virtio_queue/

descriptor_utils.rs

// Portions 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-BSD-3-Clause file.
//
// Copyright (C) 2024 Red Hat, Inc. All rights reserved.
//
// SPDX-License-Identifier: Apache-2.0 AND BSD-3-Clause

use std::collections::VecDeque;
use std::io::{self, Read, Write};
use std::mem::{size_of, MaybeUninit};
use std::ops::Deref;
use std::ptr::copy_nonoverlapping;
use std::{cmp, result};

use crate::{DescriptorChain, Error};
use vm_memory::bitmap::{BitmapSlice, WithBitmapSlice};
use vm_memory::{ByteValued, GuestMemory, Permissions, VolatileSlice};

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

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

impl<'a, B: BitmapSlice> DescriptorChainConsumer<'a, B> {
    fn available_bytes(&self) -> usize {
        // This is guaranteed not to overflow because the total length of the chain
        // is checked during all creations of `DescriptorChainConsumer` (see
        // `Reader::new()` and `Writer::new()`).
        self.buffers
            .iter()
            .fold(0usize, |count, vs| count + vs.len())
    }

    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<B>]) -> 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;
            }

            bufs.push(vs);

            let rem = count - buflen;
            if rem < vs.len() {
                buflen += rem;
            } else {
                buflen += vs.len();
            }
        }

        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 < vs.len() {
                // 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).unwrap());
                break;
            }

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

        self.bytes_consumed = total_bytes_consumed;

        Ok(bytes_consumed)
    }

    fn split_at(&mut self, offset: usize) -> Result<DescriptorChainConsumer<'a, B>> {
        let mut rem = offset;
        let pos = self.buffers.iter().position(|vs| {
            if rem < vs.len() {
                true
            } else {
                rem -= vs.len();
                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.subslice(0, rem).map_err(Error::VolatileMemoryError)?);
                other.push_front(front.offset(rem).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, B = ()> {
    buffer: DescriptorChainConsumer<'a, B>,
}

impl<'a, B: BitmapSlice> Reader<'a, B> {
    /// Construct a new Reader wrapper over `desc_chain`.
    pub fn new<M, T>(mem: &'a M, desc_chain: DescriptorChain<T>) -> Result<Reader<'a, B>>
    where
        M: GuestMemory,
        <M as GuestMemory>::Bitmap: WithBitmapSlice<'a, S = B>,
        T: Deref,
        T::Target: GuestMemory + Sized,
    {
        let mut total_len: usize = 0;
        let mut buffers = VecDeque::<VolatileSlice<'a, B>>::new();

        for desc in desc_chain.readable() {
            // Verify that summing the descriptor sizes does not overflow.
            // This can happen if a driver tricks a device into reading more data than
            // fits in a `usize`.
            total_len = total_len
                .checked_add(desc.len() as usize)
                .ok_or(Error::DescriptorChainOverflow)?;

            let slices = mem
                .get_slices(desc.addr(), desc.len() as usize, Permissions::Read)
                .map_err(Error::GuestMemoryError)?;
            for slice in slices {
                buffers.push_back(slice.map_err(Error::GuestMemoryError)?);
            }
        }

        Ok(Reader {
            buffer: DescriptorChainConsumer {
                buffers,
                bytes_consumed: 0,
            },
        })
    }

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

        // SAFETY: `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)?;

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

    /// 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) -> 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, B>> {
        self.buffer.split_at(offset).map(|buffer| Reader { buffer })
    }
}

impl<B: BitmapSlice> io::Read for Reader<'_, B> {
    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
        self.buffer.consume(buf.len(), |bufs| {
            let mut rem = buf;
            let mut total = 0;
            for vs in bufs {
                let copy_len = cmp::min(rem.len(), vs.len());

                // SAFETY: Safe because we verify that we do not read outside
                // of the slice's bound. The slice guard will only get dropped
                // after the function returns. This will keep the pointer valid
                // while reads are happening.
                unsafe {
                    copy_nonoverlapping(vs.ptr_guard().as_ptr(), rem.as_mut_ptr(), copy_len);
                }
                rem = &mut rem[copy_len..];
                total += copy_len;
            }
            Ok(total)
        })
    }
}

/// 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, B = ()> {
    buffer: DescriptorChainConsumer<'a, B>,
}

impl<'a, B: BitmapSlice> Writer<'a, B> {
    /// Construct a new Writer wrapper over `desc_chain`.
    pub fn new<M, T>(mem: &'a M, desc_chain: DescriptorChain<T>) -> Result<Writer<'a, B>>
    where
        M: GuestMemory,
        <M as GuestMemory>::Bitmap: WithBitmapSlice<'a, S = B>,
        T: Deref,
        T::Target: GuestMemory + Sized,
    {
        let mut total_len: usize = 0;
        let mut buffers = VecDeque::<VolatileSlice<'a, B>>::new();

        for desc in desc_chain.writable() {
            // Verify that summing the descriptor sizes does not overflow.
            // This can happen if a driver tricks a device into writing more data than
            // fits in a `usize`.
            total_len = total_len
                .checked_add(desc.len() as usize)
                .ok_or(Error::DescriptorChainOverflow)?;

            let slices = mem
                .get_slices(desc.addr(), desc.len() as usize, Permissions::Write)
                .map_err(Error::GuestMemoryError)?;
            for slice in slices {
                buffers.push_back(slice.map_err(Error::GuestMemoryError)?);
            }
        }

        Ok(Writer {
            buffer: DescriptorChainConsumer {
                buffers,
                bytes_consumed: 0,
            },
        })
    }

    /// Writes an object to the descriptor chain buffer.
    pub fn write_obj<T: ByteValued>(&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) -> usize {
        self.buffer.available_bytes()
    }

    /// 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, B>> {
        self.buffer.split_at(offset).map(|buffer| Writer { buffer })
    }
}

impl<B: BitmapSlice> io::Write for Writer<'_, B> {
    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
        self.buffer.consume(buf.len(), |bufs| {
            let mut rem = buf;
            let mut total = 0;
            for vs in bufs {
                let copy_len = cmp::min(rem.len(), vs.len());

                // SAFETY: Safe because we ensure that we do not write over the
                // slice's bounds. The slice guard will only get dropped after
                // the function returns. This will keep the pointer valid while
                // writes are happening.
                unsafe {
                    copy_nonoverlapping(rem.as_ptr(), vs.ptr_guard_mut().as_ptr(), copy_len);
                }
                vs.bitmap().mark_dirty(0, copy_len);
                rem = &rem[copy_len..];
                total += copy_len;
            }
            Ok(total)
        })
    }

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

#[cfg(test)]
mod tests {
    use super::*;
    use crate::{
        desc::{split::Descriptor as SplitDescriptor, RawDescriptor},
        Queue, QueueOwnedT, QueueT,
    };
    use vm_memory::{Address, GuestAddress, GuestMemoryMmap, Le32};

    use crate::mock::MockSplitQueue;
    use virtio_bindings::bindings::virtio_ring::{VRING_DESC_F_NEXT, VRING_DESC_F_WRITE};

    const MAX_QUEUE_SIZE: u16 = 16;

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

    /// Test utility function to create a descriptor chain in guest memory.
    pub fn create_descriptor_chain(
        memory: &GuestMemoryMmap,
        descriptor_array_addr: GuestAddress,
        descriptors: Vec<(DescriptorType, u32)>,
        spaces_between_regions: u32,
    ) -> Result<DescriptorChain<&GuestMemoryMmap>> {
        let descriptors_len = descriptors.len();
        let mut descs = vec![];

        let queue = MockSplitQueue::create(memory, descriptor_array_addr, MAX_QUEUE_SIZE);

        let mut buffers_start_addr = queue.end();

        for (index, (type_, size)) in descriptors.into_iter().enumerate() {
            let mut flags = 0;
            if let DescriptorType::Writable = type_ {
                flags |= VRING_DESC_F_WRITE;
            }
            if index + 1 < descriptors_len {
                flags |= VRING_DESC_F_NEXT;
            }

            descs.push(RawDescriptor::from(SplitDescriptor::new(
                buffers_start_addr.raw_value(),
                size,
                flags as u16,
                (index + 1) as u16,
            )));

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

        queue.build_desc_chain(&descs).unwrap();

        let avail_ring = queue.avail_addr();

        let mut queue: Queue = Queue::new(MAX_QUEUE_SIZE).unwrap();
        queue
            .try_set_desc_table_address(descriptor_array_addr)
            .unwrap();
        queue.try_set_avail_ring_address(avail_ring).unwrap();
        queue.set_ready(true);

        let chain = queue.iter(memory).unwrap().next().unwrap();

        Ok(chain.clone())
    }

    #[test]
    fn reader_test_inv_desc_addr() {
        let memory: GuestMemoryMmap =
            GuestMemoryMmap::from_ranges(&[(GuestAddress(0x0), 0x1000)]).unwrap();

        let queue = MockSplitQueue::create(&memory, GuestAddress(0x0), MAX_QUEUE_SIZE);

        // set addr out of memory
        let descriptor = RawDescriptor::from(SplitDescriptor::new(0x1001, 1, 0, 1_u16));
        queue.build_desc_chain(&[descriptor]).unwrap();

        let avail_ring = queue.avail_addr();

        let mut queue: Queue = Queue::new(MAX_QUEUE_SIZE).unwrap();
        queue.try_set_desc_table_address(GuestAddress(0x0)).unwrap();
        queue.try_set_avail_ring_address(avail_ring).unwrap();
        queue.set_ready(true);

        let chain = queue.iter(&memory).unwrap().next().unwrap();

        assert!(Reader::new(&memory, chain).is_err());
    }

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

        let memory_start_addr = GuestAddress(0x0);
        let memory = GuestMemoryMmap::from_ranges(&[(memory_start_addr, 0x10000)]).unwrap();

        let chain = create_descriptor_chain(
            &memory,
            GuestAddress(0x0),
            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(), 106);
        assert_eq!(reader.bytes_read(), 0);

        let mut buffer = [0_u8; 64];
        if let Err(e) = reader.read_exact(&mut buffer) {
            panic!("read_exact should not fail here: {e:?}");
        }

        assert_eq!(reader.available_bytes(), 42);
        assert_eq!(reader.bytes_read(), 64);

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

        assert_eq!(reader.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 = GuestMemoryMmap::from_ranges(&[(memory_start_addr, 0x10000)]).unwrap();

        let chain = create_descriptor_chain(
            &memory,
            GuestAddress(0x0),
            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(), 106);
        assert_eq!(writer.bytes_written(), 0);

        let buffer = [0_u8; 64];
        if let Err(e) = writer.write_all(&buffer) {
            panic!("write_all should not fail here: {e:?}");
        }

        assert_eq!(writer.available_bytes(), 42);
        assert_eq!(writer.bytes_written(), 64);

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

        assert_eq!(writer.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 = GuestMemoryMmap::from_ranges(&[(memory_start_addr, 0x10000)]).unwrap();

        let chain = create_descriptor_chain(&memory, GuestAddress(0x0), 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(), 0);
        assert_eq!(reader.bytes_read(), 0);

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

        assert_eq!(reader.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 = GuestMemoryMmap::from_ranges(&[(memory_start_addr, 0x10000)]).unwrap();

        let chain = create_descriptor_chain(&memory, GuestAddress(0x0), 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(), 0);
        assert_eq!(writer.bytes_written(), 0);

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

        assert_eq!(writer.available_bytes(), 0);
        assert_eq!(writer.bytes_written(), 0);
    }

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

        let memory_start_addr = GuestAddress(0x0);
        let memory = GuestMemoryMmap::from_ranges(&[(memory_start_addr, 0x10000)]).unwrap();

        let chain = create_descriptor_chain(
            &memory,
            GuestAddress(0x0),
            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(), 0);
        assert_eq!(reader.bytes_read(), 128);
        assert_eq!(writer.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 = GuestMemoryMmap::from_ranges(&[(memory_start_addr, 0x10000)]).unwrap();

        let secret: Le32 = 0x1234_5678.into();

        // Create a descriptor chain with memory regions that are properly separated.
        let chain_writer = create_descriptor_chain(
            &memory,
            GuestAddress(0x0),
            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(e) = writer.write_obj(secret) {
            panic!("write_obj should not fail here: {e:?}");
        }

        // Now create new descriptor chain pointing to the same memory and try to read it.
        let chain_reader = create_descriptor_chain(
            &memory,
            GuestAddress(0x0),
            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(e) => panic!("read_obj should not fail here: {e:?}"),
            Ok(read_secret) => assert_eq!(read_secret, secret),
        }
    }

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

        let memory_start_addr = GuestAddress(0x0);
        let memory = GuestMemoryMmap::from_ranges(&[(memory_start_addr, 0x10000)]).unwrap();

        let chain = create_descriptor_chain(
            &memory,
            GuestAddress(0x0),
            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![0; 1024];

        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 = GuestMemoryMmap::from_ranges(&[(memory_start_addr, 0x10000)]).unwrap();

        let chain = create_descriptor_chain(
            &memory,
            GuestAddress(0x0),
            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 other = reader.split_at(32).expect("failed to split Reader");
        assert_eq!(reader.available_bytes(), 32);
        assert_eq!(other.available_bytes(), 96);

        let mut writer = Writer::new(&memory, chain.clone()).expect("failed to create Writer");
        let other = writer.split_at(64).expect("failed to split Writer");
        assert_eq!(writer.available_bytes(), 64);
        assert_eq!(other.available_bytes(), 4);
    }

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

        let memory_start_addr = GuestAddress(0x0);
        let memory = GuestMemoryMmap::from_ranges(&[(memory_start_addr, 0x10000)]).unwrap();

        let chain = create_descriptor_chain(
            &memory,
            GuestAddress(0x0),
            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(), 24);
        assert_eq!(other.available_bytes(), 104);
    }

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

        let memory_start_addr = GuestAddress(0x0);
        let memory = GuestMemoryMmap::from_ranges(&[(memory_start_addr, 0x10000)]).unwrap();

        let chain = create_descriptor_chain(
            &memory,
            GuestAddress(0x0),
            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(), 128);
        assert_eq!(other.available_bytes(), 0);
    }

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

        let memory_start_addr = GuestAddress(0x0);
        let memory = GuestMemoryMmap::from_ranges(&[(memory_start_addr, 0x10000)]).unwrap();

        let chain = create_descriptor_chain(
            &memory,
            GuestAddress(0x0),
            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(), 0);
        assert_eq!(other.available_bytes(), 128);
    }

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

        let memory_start_addr = GuestAddress(0x0);
        let memory = GuestMemoryMmap::from_ranges(&[(memory_start_addr, 0x10000)]).unwrap();

        let chain = create_descriptor_chain(
            &memory,
            GuestAddress(0x0),
            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 reader.split_at(256).is_ok() {
            panic!("successfully split Reader with out of bounds offset");
        }
    }

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

        let memory_start_addr = GuestAddress(0x0);
        let memory = GuestMemoryMmap::from_ranges(&[(memory_start_addr, 0x10000)]).unwrap();

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

        let mut buf = [0u8; 64];
        assert_eq!(
            reader.read(&mut buf[..]).expect("failed to read to buffer"),
            48
        );
    }

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

        let memory_start_addr = GuestAddress(0x0);
        let memory = GuestMemoryMmap::from_ranges(&[(memory_start_addr, 0x10000)]).unwrap();

        let chain = create_descriptor_chain(
            &memory,
            GuestAddress(0x0),
            vec![(Writable, 16), (Writable, 16), (Writable, 16)],
            0,
        )
        .expect("create_descriptor_chain failed");
        let mut writer = Writer::new(&memory, chain).expect("failed to create Writer");

        let buf = [0xdeu8; 64];
        assert_eq!(
            writer.write(&buf[..]).expect("failed to write from buffer"),
            48
        );

        assert!(writer.flush().is_ok());
    }
}