vhost_user_fs: Add file traits to handle writing volatile memory

The vm_memory implementation for VolatileSlice is able to read and write
to a source or destination which implements a Read or Write trait.

Unfortunately, this is not enough for this specific use case as we need
to be able to write to a file at a specific offset, which is not
provided by the Read or Write trait.

This code has been ported over from crosvm commit
961461350c0b6824e5f20655031bf6c6bf6b7c30.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

vhost_user_fs/src/file_traits.rs

@@ -0,0 +1,409 @@
+// Copyright 2018 The Chromium OS Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+use std::fs::File;
+use std::io::{Error, ErrorKind, Result};
+use std::os::unix::io::AsRawFd;
+
+use vm_memory::VolatileSlice;
+
+use libc::{
+    c_int, c_void, off64_t, pread64, preadv64, pwrite64, pwritev64, read, readv, size_t, write,
+    writev,
+};
+
+/// A trait for setting the size of a file.
+/// This is equivalent to File's `set_len` method, but
+/// wrapped in a trait so that it can be implemented for
+/// other types.
+pub trait FileSetLen {
+    // Set the size of this file.
+    // This is the moral equivalent of `ftruncate()`.
+    fn set_len(&self, _len: u64) -> Result<()>;
+}
+
+impl FileSetLen for File {
+    fn set_len(&self, len: u64) -> Result<()> {
+        File::set_len(self, len)
+    }
+}
+
+/// A trait similar to `Read` and `Write`, but uses volatile memory as buffers.
+pub trait FileReadWriteVolatile {
+    /// Read bytes from this file into the given slice, returning the number of bytes read on
+    /// success.
+    fn read_volatile(&mut self, slice: VolatileSlice) -> Result<usize>;
+
+    /// Like `read_volatile`, except it reads to a slice of buffers. Data is copied to fill each
+    /// buffer in order, with the final buffer written to possibly being only partially filled. This
+    /// method must behave as a single call to `read_volatile` with the buffers concatenated would.
+    /// The default implementation calls `read_volatile` with either the first nonempty buffer
+    /// provided, or returns `Ok(0)` if none exists.
+    fn read_vectored_volatile(&mut self, bufs: &[VolatileSlice]) -> Result<usize> {
+        bufs.iter()
+            .find(|b| !b.is_empty())
+            .map(|&b| self.read_volatile(b))
+            .unwrap_or(Ok(0))
+    }
+
+    /// Reads bytes from this into the given slice until all bytes in the slice are written, or an
+    /// error is returned.
+    fn read_exact_volatile(&mut self, mut slice: VolatileSlice) -> Result<()> {
+        while !slice.is_empty() {
+            let bytes_read = self.read_volatile(slice)?;
+            if bytes_read == 0 {
+                return Err(Error::from(ErrorKind::UnexpectedEof));
+            }
+            // Will panic if read_volatile read more bytes than we gave it, which would be worthy of
+            // a panic.
+            slice = slice.offset(bytes_read).unwrap();
+        }
+        Ok(())
+    }
+
+    /// Write bytes from the slice to the given file, returning the number of bytes written on
+    /// success.
+    fn write_volatile(&mut self, slice: VolatileSlice) -> Result<usize>;
+
+    /// Like `write_volatile`, except that it writes from a slice of buffers. Data is copied from
+    /// each buffer in order, with the final buffer read from possibly being only partially
+    /// consumed. This method must behave as a call to `write_volatile` with the buffers
+    /// concatenated would. The default implementation calls `write_volatile` with either the first
+    /// nonempty buffer provided, or returns `Ok(0)` if none exists.
+    fn write_vectored_volatile(&mut self, bufs: &[VolatileSlice]) -> Result<usize> {
+        bufs.iter()
+            .find(|b| !b.is_empty())
+            .map(|&b| self.write_volatile(b))
+            .unwrap_or(Ok(0))
+    }
+
+    /// Write bytes from the slice to the given file until all the bytes from the slice have been
+    /// written, or an error is returned.
+    fn write_all_volatile(&mut self, mut slice: VolatileSlice) -> Result<()> {
+        while !slice.is_empty() {
+            let bytes_written = self.write_volatile(slice)?;
+            if bytes_written == 0 {
+                return Err(Error::from(ErrorKind::WriteZero));
+            }
+            // Will panic if read_volatile read more bytes than we gave it, which would be worthy of
+            // a panic.
+            slice = slice.offset(bytes_written).unwrap();
+        }
+        Ok(())
+    }
+}
+
+impl<'a, T: FileReadWriteVolatile + ?Sized> FileReadWriteVolatile for &'a mut T {
+    fn read_volatile(&mut self, slice: VolatileSlice) -> Result<usize> {
+        (**self).read_volatile(slice)
+    }
+
+    fn read_vectored_volatile(&mut self, bufs: &[VolatileSlice]) -> Result<usize> {
+        (**self).read_vectored_volatile(bufs)
+    }
+
+    fn read_exact_volatile(&mut self, slice: VolatileSlice) -> Result<()> {
+        (**self).read_exact_volatile(slice)
+    }
+
+    fn write_volatile(&mut self, slice: VolatileSlice) -> Result<usize> {
+        (**self).write_volatile(slice)
+    }
+
+    fn write_vectored_volatile(&mut self, bufs: &[VolatileSlice]) -> Result<usize> {
+        (**self).write_vectored_volatile(bufs)
+    }
+
+    fn write_all_volatile(&mut self, slice: VolatileSlice) -> Result<()> {
+        (**self).write_all_volatile(slice)
+    }
+}
+
+/// A trait similar to the unix `ReadExt` and `WriteExt` traits, but for volatile memory.
+pub trait FileReadWriteAtVolatile {
+    /// Reads bytes from this file at `offset` into the given slice, returning the number of bytes
+    /// read on success.
+    fn read_at_volatile(&mut self, slice: VolatileSlice, offset: u64) -> Result<usize>;
+
+    /// Like `read_at_volatile`, except it reads to a slice of buffers. Data is copied to fill each
+    /// buffer in order, with the final buffer written to possibly being only partially filled. This
+    /// method must behave as a single call to `read_at_volatile` with the buffers concatenated
+    /// would. The default implementation calls `read_at_volatile` with either the first nonempty
+    /// buffer provided, or returns `Ok(0)` if none exists.
+    fn read_vectored_at_volatile(&mut self, bufs: &[VolatileSlice], offset: u64) -> Result<usize> {
+        if let Some(&slice) = bufs.first() {
+            self.read_at_volatile(slice, offset)
+        } else {
+            Ok(0)
+        }
+    }
+
+    /// Reads bytes from this file at `offset` into the given slice until all bytes in the slice are
+    /// read, or an error is returned.
+    fn read_exact_at_volatile(&mut self, mut slice: VolatileSlice, mut offset: u64) -> Result<()> {
+        while !slice.is_empty() {
+            match self.read_at_volatile(slice, offset) {
+                Ok(0) => return Err(Error::from(ErrorKind::UnexpectedEof)),
+                Ok(n) => {
+                    slice = slice.offset(n).unwrap();
+                    offset = offset.checked_add(n as u64).unwrap();
+                }
+                Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
+                Err(e) => return Err(e),
+            }
+        }
+        Ok(())
+    }
+
+    /// Writes bytes from this file at `offset` into the given slice, returning the number of bytes
+    /// written on success.
+    fn write_at_volatile(&mut self, slice: VolatileSlice, offset: u64) -> Result<usize>;
+
+    /// Like `write_at_at_volatile`, except that it writes from a slice of buffers. Data is copied
+    /// from each buffer in order, with the final buffer read from possibly being only partially
+    /// consumed. This method must behave as a call to `write_at_volatile` with the buffers
+    /// concatenated would. The default implementation calls `write_at_volatile` with either the
+    /// first nonempty buffer provided, or returns `Ok(0)` if none exists.
+    fn write_vectored_at_volatile(&mut self, bufs: &[VolatileSlice], offset: u64) -> Result<usize> {
+        if let Some(&slice) = bufs.first() {
+            self.write_at_volatile(slice, offset)
+        } else {
+            Ok(0)
+        }
+    }
+
+    /// Writes bytes from this file at `offset` into the given slice until all bytes in the slice
+    /// are written, or an error is returned.
+    fn write_all_at_volatile(&mut self, mut slice: VolatileSlice, mut offset: u64) -> Result<()> {
+        while !slice.is_empty() {
+            match self.write_at_volatile(slice, offset) {
+                Ok(0) => return Err(Error::from(ErrorKind::WriteZero)),
+                Ok(n) => {
+                    slice = slice.offset(n).unwrap();
+                    offset = offset.checked_add(n as u64).unwrap();
+                }
+                Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
+                Err(e) => return Err(e),
+            }
+        }
+        Ok(())
+    }
+}
+
+impl<'a, T: FileReadWriteAtVolatile + ?Sized> FileReadWriteAtVolatile for &'a mut T {
+    fn read_at_volatile(&mut self, slice: VolatileSlice, offset: u64) -> Result<usize> {
+        (**self).read_at_volatile(slice, offset)
+    }
+
+    fn read_vectored_at_volatile(&mut self, bufs: &[VolatileSlice], offset: u64) -> Result<usize> {
+        (**self).read_vectored_at_volatile(bufs, offset)
+    }
+
+    fn read_exact_at_volatile(&mut self, slice: VolatileSlice, offset: u64) -> Result<()> {
+        (**self).read_exact_at_volatile(slice, offset)
+    }
+
+    fn write_at_volatile(&mut self, slice: VolatileSlice, offset: u64) -> Result<usize> {
+        (**self).write_at_volatile(slice, offset)
+    }
+
+    fn write_vectored_at_volatile(&mut self, bufs: &[VolatileSlice], offset: u64) -> Result<usize> {
+        (**self).write_vectored_at_volatile(bufs, offset)
+    }
+
+    fn write_all_at_volatile(&mut self, slice: VolatileSlice, offset: u64) -> Result<()> {
+        (**self).write_all_at_volatile(slice, offset)
+    }
+}
+
+macro_rules! volatile_impl {
+    ($ty:ty) => {
+        impl FileReadWriteVolatile for $ty {
+            fn read_volatile(&mut self, slice: VolatileSlice) -> Result<usize> {
+                // Safe because only bytes inside the slice are accessed and the kernel is expected
+                // to handle arbitrary memory for I/O.
+                let ret =
+                    unsafe { read(self.as_raw_fd(), slice.as_ptr() as *mut c_void, slice.len()) };
+                if ret >= 0 {
+                    Ok(ret as usize)
+                } else {
+                    Err(Error::last_os_error())
+                }
+            }
+
+            fn read_vectored_volatile(&mut self, bufs: &[VolatileSlice]) -> Result<usize> {
+                let iovecs: Vec<libc::iovec> = bufs
+                    .iter()
+                    .map(|s| libc::iovec {
+                        iov_base: s.as_ptr() as *mut c_void,
+                        iov_len: s.len() as size_t,
+                    })
+                    .collect();
+
+                if iovecs.is_empty() {
+                    return Ok(0);
+                }
+
+                // Safe because only bytes inside the buffers are accessed and the kernel is
+                // expected to handle arbitrary memory for I/O.
+                let ret = unsafe { readv(self.as_raw_fd(), &iovecs[0], iovecs.len() as c_int) };
+                if ret >= 0 {
+                    Ok(ret as usize)
+                } else {
+                    Err(Error::last_os_error())
+                }
+            }
+
+            fn write_volatile(&mut self, slice: VolatileSlice) -> Result<usize> {
+                // Safe because only bytes inside the slice are accessed and the kernel is expected
+                // to handle arbitrary memory for I/O.
+                let ret = unsafe {
+                    write(
+                        self.as_raw_fd(),
+                        slice.as_ptr() as *const c_void,
+                        slice.len(),
+                    )
+                };
+                if ret >= 0 {
+                    Ok(ret as usize)
+                } else {
+                    Err(Error::last_os_error())
+                }
+            }
+
+            fn write_vectored_volatile(&mut self, bufs: &[VolatileSlice]) -> Result<usize> {
+                let iovecs: Vec<libc::iovec> = bufs
+                    .iter()
+                    .map(|s| libc::iovec {
+                        iov_base: s.as_ptr() as *mut c_void,
+                        iov_len: s.len() as size_t,
+                    })
+                    .collect();
+
+                if iovecs.is_empty() {
+                    return Ok(0);
+                }
+
+                // Safe because only bytes inside the buffers are accessed and the kernel is
+                // expected to handle arbitrary memory for I/O.
+                let ret = unsafe { writev(self.as_raw_fd(), &iovecs[0], iovecs.len() as c_int) };
+                if ret >= 0 {
+                    Ok(ret as usize)
+                } else {
+                    Err(Error::last_os_error())
+                }
+            }
+        }
+
+        impl FileReadWriteAtVolatile for $ty {
+            fn read_at_volatile(&mut self, slice: VolatileSlice, offset: u64) -> Result<usize> {
+                // Safe because only bytes inside the slice are accessed and the kernel is expected
+                // to handle arbitrary memory for I/O.
+                let ret = unsafe {
+                    pread64(
+                        self.as_raw_fd(),
+                        slice.as_ptr() as *mut c_void,
+                        slice.len(),
+                        offset as off64_t,
+                    )
+                };
+
+                if ret >= 0 {
+                    Ok(ret as usize)
+                } else {
+                    Err(Error::last_os_error())
+                }
+            }
+
+            fn read_vectored_at_volatile(
+                &mut self,
+                bufs: &[VolatileSlice],
+                offset: u64,
+            ) -> Result<usize> {
+                let iovecs: Vec<libc::iovec> = bufs
+                    .iter()
+                    .map(|s| libc::iovec {
+                        iov_base: s.as_ptr() as *mut c_void,
+                        iov_len: s.len() as size_t,
+                    })
+                    .collect();
+
+                if iovecs.is_empty() {
+                    return Ok(0);
+                }
+
+                // Safe because only bytes inside the buffers are accessed and the kernel is
+                // expected to handle arbitrary memory for I/O.
+                let ret = unsafe {
+                    preadv64(
+                        self.as_raw_fd(),
+                        &iovecs[0],
+                        iovecs.len() as c_int,
+                        offset as off64_t,
+                    )
+                };
+                if ret >= 0 {
+                    Ok(ret as usize)
+                } else {
+                    Err(Error::last_os_error())
+                }
+            }
+
+            fn write_at_volatile(&mut self, slice: VolatileSlice, offset: u64) -> Result<usize> {
+                // Safe because only bytes inside the slice are accessed and the kernel is expected
+                // to handle arbitrary memory for I/O.
+                let ret = unsafe {
+                    pwrite64(
+                        self.as_raw_fd(),
+                        slice.as_ptr() as *const c_void,
+                        slice.len(),
+                        offset as off64_t,
+                    )
+                };
+
+                if ret >= 0 {
+                    Ok(ret as usize)
+                } else {
+                    Err(Error::last_os_error())
+                }
+            }
+
+            fn write_vectored_at_volatile(
+                &mut self,
+                bufs: &[VolatileSlice],
+                offset: u64,
+            ) -> Result<usize> {
+                let iovecs: Vec<libc::iovec> = bufs
+                    .iter()
+                    .map(|s| libc::iovec {
+                        iov_base: s.as_ptr() as *mut c_void,
+                        iov_len: s.len() as size_t,
+                    })
+                    .collect();
+
+                if iovecs.is_empty() {
+                    return Ok(0);
+                }
+
+                // Safe because only bytes inside the buffers are accessed and the kernel is
+                // expected to handle arbitrary memory for I/O.
+                let ret = unsafe {
+                    pwritev64(
+                        self.as_raw_fd(),
+                        &iovecs[0],
+                        iovecs.len() as c_int,
+                        offset as off64_t,
+                    )
+                };
+                if ret >= 0 {
+                    Ok(ret as usize)
+                } else {
+                    Err(Error::last_os_error())
+                }
+            }
+        }
+    };
+}
+
+volatile_impl!(File);

vhost_user_fs/src/lib.rs

@@ -5,6 +5,7 @@
 #[macro_use]
 extern crate log;
 
+pub mod file_traits;
 pub mod filesystem;
 pub mod fuse;
 pub mod multikey;