path: root/devices/src/virtio/descriptor_utils.rs

// Copyright 2019 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::cmp;
use std::collections::VecDeque;
use std::fmt::{self, Display};
use std::io::{self, Read, Write};
use std::mem::{size_of, MaybeUninit};
use std::result;

use data_model::{DataInit, Le16, Le32, Le64, VolatileMemory, VolatileMemoryError, VolatileSlice};
use sys_util::{FileReadWriteAtVolatile, FileReadWriteVolatile, GuestAddress, GuestMemory};

use super::DescriptorChain;

#[derive(Debug)]
pub enum Error {
    DescriptorChainOverflow,
    GuestMemoryError(sys_util::GuestMemoryError),
    InvalidChain,
    IoError(io::Error),
    SplitOutOfBounds(usize),
    VolatileMemoryError(VolatileMemoryError),
}

impl Display for Error {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        use self::Error::*;

        match self {
            DescriptorChainOverflow => write!(
                f,
                "the combined length of all the buffers in a `DescriptorChain` would overflow"
            ),
            GuestMemoryError(e) => write!(f, "descriptor guest memory error: {}", e),
            InvalidChain => write!(f, "invalid descriptor chain"),
            IoError(e) => write!(f, "descriptor I/O error: {}", e),
            SplitOutOfBounds(off) => write!(f, "`DescriptorChain` split is out of bounds: {}", off),
            VolatileMemoryError(e) => write!(f, "volatile memory error: {}", e),
        }
    }
}

pub type Result<T> = result::Result<T, Error>;

impl std::error::Error for Error {}

#[derive(Clone)]
struct DescriptorChainConsumer<'a> {
    buffers: VecDeque<VolatileSlice<'a>>,
    bytes_consumed: usize,
}

impl<'a> DescriptorChainConsumer<'a> {
    fn available_bytes(&self) -> Result<usize> {
        self.buffers
            .iter()
            .try_fold(0usize, |count, vs| count.checked_add(vs.size() as usize))
            .ok_or(Error::DescriptorChainOverflow)
    }

    fn bytes_consumed(&self) -> usize {
        self.bytes_consumed
    }

    /// Consumes at most `count` bytes from the `DescriptorChain`. Callers must provide a function
    /// that takes a `&[VolatileSlice]` and returns the total number of bytes consumed. This
    /// function guarantees that the combined length of all the slices in the `&[VolatileSlice]` is
    /// less than or equal to `count`.
    ///
    /// # Errors
    ///
    /// If the provided function returns any error then no bytes are consumed from the buffer and
    /// the error is returned to the caller.
    fn consume<F>(&mut self, count: usize, f: F) -> io::Result<usize>
    where
        F: FnOnce(&[VolatileSlice]) -> io::Result<usize>,
    {
        let mut buflen = 0;
        let mut bufs = Vec::with_capacity(self.buffers.len());
        for &vs in &self.buffers {
            if buflen >= count {
                break;
            }

            let rem = count - buflen;
            if (rem as u64) < vs.size() {
                let buf = vs.sub_slice(0, rem as u64).map_err(|e| {
                    io::Error::new(io::ErrorKind::InvalidData, Error::VolatileMemoryError(e))
                })?;
                bufs.push(buf);
                buflen += rem;
            } else {
                bufs.push(vs);
                buflen += vs.size() as usize;
            }
        }

        if bufs.is_empty() {
            return Ok(0);
        }

        let bytes_consumed = f(&*bufs)?;

        // This can happen if a driver tricks a device into reading/writing more data than
        // fits in a `usize`.
        let total_bytes_consumed =
            self.bytes_consumed
                .checked_add(bytes_consumed)
                .ok_or_else(|| {
                    io::Error::new(io::ErrorKind::InvalidData, Error::DescriptorChainOverflow)
                })?;

        let mut rem = bytes_consumed;
        while let Some(vs) = self.buffers.pop_front() {
            if (rem as u64) < vs.size() {
                // Split the slice and push the remainder back into the buffer list. Safe because we
                // know that `rem` is not out of bounds due to the check and we checked the bounds
                // on `vs` when we added it to the buffer list.
                self.buffers.push_front(vs.offset(rem as u64).unwrap());
                break;
            }

            // No need for checked math because we know that `vs.size() <= rem`.
            rem -= vs.size() as usize;
        }

        self.bytes_consumed = total_bytes_consumed;

        Ok(bytes_consumed)
    }

    fn split_at(&mut self, offset: usize) -> Result<DescriptorChainConsumer<'a>> {
        let mut rem = offset;
        let pos = self.buffers.iter().position(|vs| {
            if (rem as u64) < vs.size() {
                true
            } else {
                rem -= vs.size() as usize;
                false
            }
        });

        if let Some(at) = pos {
            let mut other = self.buffers.split_off(at);

            if rem > 0 {
                // There must be at least one element in `other` because we checked
                // its `size` value in the call to `position` above.
                let front = other.pop_front().expect("empty VecDeque after split");
                self.buffers.push_back(
                    front
                        .sub_slice(0, rem as u64)
                        .map_err(Error::VolatileMemoryError)?,
                );
                other.push_front(
                    front
                        .offset(rem as u64)
                        .map_err(Error::VolatileMemoryError)?,
                );
            }

            Ok(DescriptorChainConsumer {
                buffers: other,
                bytes_consumed: 0,
            })
        } else if rem == 0 {
            Ok(DescriptorChainConsumer {
                buffers: VecDeque::new(),
                bytes_consumed: 0,
            })
        } else {
            Err(Error::SplitOutOfBounds(offset))
        }
    }
}

/// Provides high-level interface over the sequence of memory regions
/// defined by readable descriptors in the descriptor chain.
///
/// Note that virtio spec requires driver to place any device-writable
/// descriptors after any device-readable descriptors (2.6.4.2 in Virtio Spec v1.1).
/// Reader will skip iterating over descriptor chain when first writable
/// descriptor is encountered.
#[derive(Clone)]
pub struct Reader<'a> {
    buffer: DescriptorChainConsumer<'a>,
}

impl<'a> Reader<'a> {
    /// Construct a new Reader wrapper over `desc_chain`.
    pub fn new(mem: &'a GuestMemory, desc_chain: DescriptorChain<'a>) -> Result<Reader<'a>> {
        // TODO(jstaron): Update this code to take the indirect descriptors into account.
        let buffers = desc_chain
            .into_iter()
            .readable()
            .map(|desc| {
                mem.get_slice(desc.addr.offset(), desc.len.into())
                    .map_err(Error::VolatileMemoryError)
            })
            .collect::<Result<VecDeque<VolatileSlice<'a>>>>()?;
        Ok(Reader {
            buffer: DescriptorChainConsumer {
                buffers,
                bytes_consumed: 0,
            },
        })
    }

    /// Reads an object from the descriptor chain buffer.
    pub fn read_obj<T: DataInit>(&mut self) -> io::Result<T> {
        let mut obj = MaybeUninit::<T>::uninit();

        // Safe because `MaybeUninit` guarantees that the pointer is valid for
        // `size_of::<T>()` bytes.
        let buf = unsafe {
            ::std::slice::from_raw_parts_mut(obj.as_mut_ptr() as *mut u8, size_of::<T>())
        };

        self.read_exact(buf)?;

        // Safe because any type that implements `DataInit` can be considered initialized
        // even if it is filled with random data.
        Ok(unsafe { obj.assume_init() })
    }

    /// Reads data from the descriptor chain buffer into a file descriptor.
    /// Returns the number of bytes read from the descriptor chain buffer.
    /// The number of bytes read can be less than `count` if there isn't
    /// enough data in the descriptor chain buffer.
    pub fn read_to<F: FileReadWriteVolatile>(
        &mut self,
        mut dst: F,
        count: usize,
    ) -> io::Result<usize> {
        self.buffer
            .consume(count, |bufs| dst.write_vectored_volatile(bufs))
    }

    /// Reads data from the descriptor chain buffer into a File at offset `off`.
    /// Returns the number of bytes read from the descriptor chain buffer.
    /// The number of bytes read can be less than `count` if there isn't
    /// enough data in the descriptor chain buffer.
    pub fn read_to_at<F: FileReadWriteAtVolatile>(
        &mut self,
        mut dst: F,
        count: usize,
        off: u64,
    ) -> io::Result<usize> {
        self.buffer
            .consume(count, |bufs| dst.write_vectored_at_volatile(bufs, off))
    }

    pub fn read_exact_to<F: FileReadWriteVolatile>(
        &mut self,
        mut dst: F,
        mut count: usize,
    ) -> io::Result<()> {
        while count > 0 {
            match self.read_to(&mut dst, count) {
                Ok(0) => {
                    return Err(io::Error::new(
                        io::ErrorKind::UnexpectedEof,
                        "failed to fill whole buffer",
                    ))
                }
                Ok(n) => count -= n,
                Err(ref e) if e.kind() == io::ErrorKind::Interrupted => {}
                Err(e) => return Err(e),
            }
        }

        Ok(())
    }

    /// Returns number of bytes available for reading.  May return an error if the combined
    /// lengths of all the buffers in the DescriptorChain would cause an integer overflow.
    pub fn available_bytes(&self) -> Result<usize> {
        self.buffer.available_bytes()
    }

    /// Returns number of bytes already read from the descriptor chain buffer.
    pub fn bytes_read(&self) -> usize {
        self.buffer.bytes_consumed()
    }

    /// Splits this `Reader` into two at the given offset in the `DescriptorChain` buffer.
    /// After the split, `self` will be able to read up to `offset` bytes while the returned
    /// `Reader` can read up to `available_bytes() - offset` bytes.  Returns an error if
    /// `offset > self.available_bytes()`.
    pub fn split_at(&mut self, offset: usize) -> Result<Reader<'a>> {
        self.buffer.split_at(offset).map(|buffer| Reader { buffer })
    }
}

impl<'a> io::Read for Reader<'a> {
    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
        self.buffer.consume(buf.len(), |bufs| {
            if let Some(vs) = bufs.first() {
                // This is guaranteed by the implementation of `consume`.
                debug_assert_eq!(vs.size(), cmp::min(buf.len() as u64, vs.size()));
                vs.copy_to(buf);
                Ok(vs.size() as usize)
            } else {
                Ok(0)
            }
        })
    }
}

/// Provides high-level interface over the sequence of memory regions
/// defined by writable descriptors in the descriptor chain.
///
/// Note that virtio spec requires driver to place any device-writable
/// descriptors after any device-readable descriptors (2.6.4.2 in Virtio Spec v1.1).
/// Writer will start iterating the descriptors from the first writable one and will
/// assume that all following descriptors are writable.
#[derive(Clone)]
pub struct Writer<'a> {
    buffer: DescriptorChainConsumer<'a>,
}

impl<'a> Writer<'a> {
    /// Construct a new Writer wrapper over `desc_chain`.
    pub fn new(mem: &'a GuestMemory, desc_chain: DescriptorChain<'a>) -> Result<Writer<'a>> {
        let buffers = desc_chain
            .into_iter()
            .writable()
            .map(|desc| {
                mem.get_slice(desc.addr.offset(), desc.len.into())
                    .map_err(Error::VolatileMemoryError)
            })
            .collect::<Result<VecDeque<VolatileSlice<'a>>>>()?;
        Ok(Writer {
            buffer: DescriptorChainConsumer {
                buffers,
                bytes_consumed: 0,
            },
        })
    }

    /// Writes an object to the descriptor chain buffer.
    pub fn write_obj<T: DataInit>(&mut self, val: T) -> io::Result<()> {
        self.write_all(val.as_slice())
    }

    /// Returns number of bytes available for writing.  May return an error if the combined
    /// lengths of all the buffers in the DescriptorChain would cause an overflow.
    pub fn available_bytes(&self) -> Result<usize> {
        self.buffer.available_bytes()
    }

    /// Writes data to the descriptor chain buffer from a file descriptor.
    /// Returns the number of bytes written to the descriptor chain buffer.
    /// The number of bytes written can be less than `count` if
    /// there isn't enough data in the descriptor chain buffer.
    pub fn write_from<F: FileReadWriteVolatile>(
        &mut self,
        mut src: F,
        count: usize,
    ) -> io::Result<usize> {
        self.buffer
            .consume(count, |bufs| src.read_vectored_volatile(bufs))
    }

    /// Writes data to the descriptor chain buffer from a File at offset `off`.
    /// Returns the number of bytes written to the descriptor chain buffer.
    /// The number of bytes written can be less than `count` if
    /// there isn't enough data in the descriptor chain buffer.
    pub fn write_from_at<F: FileReadWriteAtVolatile>(
        &mut self,
        mut src: F,
        count: usize,
        off: u64,
    ) -> io::Result<usize> {
        self.buffer
            .consume(count, |bufs| src.read_vectored_at_volatile(bufs, off))
    }

    pub fn write_all_from<F: FileReadWriteVolatile>(
        &mut self,
        mut src: F,
        mut count: usize,
    ) -> io::Result<()> {
        while count > 0 {
            match self.write_from(&mut src, count) {
                Ok(0) => {
                    return Err(io::Error::new(
                        io::ErrorKind::WriteZero,
                        "failed to write whole buffer",
                    ))
                }
                Ok(n) => count -= n,
                Err(ref e) if e.kind() == io::ErrorKind::Interrupted => {}
                Err(e) => return Err(e),
            }
        }

        Ok(())
    }

    /// Returns number of bytes already written to the descriptor chain buffer.
    pub fn bytes_written(&self) -> usize {
        self.buffer.bytes_consumed()
    }

    /// Splits this `Writer` into two at the given offset in the `DescriptorChain` buffer.
    /// After the split, `self` will be able to write up to `offset` bytes while the returned
    /// `Writer` can write up to `available_bytes() - offset` bytes.  Returns an error if
    /// `offset > self.available_bytes()`.
    pub fn split_at(&mut self, offset: usize) -> Result<Writer<'a>> {
        self.buffer.split_at(offset).map(|buffer| Writer { buffer })
    }
}

impl<'a> io::Write for Writer<'a> {
    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
        self.buffer.consume(buf.len(), |bufs| {
            if let Some(vs) = bufs.first() {
                // This is guaranteed by the implementation of `consume`.
                debug_assert_eq!(vs.size(), cmp::min(buf.len() as u64, vs.size()));
                vs.copy_from(buf);
                Ok(vs.size() as usize)
            } else {
                Ok(0)
            }
        })
    }

    fn flush(&mut self) -> io::Result<()> {
        // Nothing to flush since the writes go straight into the buffer.
        Ok(())
    }
}

const VIRTQ_DESC_F_NEXT: u16 = 0x1;
const VIRTQ_DESC_F_WRITE: u16 = 0x2;

#[derive(Copy, Clone, PartialEq, Eq)]
pub enum DescriptorType {
    Readable,
    Writable,
}

#[derive(Copy, Clone, Debug)]
#[repr(C)]
struct virtq_desc {
    addr: Le64,
    len: Le32,
    flags: Le16,
    next: Le16,
}

// Safe because it only has data and has no implicit padding.
unsafe impl DataInit for virtq_desc {}

/// Test utility function to create a descriptor chain in guest memory.
pub fn create_descriptor_chain(
    memory: &GuestMemory,
    descriptor_array_addr: GuestAddress,
    mut buffers_start_addr: GuestAddress,
    descriptors: Vec<(DescriptorType, u32)>,
    spaces_between_regions: u32,
) -> Result<DescriptorChain> {
    let descriptors_len = descriptors.len();
    for (index, (type_, size)) in descriptors.into_iter().enumerate() {
        let mut flags = 0;
        if let DescriptorType::Writable = type_ {
            flags |= VIRTQ_DESC_F_WRITE;
        }
        if index + 1 < descriptors_len {
            flags |= VIRTQ_DESC_F_NEXT;
        }

        let index = index as u16;
        let desc = virtq_desc {
            addr: buffers_start_addr.offset().into(),
            len: size.into(),
            flags: flags.into(),
            next: (index + 1).into(),
        };

        let offset = size + spaces_between_regions;
        buffers_start_addr = buffers_start_addr
            .checked_add(offset as u64)
            .ok_or(Error::InvalidChain)?;

        let _ = memory.write_obj_at_addr(
            desc,
            descriptor_array_addr
                .checked_add(index as u64 * std::mem::size_of::<virtq_desc>() as u64)
                .ok_or(Error::InvalidChain)?,
        );
    }

    DescriptorChain::checked_new(memory, descriptor_array_addr, 0x100, 0, 0)
        .ok_or(Error::InvalidChain)
}

#[cfg(test)]
mod tests {
    use super::*;
    use sys_util::{MemfdSeals, SharedMemory};

    #[test]
    fn reader_test_simple_chain() {
        use DescriptorType::*;

        let memory_start_addr = GuestAddress(0x0);
        let memory = GuestMemory::new(&vec![(memory_start_addr, 0x10000)]).unwrap();

        let chain = create_descriptor_chain(
            &memory,
            GuestAddress(0x0),
            GuestAddress(0x100),
            vec![
                (Readable, 8),
                (Readable, 16),
                (Readable, 18),
                (Readable, 64),
            ],
            0,
        )
        .expect("create_descriptor_chain failed");
        let mut reader = Reader::new(&memory, chain).expect("failed to create Reader");
        assert_eq!(
            reader
                .available_bytes()
                .expect("failed to get available bytes"),
            106
        );
        assert_eq!(reader.bytes_read(), 0);

        let mut buffer = [0 as u8; 64];
        if let Err(_) = reader.read_exact(&mut buffer) {
            panic!("read_exact should not fail here");
        }

        assert_eq!(
            reader
                .available_bytes()
                .expect("failed to get available bytes"),
            42
        );
        assert_eq!(reader.bytes_read(), 64);

        match reader.read(&mut buffer) {
            Err(_) => panic!("read should not fail here"),
            Ok(length) => assert_eq!(length, 42),
        }

        assert_eq!(
            reader
                .available_bytes()
                .expect("failed to get available bytes"),
            0
        );
        assert_eq!(reader.bytes_read(), 106);
    }

    #[test]
    fn writer_test_simple_chain() {
        use DescriptorType::*;

        let memory_start_addr = GuestAddress(0x0);
        let memory = GuestMemory::new(&vec![(memory_start_addr, 0x10000)]).unwrap();

        let chain = create_descriptor_chain(
            &memory,
            GuestAddress(0x0),
            GuestAddress(0x100),
            vec![
                (Writable, 8),
                (Writable, 16),
                (Writable, 18),
                (Writable, 64),
            ],
            0,
        )
        .expect("create_descriptor_chain failed");;
        let mut writer = Writer::new(&memory, chain).expect("failed to create Writer");
        assert_eq!(
            writer
                .available_bytes()
                .expect("failed to get available bytes"),
            106
        );
        assert_eq!(writer.bytes_written(), 0);

        let mut buffer = [0 as u8; 64];
        if let Err(_) = writer.write_all(&mut buffer) {
            panic!("write_all should not fail here");
        }

        assert_eq!(
            writer
                .available_bytes()
                .expect("failed to get available bytes"),
            42
        );
        assert_eq!(writer.bytes_written(), 64);

        match writer.write(&mut buffer) {
            Err(_) => panic!("write should not fail here"),
            Ok(length) => assert_eq!(length, 42),
        }

        assert_eq!(
            writer
                .available_bytes()
                .expect("failed to get available bytes"),
            0
        );
        assert_eq!(writer.bytes_written(), 106);
    }

    #[test]
    fn reader_test_incompatible_chain() {
        use DescriptorType::*;

        let memory_start_addr = GuestAddress(0x0);
        let memory = GuestMemory::new(&vec![(memory_start_addr, 0x10000)]).unwrap();

        let chain = create_descriptor_chain(
            &memory,
            GuestAddress(0x0),
            GuestAddress(0x100),
            vec![(Writable, 8)],
            0,
        )
        .expect("create_descriptor_chain failed");;
        let mut reader = Reader::new(&memory, chain).expect("failed to create Reader");
        assert_eq!(
            reader
                .available_bytes()
                .expect("failed to get available bytes"),
            0
        );
        assert_eq!(reader.bytes_read(), 0);

        assert!(reader.read_obj::<u8>().is_err());

        assert_eq!(
            reader
                .available_bytes()
                .expect("failed to get available bytes"),
            0
        );
        assert_eq!(reader.bytes_read(), 0);
    }

    #[test]
    fn writer_test_incompatible_chain() {
        use DescriptorType::*;

        let memory_start_addr = GuestAddress(0x0);
        let memory = GuestMemory::new(&vec![(memory_start_addr, 0x10000)]).unwrap();

        let chain = create_descriptor_chain(
            &memory,
            GuestAddress(0x0),
            GuestAddress(0x100),
            vec![(Readable, 8)],
            0,
        )
        .expect("create_descriptor_chain failed");;
        let mut writer = Writer::new(&memory, chain).expect("failed to create Writer");
        assert_eq!(
            writer
                .available_bytes()
                .expect("failed to get available bytes"),
            0
        );
        assert_eq!(writer.bytes_written(), 0);

        assert!(writer.write_obj(0u8).is_err());

        assert_eq!(
            writer
                .available_bytes()
                .expect("failed to get available bytes"),
            0
        );
        assert_eq!(writer.bytes_written(), 0);
    }

    #[test]
    fn reader_failing_io() {
        use DescriptorType::*;

        let memory_start_addr = GuestAddress(0x0);
        let memory = GuestMemory::new(&vec![(memory_start_addr, 0x10000)]).unwrap();

        let chain = create_descriptor_chain(
            &memory,
            GuestAddress(0x0),
            GuestAddress(0x100),
            vec![(Readable, 256), (Readable, 256)],
            0,
        )
        .expect("create_descriptor_chain failed");;

        let mut reader = Reader::new(&memory, chain).expect("failed to create Reader");

        // GuestMemory's write_from_memory requires raw file descriptor.
        let mut shm = SharedMemory::anon().unwrap();
        shm.set_size(384).unwrap();

        // Prevent shared memory from growing on `write` call.
        let mut fd_seals = MemfdSeals::new();
        fd_seals.set_grow_seal();
        shm.add_seals(fd_seals).unwrap();

        reader
            .read_exact_to(&mut shm, 512)
            .expect_err("successfully read more bytes than SharedMemory size");

        // Linux doesn't do partial writes if you give a buffer larger than the remaining length of
        // the shared memory. And since we passed an iovec with the full contents of the
        // DescriptorChain we ended up not writing any bytes at all.
        assert_eq!(
            reader
                .available_bytes()
                .expect("failed to get available bytes"),
            512
        );
        assert_eq!(reader.bytes_read(), 0);
    }

    #[test]
    fn writer_failing_io() {
        use DescriptorType::*;

        let memory_start_addr = GuestAddress(0x0);
        let memory = GuestMemory::new(&vec![(memory_start_addr, 0x10000)]).unwrap();

        let chain = create_descriptor_chain(
            &memory,
            GuestAddress(0x0),
            GuestAddress(0x100),
            vec![(Writable, 256), (Writable, 256)],
            0,
        )
        .expect("create_descriptor_chain failed");

        let mut writer = Writer::new(&memory, chain).expect("failed to create Writer");

        // GuestMemory's read_to_memory requires raw file descriptor.
        let mut shm = SharedMemory::anon().unwrap();
        shm.set_size(384).unwrap();

        writer
            .write_all_from(&mut shm, 512)
            .expect_err("successfully wrote more bytes than in SharedMemory");

        assert_eq!(
            writer
                .available_bytes()
                .expect("failed to get available bytes"),
            128
        );
        assert_eq!(writer.bytes_written(), 384);
    }

    #[test]
    fn reader_writer_shared_chain() {
        use DescriptorType::*;

        let memory_start_addr = GuestAddress(0x0);
        let memory = GuestMemory::new(&vec![(memory_start_addr, 0x10000)]).unwrap();

        let chain = create_descriptor_chain(
            &memory,
            GuestAddress(0x0),
            GuestAddress(0x100),
            vec![
                (Readable, 16),
                (Readable, 16),
                (Readable, 96),
                (Writable, 64),
                (Writable, 1),
                (Writable, 3),
            ],
            0,
        )
        .expect("create_descriptor_chain failed");
        let mut reader = Reader::new(&memory, chain.clone()).expect("failed to create Reader");
        let mut writer = Writer::new(&memory, chain).expect("failed to create Writer");

        assert_eq!(reader.bytes_read(), 0);
        assert_eq!(writer.bytes_written(), 0);

        let mut buffer = Vec::with_capacity(200);

        assert_eq!(
            reader
                .read_to_end(&mut buffer)
                .expect("read should not fail here"),
            128
        );

        // The writable descriptors are only 68 bytes long.
        writer
            .write_all(&buffer[..68])
            .expect("write should not fail here");

        assert_eq!(
            reader
                .available_bytes()
                .expect("failed to get available bytes"),
            0
        );
        assert_eq!(reader.bytes_read(), 128);
        assert_eq!(
            writer
                .available_bytes()
                .expect("failed to get available bytes"),
            0
        );
        assert_eq!(writer.bytes_written(), 68);
    }

    #[test]
    fn reader_writer_shattered_object() {
        use DescriptorType::*;

        let memory_start_addr = GuestAddress(0x0);
        let memory = GuestMemory::new(&vec![(memory_start_addr, 0x10000)]).unwrap();

        let secret: Le32 = 0x12345678.into();

        // Create a descriptor chain with memory regions that are properly separated.
        let chain_writer = create_descriptor_chain(
            &memory,
            GuestAddress(0x0),
            GuestAddress(0x100),
            vec![(Writable, 1), (Writable, 1), (Writable, 1), (Writable, 1)],
            123,
        )
        .expect("create_descriptor_chain failed");
        let mut writer = Writer::new(&memory, chain_writer).expect("failed to create Writer");
        if let Err(_) = writer.write_obj(secret) {
            panic!("write_obj should not fail here");
        }

        // Now create new descriptor chain pointing to the same memory and try to read it.
        let chain_reader = create_descriptor_chain(
            &memory,
            GuestAddress(0x0),
            GuestAddress(0x100),
            vec![(Readable, 1), (Readable, 1), (Readable, 1), (Readable, 1)],
            123,
        )
        .expect("create_descriptor_chain failed");
        let mut reader = Reader::new(&memory, chain_reader).expect("failed to create Reader");
        match reader.read_obj::<Le32>() {
            Err(_) => panic!("read_obj should not fail here"),
            Ok(read_secret) => assert_eq!(read_secret, secret),
        }
    }

    #[test]
    fn reader_unexpected_eof() {
        use DescriptorType::*;

        let memory_start_addr = GuestAddress(0x0);
        let memory = GuestMemory::new(&vec![(memory_start_addr, 0x10000)]).unwrap();

        let chain = create_descriptor_chain(
            &memory,
            GuestAddress(0x0),
            GuestAddress(0x100),
            vec![(Readable, 256), (Readable, 256)],
            0,
        )
        .expect("create_descriptor_chain failed");

        let mut reader = Reader::new(&memory, chain).expect("failed to create Reader");

        let mut buf = Vec::with_capacity(1024);
        buf.resize(1024, 0);

        assert_eq!(
            reader
                .read_exact(&mut buf[..])
                .expect_err("read more bytes than available")
                .kind(),
            io::ErrorKind::UnexpectedEof
        );
    }

    #[test]
    fn split_border() {
        use DescriptorType::*;

        let memory_start_addr = GuestAddress(0x0);
        let memory = GuestMemory::new(&vec![(memory_start_addr, 0x10000)]).unwrap();

        let chain = create_descriptor_chain(
            &memory,
            GuestAddress(0x0),
            GuestAddress(0x100),
            vec![
                (Readable, 16),
                (Readable, 16),
                (Readable, 96),
                (Writable, 64),
                (Writable, 1),
                (Writable, 3),
            ],
            0,
        )
        .expect("create_descriptor_chain failed");;
        let mut reader = Reader::new(&memory, chain).expect("failed to create Reader");

        let other = reader.split_at(32).expect("failed to split Reader");
        assert_eq!(
            reader
                .available_bytes()
                .expect("failed to get available bytes"),
            32
        );
        assert_eq!(
            other
                .available_bytes()
                .expect("failed to get available bytes"),
            96
        );
    }

    #[test]
    fn split_middle() {
        use DescriptorType::*;

        let memory_start_addr = GuestAddress(0x0);
        let memory = GuestMemory::new(&vec![(memory_start_addr, 0x10000)]).unwrap();

        let chain = create_descriptor_chain(
            &memory,
            GuestAddress(0x0),
            GuestAddress(0x100),
            vec![
                (Readable, 16),
                (Readable, 16),
                (Readable, 96),
                (Writable, 64),
                (Writable, 1),
                (Writable, 3),
            ],
            0,
        )
        .expect("create_descriptor_chain failed");
        let mut reader = Reader::new(&memory, chain).expect("failed to create Reader");

        let other = reader.split_at(24).expect("failed to split Reader");
        assert_eq!(
            reader
                .available_bytes()
                .expect("failed to get available bytes"),
            24
        );
        assert_eq!(
            other
                .available_bytes()
                .expect("failed to get available bytes"),
            104
        );
    }

    #[test]
    fn split_end() {
        use DescriptorType::*;

        let memory_start_addr = GuestAddress(0x0);
        let memory = GuestMemory::new(&vec![(memory_start_addr, 0x10000)]).unwrap();

        let chain = create_descriptor_chain(
            &memory,
            GuestAddress(0x0),
            GuestAddress(0x100),
            vec![
                (Readable, 16),
                (Readable, 16),
                (Readable, 96),
                (Writable, 64),
                (Writable, 1),
                (Writable, 3),
            ],
            0,
        )
        .expect("create_descriptor_chain failed");
        let mut reader = Reader::new(&memory, chain).expect("failed to create Reader");

        let other = reader.split_at(128).expect("failed to split Reader");
        assert_eq!(
            reader
                .available_bytes()
                .expect("failed to get available bytes"),
            128
        );
        assert_eq!(
            other
                .available_bytes()
                .expect("failed to get available bytes"),
            0
        );
    }

    #[test]
    fn split_beginning() {
        use DescriptorType::*;

        let memory_start_addr = GuestAddress(0x0);
        let memory = GuestMemory::new(&vec![(memory_start_addr, 0x10000)]).unwrap();

        let chain = create_descriptor_chain(
            &memory,
            GuestAddress(0x0),
            GuestAddress(0x100),
            vec![
                (Readable, 16),
                (Readable, 16),
                (Readable, 96),
                (Writable, 64),
                (Writable, 1),
                (Writable, 3),
            ],
            0,
        )
        .expect("create_descriptor_chain failed");
        let mut reader = Reader::new(&memory, chain).expect("failed to create Reader");

        let other = reader.split_at(0).expect("failed to split Reader");
        assert_eq!(
            reader
                .available_bytes()
                .expect("failed to get available bytes"),
            0
        );
        assert_eq!(
            other
                .available_bytes()
                .expect("failed to get available bytes"),
            128
        );
    }

    #[test]
    fn split_outofbounds() {
        use DescriptorType::*;

        let memory_start_addr = GuestAddress(0x0);
        let memory = GuestMemory::new(&vec![(memory_start_addr, 0x10000)]).unwrap();

        let chain = create_descriptor_chain(
            &memory,
            GuestAddress(0x0),
            GuestAddress(0x100),
            vec![
                (Readable, 16),
                (Readable, 16),
                (Readable, 96),
                (Writable, 64),
                (Writable, 1),
                (Writable, 3),
            ],
            0,
        )
        .expect("create_descriptor_chain failed");
        let mut reader = Reader::new(&memory, chain).expect("failed to create Reader");

        if let Ok(_) = reader.split_at(256) {
            panic!("successfully split Reader with out of bounds offset");
        }
    }
}