)
+    }
+
     fn lookup(&self, _ctx: Context, _parent: Inode, name: &CStr) -> io::Result<Entry> {
         // Only accept file name that looks like an integrer. Files in the pool are simply exposed
         // by their inode number. Also, there is currently no directory structure.
@@ -206,10 +191,15 @@
         // be static.
         let inode = num.parse::<Inode>().map_err(|_| io::Error::from_raw_os_error(libc::ENOENT))?;
         let st = match self.get_file_config(&inode)? {
-            FileConfig::LocalVerifiedFile(_, file_size)
-            | FileConfig::LocalUnverifiedFile(_, file_size)
-            | FileConfig::RemoteUnverifiedFile(_, file_size)
-            | FileConfig::RemoteVerifiedFile(_, file_size) => create_stat(inode, *file_size)?,
+            FileConfig::LocalVerifiedReadonlyFile(_, file_size)
+            | FileConfig::LocalUnverifiedReadonlyFile(_, file_size)
+            | FileConfig::RemoteUnverifiedReadonlyFile(_, file_size)
+            | FileConfig::RemoteVerifiedReadonlyFile(_, file_size) => {
+                create_stat(inode, *file_size, FileMode::ReadOnly)?
+            }
+            FileConfig::RemoteVerifiedNewFile(file) => {
+                create_stat(inode, file.size(), FileMode::ReadWrite)?
+            }
         };
         Ok(Entry {
             inode,
@@ -228,10 +218,15 @@
     ) -> io::Result<(libc::stat64, Duration)> {
         Ok((
             match self.get_file_config(&inode)? {
-                FileConfig::LocalVerifiedFile(_, file_size)
-                | FileConfig::LocalUnverifiedFile(_, file_size)
-                | FileConfig::RemoteUnverifiedFile(_, file_size)
-                | FileConfig::RemoteVerifiedFile(_, file_size) => create_stat(inode, *file_size)?,
+                FileConfig::LocalVerifiedReadonlyFile(_, file_size)
+                | FileConfig::LocalUnverifiedReadonlyFile(_, file_size)
+                | FileConfig::RemoteUnverifiedReadonlyFile(_, file_size)
+                | FileConfig::RemoteVerifiedReadonlyFile(_, file_size) => {
+                    create_stat(inode, *file_size, FileMode::ReadOnly)?
+                }
+                FileConfig::RemoteVerifiedNewFile(file) => {
+                    create_stat(inode, file.size(), FileMode::ReadWrite)?
+                }
             },
             DEFAULT_METADATA_TIMEOUT,
         ))
@@ -244,21 +239,29 @@
         flags: u32,
     ) -> io::Result<(Option<Self::Handle>, fuse::sys::OpenOptions)> {
         // Since file handle is not really used in later operations (which use Inode directly),
-        // return None as the handle..
+        // return None as the handle.
         match self.get_file_config(&inode)? {
-            FileConfig::LocalVerifiedFile(_, _) | FileConfig::RemoteVerifiedFile(_, _) => {
+            FileConfig::LocalVerifiedReadonlyFile(_, _)
+            | FileConfig::RemoteVerifiedReadonlyFile(_, _) => {
                 check_access_mode(flags, libc::O_RDONLY)?;
                 // Once verified, and only if verified, the file content can be cached. This is not
-                // really needed for a local file, but is the behavior of RemoteVerifiedFile later.
+                // really needed for a local file, but is the behavior of RemoteVerifiedReadonlyFile
+                // later.
                 Ok((None, fuse::sys::OpenOptions::KEEP_CACHE))
             }
-            FileConfig::LocalUnverifiedFile(_, _) | FileConfig::RemoteUnverifiedFile(_, _) => {
+            FileConfig::LocalUnverifiedReadonlyFile(_, _)
+            | FileConfig::RemoteUnverifiedReadonlyFile(_, _) => {
                 check_access_mode(flags, libc::O_RDONLY)?;
                 // Do not cache the content. This type of file is supposed to be verified using
                 // dm-verity. The filesystem mount over dm-verity already is already cached, so use
                 // direct I/O here to avoid double cache.
                 Ok((None, fuse::sys::OpenOptions::DIRECT_IO))
             }
+            FileConfig::RemoteVerifiedNewFile(_) => {
+                // No need to check access modes since all the modes are allowed to the
+                // read-writable file.
+                Ok((None, fuse::sys::OpenOptions::KEEP_CACHE))
+            }
         }
     }
 
@@ -274,18 +277,45 @@
         _flags: u32,
     ) -> io::Result<usize> {
         match self.get_file_config(&inode)? {
-            FileConfig::LocalVerifiedFile(file, file_size) => {
+            FileConfig::LocalVerifiedReadonlyFile(file, file_size) => {
                 read_chunks(w, file, *file_size, offset, size)
             }
-            FileConfig::LocalUnverifiedFile(file, file_size) => {
+            FileConfig::LocalUnverifiedReadonlyFile(file, file_size) => {
                 read_chunks(w, file, *file_size, offset, size)
             }
-            FileConfig::RemoteVerifiedFile(file, file_size) => {
+            FileConfig::RemoteVerifiedReadonlyFile(file, file_size) => {
                 read_chunks(w, file, *file_size, offset, size)
             }
-            FileConfig::RemoteUnverifiedFile(file, file_size) => {
+            FileConfig::RemoteUnverifiedReadonlyFile(file, file_size) => {
                 read_chunks(w, file, *file_size, offset, size)
             }
+            FileConfig::RemoteVerifiedNewFile(file) => {
+                // Note that with FsOptions::WRITEBACK_CACHE, it's possible for the kernel to
+                // request a read even if the file is open with O_WRONLY.
+                read_chunks(w, file, file.size(), offset, size)
+            }
+        }
+    }
+
+    fn write<R: io::Read + ZeroCopyReader>(
+        &self,
+        _ctx: Context,
+        inode: Self::Inode,
+        _handle: Self::Handle,
+        mut r: R,
+        size: u32,
+        offset: u64,
+        _lock_owner: Option<u64>,
+        _delayed_write: bool,
+        _flags: u32,
+    ) -> io::Result<usize> {
+        match self.get_file_config(&inode)? {
+            FileConfig::RemoteVerifiedNewFile(file) => {
+                let mut buf = vec![0; size as usize];
+                r.read_exact(&mut buf)?;
+                file.write_at(&buf, offset)
+            }
+            _ => Err(io::Error::from_raw_os_error(libc::EBADF)),
         }
     }
 }
@@ -325,27 +355,3 @@
         AuthFs::new(file_pool, max_write),
     )
 }
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-
-    fn collect_chunk_read_iter(remaining: usize, offset: u64) -> Vec<(u64, usize)> {
-        ChunkReadIter::new(remaining, offset).collect::<Vec<_>>()
-    }
-
-    #[test]
-    fn test_chunk_read_iter() {
-        assert_eq!(collect_chunk_read_iter(4096, 0), [(0, 4096)]);
-        assert_eq!(collect_chunk_read_iter(8192, 0), [(0, 4096), (4096, 4096)]);
-        assert_eq!(collect_chunk_read_iter(8192, 4096), [(4096, 4096), (8192, 4096)]);
-
-        assert_eq!(
-            collect_chunk_read_iter(16384, 1),
-            [(1, 4095), (4096, 4096), (8192, 4096), (12288, 4096), (16384, 1)]
-        );
-
-        assert_eq!(collect_chunk_read_iter(0, 0), []);
-        assert_eq!(collect_chunk_read_iter(0, 100), []);
-    }
-}
diff --git a/authfs/src/main.rs b/authfs/src/main.rs
index 41b922d..a4b0d40 100644
--- a/authfs/src/main.rs
+++ b/authfs/src/main.rs
@@ -38,16 +38,14 @@
 mod auth;
 mod common;
 mod crypto;
+mod file;
 mod fsverity;
 mod fusefs;
-mod reader;
-mod remote_file;
 
 use auth::FakeAuthenticator;
-use fsverity::FsverityChunkedFileReader;
+use file::{LocalFileReader, RemoteFileEditor, RemoteFileReader, RemoteMerkleTreeReader};
+use fsverity::{VerifiedFileEditor, VerifiedFileReader};
 use fusefs::{FileConfig, Inode};
-use reader::ChunkedFileReader;
-use remote_file::{RemoteChunkedFileReader, RemoteFsverityMerkleTreeReader};
 
 #[derive(StructOpt)]
 struct Args {
@@ -55,31 +53,38 @@
     #[structopt(parse(from_os_str))]
     mount_point: PathBuf,
 
-    /// A verifiable read-only file. Can be multiple.
+    /// A read-only remote file with integrity check. Can be multiple.
     ///
     /// For example, `--remote-verified-file 5:10:1234:/path/to/cert` tells the filesystem to
     /// associate entry 5 with a remote file 10 of size 1234 bytes, and need to be verified against
     /// the /path/to/cert.
-    #[structopt(long, parse(try_from_str = parse_remote_verified_file_option))]
-    remote_verified_file: Vec<RemoteVerifiedFileConfig>,
+    #[structopt(long, parse(try_from_str = parse_remote_ro_file_option))]
+    remote_ro_file: Vec<OptionRemoteRoFile>,
 
-    /// An unverifiable read-only file. Can be multiple.
+    /// A read-only remote file without integrity check. Can be multiple.
     ///
     /// For example, `--remote-unverified-file 5:10:1234` tells the filesystem to associate entry 5
     /// with a remote file 10 of size 1234 bytes.
-    #[structopt(long, parse(try_from_str = parse_remote_unverified_file_option))]
-    remote_unverified_file: Vec<RemoteUnverifiedFileConfig>,
+    #[structopt(long, parse(try_from_str = parse_remote_ro_file_unverified_option))]
+    remote_ro_file_unverified: Vec<OptionRemoteRoFileUnverified>,
 
-    /// Debug only. A readonly file to be protected by fs-verity. Can be multiple.
-    #[structopt(long, parse(try_from_str = parse_local_verified_file_option))]
-    local_verified_file: Vec<LocalVerifiedFileConfig>,
+    /// A new read-writable remote file with integrity check. Can be multiple.
+    ///
+    /// For example, `--remote-new-verified-file 12:34` tells the filesystem to associate entry 12
+    /// with a remote file 34.
+    #[structopt(long, parse(try_from_str = parse_remote_new_rw_file_option))]
+    remote_new_rw_file: Vec<OptionRemoteRwFile>,
 
-    /// Debug only. An unverified read-only file. Can be multiple.
-    #[structopt(long, parse(try_from_str = parse_local_unverified_file_option))]
-    local_unverified_file: Vec<LocalUnverifiedFileConfig>,
+    /// Debug only. A read-only local file with integrity check. Can be multiple.
+    #[structopt(long, parse(try_from_str = parse_local_file_ro_option))]
+    local_ro_file: Vec<OptionLocalFileRo>,
+
+    /// Debug only. A read-only local file without integrity check. Can be multiple.
+    #[structopt(long, parse(try_from_str = parse_local_ro_file_unverified_ro_option))]
+    local_ro_file_unverified: Vec<OptionLocalRoFileUnverified>,
 }
 
-struct RemoteVerifiedFileConfig {
+struct OptionRemoteRoFile {
     ino: Inode,
 
     /// ID to refer to the remote file.
@@ -94,7 +99,7 @@
     _certificate_path: PathBuf,
 }
 
-struct RemoteUnverifiedFileConfig {
+struct OptionRemoteRoFileUnverified {
     ino: Inode,
 
     /// ID to refer to the remote file.
@@ -104,7 +109,14 @@
     file_size: u64,
 }
 
-struct LocalVerifiedFileConfig {
+struct OptionRemoteRwFile {
+    ino: Inode,
+
+    /// ID to refer to the remote file.
+    remote_id: i32,
+}
+
+struct OptionLocalFileRo {
     ino: Inode,
 
     /// Local path of the backing file.
@@ -121,19 +133,19 @@
     _certificate_path: PathBuf,
 }
 
-struct LocalUnverifiedFileConfig {
+struct OptionLocalRoFileUnverified {
     ino: Inode,
 
     /// Local path of the backing file.
     file_path: PathBuf,
 }
 
-fn parse_remote_verified_file_option(option: &str) -> Result<RemoteVerifiedFileConfig> {
+fn parse_remote_ro_file_option(option: &str) -> Result<OptionRemoteRoFile> {
     let strs: Vec<&str> = option.split(':').collect();
     if strs.len() != 4 {
         bail!("Invalid option: {}", option);
     }
-    Ok(RemoteVerifiedFileConfig {
+    Ok(OptionRemoteRoFile {
         ino: strs[0].parse::<Inode>()?,
         remote_id: strs[1].parse::<i32>()?,
         file_size: strs[2].parse::<u64>()?,
@@ -141,24 +153,35 @@
     })
 }
 
-fn parse_remote_unverified_file_option(option: &str) -> Result<RemoteUnverifiedFileConfig> {
+fn parse_remote_ro_file_unverified_option(option: &str) -> Result<OptionRemoteRoFileUnverified> {
     let strs: Vec<&str> = option.split(':').collect();
     if strs.len() != 3 {
         bail!("Invalid option: {}", option);
     }
-    Ok(RemoteUnverifiedFileConfig {
+    Ok(OptionRemoteRoFileUnverified {
         ino: strs[0].parse::<Inode>()?,
         remote_id: strs[1].parse::<i32>()?,
         file_size: strs[2].parse::<u64>()?,
     })
 }
 
-fn parse_local_verified_file_option(option: &str) -> Result<LocalVerifiedFileConfig> {
+fn parse_remote_new_rw_file_option(option: &str) -> Result<OptionRemoteRwFile> {
+    let strs: Vec<&str> = option.split(':').collect();
+    if strs.len() != 2 {
+        bail!("Invalid option: {}", option);
+    }
+    Ok(OptionRemoteRwFile {
+        ino: strs[0].parse::<Inode>().unwrap(),
+        remote_id: strs[1].parse::<i32>().unwrap(),
+    })
+}
+
+fn parse_local_file_ro_option(option: &str) -> Result<OptionLocalFileRo> {
     let strs: Vec<&str> = option.split(':').collect();
     if strs.len() != 5 {
         bail!("Invalid option: {}", option);
     }
-    Ok(LocalVerifiedFileConfig {
+    Ok(OptionLocalFileRo {
         ino: strs[0].parse::<Inode>()?,
         file_path: PathBuf::from(strs[1]),
         merkle_tree_dump_path: PathBuf::from(strs[2]),
@@ -167,92 +190,95 @@
     })
 }
 
-fn parse_local_unverified_file_option(option: &str) -> Result<LocalUnverifiedFileConfig> {
+fn parse_local_ro_file_unverified_ro_option(option: &str) -> Result<OptionLocalRoFileUnverified> {
     let strs: Vec<&str> = option.split(':').collect();
     if strs.len() != 2 {
         bail!("Invalid option: {}", option);
     }
-    Ok(LocalUnverifiedFileConfig {
+    Ok(OptionLocalRoFileUnverified {
         ino: strs[0].parse::<Inode>()?,
         file_path: PathBuf::from(strs[1]),
     })
 }
 
 fn new_config_remote_verified_file(remote_id: i32, file_size: u64) -> Result<FileConfig> {
-    let service = remote_file::server::get_local_service();
+    let service = file::get_local_binder();
     let signature = service.readFsveritySignature(remote_id).context("Failed to read signature")?;
 
     let service = Arc::new(Mutex::new(service));
     let authenticator = FakeAuthenticator::always_succeed();
-    Ok(FileConfig::RemoteVerifiedFile(
-        FsverityChunkedFileReader::new(
+    Ok(FileConfig::RemoteVerifiedReadonlyFile(
+        VerifiedFileReader::new(
             &authenticator,
-            RemoteChunkedFileReader::new(Arc::clone(&service), remote_id),
+            RemoteFileReader::new(Arc::clone(&service), remote_id),
             file_size,
             signature,
-            RemoteFsverityMerkleTreeReader::new(Arc::clone(&service), remote_id),
+            RemoteMerkleTreeReader::new(Arc::clone(&service), remote_id),
         )?,
         file_size,
     ))
 }
 
 fn new_config_remote_unverified_file(remote_id: i32, file_size: u64) -> Result<FileConfig> {
-    let file_reader = RemoteChunkedFileReader::new(
-        Arc::new(Mutex::new(remote_file::server::get_local_service())),
-        remote_id,
-    );
-    Ok(FileConfig::RemoteUnverifiedFile(file_reader, file_size))
+    let file_reader =
+        RemoteFileReader::new(Arc::new(Mutex::new(file::get_local_binder())), remote_id);
+    Ok(FileConfig::RemoteUnverifiedReadonlyFile(file_reader, file_size))
 }
 
-fn new_config_local_verified_file(
+fn new_config_local_ro_file(
     protected_file: &PathBuf,
     merkle_tree_dump: &PathBuf,
     signature: &PathBuf,
 ) -> Result<FileConfig> {
     let file = File::open(&protected_file)?;
     let file_size = file.metadata()?.len();
-    let file_reader = ChunkedFileReader::new(file)?;
-    let merkle_tree_reader = ChunkedFileReader::new(File::open(merkle_tree_dump)?)?;
+    let file_reader = LocalFileReader::new(file)?;
+    let merkle_tree_reader = LocalFileReader::new(File::open(merkle_tree_dump)?)?;
     let authenticator = FakeAuthenticator::always_succeed();
     let mut sig = Vec::new();
     let _ = File::open(signature)?.read_to_end(&mut sig)?;
-    let file_reader = FsverityChunkedFileReader::new(
-        &authenticator,
-        file_reader,
-        file_size,
-        sig,
-        merkle_tree_reader,
-    )?;
-    Ok(FileConfig::LocalVerifiedFile(file_reader, file_size))
+    let file_reader =
+        VerifiedFileReader::new(&authenticator, file_reader, file_size, sig, merkle_tree_reader)?;
+    Ok(FileConfig::LocalVerifiedReadonlyFile(file_reader, file_size))
 }
 
-fn new_config_local_unverified_file(file_path: &PathBuf) -> Result<FileConfig> {
-    let file_reader = ChunkedFileReader::new(File::open(file_path)?)?;
+fn new_config_local_ro_file_unverified(file_path: &PathBuf) -> Result<FileConfig> {
+    let file_reader = LocalFileReader::new(File::open(file_path)?)?;
     let file_size = file_reader.len();
-    Ok(FileConfig::LocalUnverifiedFile(file_reader, file_size))
+    Ok(FileConfig::LocalUnverifiedReadonlyFile(file_reader, file_size))
+}
+
+fn new_config_remote_new_verified_file(remote_id: i32) -> Result<FileConfig> {
+    let remote_file =
+        RemoteFileEditor::new(Arc::new(Mutex::new(file::get_local_binder())), remote_id);
+    Ok(FileConfig::RemoteVerifiedNewFile(VerifiedFileEditor::new(remote_file)))
 }
 
 fn prepare_file_pool(args: &Args) -> Result<BTreeMap<Inode, FileConfig>> {
     let mut file_pool = BTreeMap::new();
 
-    for config in &args.remote_verified_file {
+    for config in &args.remote_ro_file {
         file_pool.insert(
             config.ino,
             new_config_remote_verified_file(config.remote_id, config.file_size)?,
         );
     }
 
-    for config in &args.remote_unverified_file {
+    for config in &args.remote_ro_file_unverified {
         file_pool.insert(
             config.ino,
             new_config_remote_unverified_file(config.remote_id, config.file_size)?,
         );
     }
 
-    for config in &args.local_verified_file {
+    for config in &args.remote_new_rw_file {
+        file_pool.insert(config.ino, new_config_remote_new_verified_file(config.remote_id)?);
+    }
+
+    for config in &args.local_ro_file {
         file_pool.insert(
             config.ino,
-            new_config_local_verified_file(
+            new_config_local_ro_file(
                 &config.file_path,
                 &config.merkle_tree_dump_path,
                 &config.signature_path,
@@ -260,8 +286,8 @@
         );
     }
 
-    for config in &args.local_unverified_file {
-        file_pool.insert(config.ino, new_config_local_unverified_file(&config.file_path)?);
+    for config in &args.local_ro_file_unverified {
+        file_pool.insert(config.ino, new_config_local_ro_file_unverified(&config.file_path)?);
     }
 
     Ok(file_pool)
diff --git a/authfs/src/remote_file.rs b/authfs/src/remote_file.rs
deleted file mode 100644
index 01e803c..0000000
--- a/authfs/src/remote_file.rs
+++ /dev/null
@@ -1,92 +0,0 @@
-/*
- * Copyright (C) 2021 The Android Open Source Project
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *      http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-use std::convert::TryFrom;
-use std::io;
-use std::io::Write;
-use std::sync::{Arc, Mutex};
-
-use crate::common::CHUNK_SIZE;
-use crate::reader::ReadOnlyDataByChunk;
-
-use authfs_aidl_interface::aidl::com::android::virt::fs::IVirtFdService;
-use authfs_aidl_interface::binder::Strong;
-
-type VirtFdService = Strong<dyn IVirtFdService::IVirtFdService>;
-
-pub mod server {
-    // TODO(victorhsieh): use remote binder.
-    pub fn get_local_service() -> super::VirtFdService {
-        let service_name = "authfs_fd_server";
-        authfs_aidl_interface::binder::get_interface(&service_name)
-            .expect("Cannot reach authfs_fd_server binder service")
-    }
-}
-
-pub struct RemoteChunkedFileReader {
-    // This needs to have Sync trait to be used in fuse::worker::start_message_loop.
-    service: Arc<Mutex<VirtFdService>>,
-    file_fd: i32,
-}
-
-impl RemoteChunkedFileReader {
-    pub fn new(service: Arc<Mutex<VirtFdService>>, file_fd: i32) -> Self {
-        RemoteChunkedFileReader { service, file_fd }
-    }
-}
-
-impl ReadOnlyDataByChunk for RemoteChunkedFileReader {
-    fn read_chunk(&self, chunk_index: u64, mut buf: &mut [u8]) -> io::Result<usize> {
-        let offset = i64::try_from(chunk_index * CHUNK_SIZE)
-            .map_err(|_| io::Error::from_raw_os_error(libc::EOVERFLOW))?;
-
-        let service = Arc::clone(&self.service);
-        let chunk = service
-            .lock()
-            .unwrap()
-            .readFile(self.file_fd, offset, buf.len() as i32)
-            .map_err(|e| io::Error::new(io::ErrorKind::Other, e.get_description()))?;
-        buf.write(&chunk)
-    }
-}
-
-pub struct RemoteFsverityMerkleTreeReader {
-    // This needs to be a Sync to be used in fuse::worker::start_message_loop.
-    // TODO(victorhsieh): change to Strong<> once binder supports it.
-    service: Arc<Mutex<VirtFdService>>,
-    file_fd: i32,
-}
-
-impl RemoteFsverityMerkleTreeReader {
-    pub fn new(service: Arc<Mutex<VirtFdService>>, file_fd: i32) -> Self {
-        RemoteFsverityMerkleTreeReader { service, file_fd }
-    }
-}
-
-impl ReadOnlyDataByChunk for RemoteFsverityMerkleTreeReader {
-    fn read_chunk(&self, chunk_index: u64, mut buf: &mut [u8]) -> io::Result<usize> {
-        let offset = i64::try_from(chunk_index * CHUNK_SIZE)
-            .map_err(|_| io::Error::from_raw_os_error(libc::EOVERFLOW))?;
-
-        let service = Arc::clone(&self.service);
-        let chunk = service
-            .lock()
-            .unwrap()
-            .readFsverityMerkleTree(self.file_fd, offset, buf.len() as i32)
-            .map_err(|e| io::Error::new(io::ErrorKind::Other, e.get_description()))?;
-        buf.write(&chunk)
-    }
-}
diff --git a/authfs/tools/device-test.sh b/authfs/tools/device-test.sh
index 5cf5f10..82aa6bc 100755
--- a/authfs/tools/device-test.sh
+++ b/authfs/tools/device-test.sh
@@ -6,20 +6,28 @@
 #
 # Setup:
 # $ adb push testdata/input.4m* /data/local/tmp
+# $ adb push tools/device-test.sh /data/local/tmp/
 #
 # Shell 1:
-# $ adb shell 'cd /data/local/tmp && exec 9</system/bin/sh 8<input.4m 7<input.4m.merkle_dump 6<input.4m.fsv_sig 5<input.4m 4<input.4m.merkle_dump.bad 3<input.4m.fsv_sig fd_server --ro-fds 9 --ro-fds 8:7:6 --ro-fds 5:4:3'
+# $ adb shell /data/local/tmp/device-test.sh --run-fd-server
 #
 # Shell 2:
-# $ adb push tools/device-test.sh /data/local/tmp/ && adb shell /data/local/tmp/device-test.sh
+# $ adb shell /data/local/tmp/device-test.sh
+
+cd /data/local/tmp
+cat /dev/null > output
+
+if [[ $1 == "--run-fd-server" ]]; then
+  exec 9</system/bin/sh 8<input.4m 7<input.4m.merkle_dump 6<input.4m \
+    5<input.4m.merkle_dump.bad 4<input.4m.fsv_sig 3<>output \
+    fd_server --ro-fds 9 --ro-fds 8:7:4 --ro-fds 6:5:4 --rw-fds 3
+fi
 
 # Run with -u to enter new namespace.
 if [[ $1 == "-u" ]]; then
   exec unshare -mUr $0
 fi
 
-cd /data/local/tmp
-
 MOUNTPOINT=/data/local/tmp/authfs
 trap "umount ${MOUNTPOINT}" EXIT;
 mkdir -p ${MOUNTPOINT}
@@ -33,13 +41,14 @@
 # certificate.
 authfs \
   ${MOUNTPOINT} \
-  --local-verified-file 2:input.4m:input.4m.merkle_dump:input.4m.fsv_sig:/dev/null \
-  --local-verified-file 3:input.4k1:input.4k1.merkle_dump:input.4k1.fsv_sig:/dev/null \
-  --local-verified-file 4:input.4k:input.4k.merkle_dump:input.4k.fsv_sig:/dev/null \
-  --local-unverified-file 5:/system/bin/sh \
-  --remote-unverified-file 6:9:${size} \
-  --remote-verified-file 7:8:${size2}:/dev/null \
-  --remote-verified-file 8:5:${size2}:/dev/null \
+  --local-ro-file 2:input.4m:input.4m.merkle_dump:input.4m.fsv_sig:/dev/null \
+  --local-ro-file 3:input.4k1:input.4k1.merkle_dump:input.4k1.fsv_sig:/dev/null \
+  --local-ro-file 4:input.4k:input.4k.merkle_dump:input.4k.fsv_sig:/dev/null \
+  --local-ro-file-unverified 5:/system/bin/sh \
+  --remote-ro-file-unverified 6:9:${size} \
+  --remote-ro-file 7:8:${size2}:/dev/null \
+  --remote-ro-file 8:6:${size2}:/dev/null \
+  --remote-new-rw-file 9:3 \
   &
 sleep 0.1
 
@@ -55,6 +64,9 @@
 echo
 md5sum ${MOUNTPOINT}/7 input.4m
 echo
+cat input.4m > ${MOUNTPOINT}/9
+md5sum ${MOUNTPOINT}/9 output
+echo
 echo Checking error cases...
 cat /data/local/tmp/authfs/8 2>&1 |grep -q ": I/O error" || echo "Failed to catch the problem"
 echo "Done!"
diff --git a/microdroid/Android.bp b/microdroid/Android.bp
index ad4e8d5..f853b75 100644
--- a/microdroid/Android.bp
+++ b/microdroid/Android.bp
@@ -37,6 +37,7 @@
     use_avb: true,
     avb_private_key: "microdroid.pem",
     avb_algorithm: "SHA256_RSA4096",
+    partition_name: "system",
     deps: [
         "init_second_stage",
         "microdroid_init_rc",
@@ -162,6 +163,8 @@
     dtb_prebuilt: "dummy_dtb.img",
     header_version: "4",
     partition_name: "boot",
+    use_avb: true,
+    avb_private_key: "microdroid.pem",
 }
 
 android_filesystem {
@@ -189,6 +192,8 @@
     header_version: "4",
     vendor_boot: true,
     partition_name: "vendor_boot",
+    use_avb: true,
+    avb_private_key: "microdroid.pem",
 }
 
 android_filesystem {
@@ -287,3 +292,34 @@
     relative_install_path: "selinux",
     installable: false,
 }
+
+vbmeta {
+    name: "microdroid_vbmeta",
+    partition_name: "vbmeta",
+    private_key: "microdroid.pem",
+    partitions: [
+        "microdroid_vendor",
+        "microdroid_vendor_boot-5.10",
+    ],
+    chained_partitions: [
+        {
+            name: "vbmeta_system",
+            rollback_index_location: 1,
+            private_key: "microdroid.pem",
+        },
+        {
+            name: "boot",
+            rollback_index_location: 2,
+            private_key: "microdroid.pem",
+        },
+    ],
+}
+
+vbmeta {
+    name: "microdroid_vbmeta_system",
+    partition_name: "vbmeta_system",
+    private_key: "microdroid.pem",
+    partitions: [
+        "microdroid",
+    ],
+}
diff --git a/microdroid/README.md b/microdroid/README.md
index 465d234..363d623 100644
--- a/microdroid/README.md
+++ b/microdroid/README.md
@@ -22,6 +22,8 @@
 $ m microdroid_boot-5.10
 $ m microdroid_vendor_boot-5.10
 $ m microdroid_uboot_env
+$ m microdroid_vbmeta
+$ m microdroid_vbmeta_system
 ```
 
 ## Installing
@@ -34,6 +36,8 @@
 $ adb push $ANDROID_PRODUCT_OUT/system/etc/microdroid_super.img /data/local/tmp/super.img
 $ adb push $ANDROID_PRODUCT_OUT/system/etc/microdroid_boot-5.10.img /data/local/tmp/boot.img
 $ adb push $ANDROID_PRODUCT_OUT/system/etc/microdroid_vendor_boot-5.10.img /data/local/tmp/vendor_boot.img
+$ adb push $ANDROID_PRODUCT_OUT/system/etc/microdroid_vbmeta.img /data/local/tmp/vbmeta.img
+$ adb push $ANDROID_PRODUCT_OUT/system/etc/microdroid_vbmeta_system.img /data/local/tmp/vbmeta_system.img
 $ adb shell mkdir /data/local/tmp/cuttlefish_runtime.1/
 $ adb push $ANDROID_PRODUCT_OUT/system/etc/uboot_env.img /data/local/tmp/cuttlefish_runtime.1/
 $ adb shell mkdir -p /data/local/tmp/etc/cvd_config
@@ -41,8 +45,6 @@
 $ dd if=/dev/zero of=empty.img bs=4k count=600
 $ mkfs.ext4 -F empty.img
 $ adb push empty.img /data/local/tmp/userdata.img
-$ adb push empty.img /data/local/tmp/vbmeta.img
-$ adb push empty.img /data/local/tmp/vbmeta_system.img
 $ adb push empty.img /data/local/tmp/cache.img
 ```
 
diff --git a/tests/hostside/Android.bp b/tests/hostside/Android.bp
index e07459a..c030e8d 100644
--- a/tests/hostside/Android.bp
+++ b/tests/hostside/Android.bp
@@ -14,6 +14,8 @@
         ":microdroid_uboot_env",
         ":cuttlefish_crosvm_bootloader",
         ":MicrodroidHostTestCase_EmptyImage",
+        ":microdroid_vbmeta",
+        ":microdroid_vbmeta_system",
     ],
 }
 
diff --git a/tests/hostside/java/android/virt/test/MicrodroidTestCase.java b/tests/hostside/java/android/virt/test/MicrodroidTestCase.java
index 092a1dd..32b3c11 100644
--- a/tests/hostside/java/android/virt/test/MicrodroidTestCase.java
+++ b/tests/hostside/java/android/virt/test/MicrodroidTestCase.java
@@ -75,8 +75,8 @@
         pushFile("microdroid_vendor_boot-5.10.img", "vendor_boot.img");
         pushFile("uboot_env.img", "cuttlefish_runtime.1/uboot_env.img");
         pushFile("empty.img", "userdata.img");
-        pushFile("empty.img", "vbmeta.img");
-        pushFile("empty.img", "vbmeta_system.img");
+        pushFile("microdroid_vbmeta.img", "vbmeta.img");
+        pushFile("microdroid_vbmeta_system.img", "vbmeta_system.img");
         pushFile("empty.img", "cache.img");
         getDevice().executeShellCommand("mkdir -p " + TEST_ROOT + "etc/cvd_config");
         getDevice().pushString("{}", TEST_ROOT + "etc/cvd_config/cvd_config_phone.json");
diff --git a/virtmanager/src/main.rs b/virtmanager/src/main.rs
index 3ea33d9..486efeb 100644
--- a/virtmanager/src/main.rs
+++ b/virtmanager/src/main.rs
@@ -33,9 +33,9 @@
 type Cid = u32;
 
 fn main() {
-    android_logger::init_once(android_logger::Config::default().with_tag(LOG_TAG).with_min_level(
-        if env!("TARGET_BUILD_VARIANT") == "user" { Level::Info } else { Level::Trace },
-    ));
+    android_logger::init_once(
+        android_logger::Config::default().with_tag(LOG_TAG).with_min_level(Level::Trace),
+    );
 
     let virt_manager = VirtManager::default();
     let virt_manager = BnVirtManager::new_binder(virt_manager);