src/linux.rs

// Copyright 2017 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;
use std::cmp::min;
use std::ffi::CStr;
use std::fmt;
use std::error;
use std::fs::{File, OpenOptions};
use std::io::{self, Read, stdin};
use std::mem;
use std::os::unix::io::{FromRawFd, RawFd};
use std::os::unix::net::UnixDatagram;
use std::path::{Path, PathBuf};
use std::str;
use std::sync::atomic::{AtomicBool, Ordering};
use std::sync::{Arc, Barrier};
use std::thread;
use std::time::Duration;
use std::thread::JoinHandle;

use libc::{self, c_int};
use rand::thread_rng;
use rand::distributions::{IndependentSample, Range};

use byteorder::{ByteOrder, LittleEndian};
use devices;
use io_jail::{self, Minijail};
use kvm::*;
use net_util::Tap;
use qcow::{self, QcowFile};
use sys_util::*;
use sys_util;
use vhost;
use vm_control::VmRequest;

use Config;
use DiskType;
use VirtIoDeviceInfo;

use arch::{self, LinuxArch, RunnableLinuxVm, VirtioDeviceStub, VmComponents};

#[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
use x86_64::X8664arch as Arch;
#[cfg(any(target_arch = "arm", target_arch = "aarch64"))]
use aarch64::AArch64 as Arch;

#[derive(Debug)]
pub enum Error {
    BalloonDeviceNew(devices::virtio::BalloonError),
    BlockDeviceNew(sys_util::Error),
    BlockSignal(sys_util::signal::Error),
    BuildingVm(Box<error::Error>),
    CloneEventFd(sys_util::Error),
    CreateEventFd(sys_util::Error),
    CreatePollContext(sys_util::Error),
    CreateSignalFd(sys_util::SignalFdError),
    CreateSocket(io::Error),
    CreateTimerFd(sys_util::Error),
    DeviceJail(io_jail::Error),
    DevicePivotRoot(io_jail::Error),
    Disk(io::Error),
    DiskImageLock(sys_util::Error),
    FailedCLOEXECCheck,
    FailedToDupFd,
    InvalidFdPath,
    NetDeviceNew(devices::virtio::NetError),
    NoVarEmpty,
    OpenKernel(PathBuf, io::Error),
    P9DeviceNew(devices::virtio::P9Error),
    PollContextAdd(sys_util::Error),
    PollContextDelete(sys_util::Error),
    QcowDeviceCreate(qcow::Error),
    ReadLowmemAvailable(io::Error),
    ReadLowmemMargin(io::Error),
    RegisterBalloon(arch::MmioRegisterError),
    RegisterBlock(arch::MmioRegisterError),
    RegisterGpu(arch::MmioRegisterError),
    RegisterNet(arch::MmioRegisterError),
    RegisterP9(arch::MmioRegisterError),
    RegisterRng(arch::MmioRegisterError),
    RegisterSignalHandler(sys_util::Error),
    RegisterWayland(arch::MmioRegisterError),
    ResetTimerFd(sys_util::Error),
    RngDeviceNew(devices::virtio::RngError),
    SettingGidMap(io_jail::Error),
    SettingUidMap(io_jail::Error),
    SignalFd(sys_util::SignalFdError),
    SpawnVcpu(io::Error),
    TimerFd(sys_util::Error),
    VhostNetDeviceNew(devices::virtio::vhost::Error),
    VhostVsockDeviceNew(devices::virtio::vhost::Error),
    WaylandDeviceNew(sys_util::Error),
    LoadKernel(Box<error::Error>),
}

impl fmt::Display for Error {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self {
            &Error::BalloonDeviceNew(ref e) => write!(f, "failed to create balloon: {:?}", e),
            &Error::BlockDeviceNew(ref e) => write!(f, "failed to create block device: {:?}", e),
            &Error::BlockSignal(ref e) => write!(f, "failed to block signal: {:?}", e),
            &Error::BuildingVm(ref e) => {
                write!(f, "The architecture failed to build the vm: {:?}", e)
            }
            &Error::CloneEventFd(ref e) => write!(f, "failed to clone eventfd: {:?}", e),
            &Error::CreateEventFd(ref e) => write!(f, "failed to create eventfd: {:?}", e),
            &Error::CreatePollContext(ref e) => write!(f, "failed to create poll context: {:?}", e),
            &Error::CreateSignalFd(ref e) => write!(f, "failed to create signalfd: {:?}", e),
            &Error::CreateSocket(ref e) => write!(f, "failed to create socket: {}", e),
            &Error::CreateTimerFd(ref e) => write!(f, "failed to create timerfd: {}", e),
            &Error::DeviceJail(ref e) => write!(f, "failed to jail device: {}", e),
            &Error::DevicePivotRoot(ref e) => write!(f, "failed to pivot root device: {}", e),
            &Error::Disk(ref e) => write!(f, "failed to load disk image: {}", e),
            &Error::DiskImageLock(ref e) => write!(f, "failed to lock disk image: {:?}", e),
            &Error::FailedCLOEXECCheck => {
                write!(f, "/proc/self/fd argument failed check for CLOEXEC")
            }
            &Error::FailedToDupFd => write!(f, "failed to dup fd from /proc/self/fd"),
            &Error::InvalidFdPath => write!(f, "failed parsing a /proc/self/fd/*"),
            &Error::NetDeviceNew(ref e) => write!(f, "failed to set up virtio networking: {:?}", e),
            &Error::NoVarEmpty => write!(f, "/var/empty doesn't exist, can't jail devices."),
            &Error::OpenKernel(ref p, ref e) => {
                write!(f, "failed to open kernel image {:?}: {}", p, e)
            }
            &Error::P9DeviceNew(ref e) => write!(f, "failed to create 9p device: {}", e),
            &Error::PollContextAdd(ref e) => write!(f, "failed to add fd to poll context: {:?}", e),
            &Error::PollContextDelete(ref e) => {
                write!(f, "failed to remove fd from poll context: {:?}", e)
            }
            &Error::QcowDeviceCreate(ref e) => {
                write!(f, "failed to read qcow formatted file {:?}", e)
            }
            &Error::ReadLowmemAvailable(ref e) => {
                write!(f, "failed to read /sys/kernel/mm/chromeos-low_mem/available: {}", e)
            }
            &Error::ReadLowmemMargin(ref e) => {
                write!(f, "failed to read /sys/kernel/mm/chromeos-low_mem/margin: {}", e)
            }
            &Error::RegisterBalloon(ref e) => {
                write!(f, "error registering balloon device: {:?}", e)
            },
            &Error::RegisterBlock(ref e) => write!(f, "error registering block device: {:?}", e),
            &Error::RegisterGpu(ref e) => write!(f, "error registering gpu device: {:?}", e),
            &Error::RegisterNet(ref e) => write!(f, "error registering net device: {:?}", e),
            &Error::RegisterP9(ref e) => write!(f, "error registering 9p device: {:?}", e),
            &Error::RegisterRng(ref e) => write!(f, "error registering rng device: {:?}", e),
            &Error::RegisterSignalHandler(ref e) => {
                write!(f, "error registering signal handler: {:?}", e)
            }
            &Error::RegisterWayland(ref e) => write!(f, "error registering wayland device: {}", e),
            &Error::ResetTimerFd(ref e) => write!(f, "failed to reset timerfd: {}", e),
            &Error::RngDeviceNew(ref e) => write!(f, "failed to set up rng: {:?}", e),
            &Error::SettingGidMap(ref e) => write!(f, "error setting GID map: {}", e),
            &Error::SettingUidMap(ref e) => write!(f, "error setting UID map: {}", e),
            &Error::SignalFd(ref e) => write!(f, "failed to read signal fd: {:?}", e),
            &Error::SpawnVcpu(ref e) => write!(f, "failed to spawn VCPU thread: {:?}", e),
            &Error::TimerFd(ref e) => write!(f, "failed to read timer fd: {:?}", e),
            &Error::VhostNetDeviceNew(ref e) => {
                write!(f, "failed to set up vhost networking: {:?}", e)
            }
            &Error::VhostVsockDeviceNew(ref e) => {
                write!(f, "failed to set up virtual socket device: {:?}", e)
            }
            &Error::WaylandDeviceNew(ref e) => {
                write!(f, "failed to create wayland device: {:?}", e)
            }
            &Error::LoadKernel(ref e) => write!(f, "failed to load kernel: {}", e),
        }
    }
}

impl std::error::Error for Error {
    fn description(&self) -> &str {
        "Some device failure"
    }
}

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

// Verifies that |raw_fd| is actually owned by this process and duplicates it to ensure that
// we have a unique handle to it.
fn validate_raw_fd(raw_fd: RawFd) -> std::result::Result<RawFd, Box<error::Error>> {
    // Checking that close-on-exec isn't set helps filter out FDs that were opened by
    // crosvm as all crosvm FDs are close on exec.
    // Safe because this doesn't modify any memory and we check the return value.
    let flags = unsafe { libc::fcntl(raw_fd, libc::F_GETFD) };
    if flags < 0 || (flags & libc::FD_CLOEXEC) != 0 {
        return Err(Box::new(Error::FailedCLOEXECCheck));
    }

    // Duplicate the fd to ensure that we don't accidentally close an fd previously
    // opened by another subsystem.  Safe because this doesn't modify any memory and
    // we check the return value.
    let dup_fd = unsafe { libc::fcntl(raw_fd, libc::F_DUPFD_CLOEXEC, 0) };
    if dup_fd < 0 {
        return Err(Box::new(Error::FailedToDupFd));
    }
    Ok(dup_fd as RawFd)
}

fn create_base_minijail(root: &Path, seccomp_policy: &Path) -> Result<Minijail> {
    // All child jails run in a new user namespace without any users mapped,
    // they run as nobody unless otherwise configured.
    let mut j = Minijail::new().map_err(|e| Error::DeviceJail(e))?;
    j.namespace_pids();
    j.namespace_user();
    j.namespace_user_disable_setgroups();
    // Don't need any capabilities.
    j.use_caps(0);
    // Create a new mount namespace with an empty root FS.
    j.namespace_vfs();
    j.enter_pivot_root(root)
        .map_err(|e| Error::DevicePivotRoot(e))?;
    // Run in an empty network namespace.
    j.namespace_net();
    // Apply the block device seccomp policy.
    j.no_new_privs();
    // Use TSYNC only for the side effect of it using SECCOMP_RET_TRAP, which will correctly kill
    // the entire device process if a worker thread commits a seccomp violation.
    j.set_seccomp_filter_tsync();
    #[cfg(debug_assertions)]
    j.log_seccomp_filter_failures();
    j.parse_seccomp_filters(seccomp_policy)
        .map_err(|e| Error::DeviceJail(e))?;
    j.use_seccomp_filter();
    // Don't do init setup.
    j.run_as_init();
    Ok(j)
}

fn create_virtio_devs(cfg: VirtIoDeviceInfo,
                      mem: &GuestMemory,
                      _exit_evt: &EventFd,
                      wayland_device_socket: UnixDatagram,
                      balloon_device_socket: UnixDatagram)
                      -> std::result::Result<Vec<VirtioDeviceStub>, Box<error::Error>> {
    static DEFAULT_PIVOT_ROOT: &'static str = "/var/empty";

    let mut devs = Vec::new();

    // An empty directory for jailed device's pivot root.
    let empty_root_path = Path::new(DEFAULT_PIVOT_ROOT);
    if cfg.multiprocess && !empty_root_path.exists() {
        return Err(Box::new(Error::NoVarEmpty));
    }

    for disk in &cfg.disks {
        // Special case '/proc/self/fd/*' paths. The FD is already open, just use it.
        let mut raw_image: File = if disk.path.parent() == Some(Path::new("/proc/self/fd")) {
            if !disk.path.is_file() {
                return Err(Box::new(Error::InvalidFdPath));
            }
            let raw_fd = disk.path.file_name()
                .and_then(|fd_osstr| fd_osstr.to_str())
                .and_then(|fd_str| fd_str.parse::<c_int>().ok())
                .ok_or(Error::InvalidFdPath)?;
            // Safe because we will validate |raw_fd|.
            unsafe { File::from_raw_fd(validate_raw_fd(raw_fd)?) }
        } else {
            OpenOptions::new()
                .read(true)
                .write(!disk.read_only)
                .open(&disk.path)
                .map_err(|e| Error::Disk(e))?
        };
        // Lock the disk image to prevent other crosvm instances from using it.
        let lock_op = if disk.read_only {
            FlockOperation::LockShared
        } else {
            FlockOperation::LockExclusive
        };
        flock(&raw_image, lock_op, true).map_err(Error::DiskImageLock)?;

        let block_box: Box<devices::virtio::VirtioDevice> = match disk.disk_type {
            DiskType::FlatFile => { // Access as a raw block device.
                Box::new(devices::virtio::Block::new(raw_image, disk.read_only)
                    .map_err(|e| Error::BlockDeviceNew(e))?)
            }
            DiskType::Qcow => { // Valid qcow header present
                let qcow_image = QcowFile::from(raw_image)
                    .map_err(|e| Error::QcowDeviceCreate(e))?;
                Box::new(devices::virtio::Block::new(qcow_image, disk.read_only)
                    .map_err(|e| Error::BlockDeviceNew(e))?)
            }
        };
        let jail = if cfg.multiprocess {
            let policy_path: PathBuf = cfg.seccomp_policy_dir.join("block_device.policy");
            Some(create_base_minijail(empty_root_path, &policy_path)?)
        }
        else {
            None
        };

        devs.push(VirtioDeviceStub {dev: block_box, jail});
    }

    let rng_box = Box::new(devices::virtio::Rng::new().map_err(Error::RngDeviceNew)?);
    let rng_jail = if cfg.multiprocess {
        let policy_path: PathBuf = cfg.seccomp_policy_dir.join("rng_device.policy");
        Some(create_base_minijail(empty_root_path, &policy_path)?)
    } else {
        None
    };
    devs.push(VirtioDeviceStub {dev: rng_box, jail: rng_jail});

    let balloon_box = Box::new(devices::virtio::Balloon::new(balloon_device_socket)
                                   .map_err(Error::BalloonDeviceNew)?);
    let balloon_jail = if cfg.multiprocess {
        let policy_path: PathBuf = cfg.seccomp_policy_dir.join("balloon_device.policy");
        Some(create_base_minijail(empty_root_path, &policy_path)?)
    } else {
        None
    };
    devs.push(VirtioDeviceStub {dev: balloon_box, jail: balloon_jail});

    // We checked above that if the IP is defined, then the netmask is, too.
    if let Some(tap_fd) = cfg.tap_fd {
        // Safe because we ensure that we get a unique handle to the fd.
        let tap = unsafe { Tap::from_raw_fd(validate_raw_fd(tap_fd)?) };
        let net_box = Box::new(devices::virtio::Net::from(tap)
            .map_err(|e| Error::NetDeviceNew(e))?);

        let jail = if cfg.multiprocess {
            let policy_path: PathBuf = cfg.seccomp_policy_dir.join("net_device.policy");

            Some(create_base_minijail(empty_root_path, &policy_path)?)
        } else {
            None
        };

        devs.push(VirtioDeviceStub {dev: net_box, jail});
    } else if let Some(host_ip) = cfg.host_ip {
        if let Some(netmask) = cfg.netmask {
            if let Some(mac_address) = cfg.mac_address {
                let net_box: Box<devices::virtio::VirtioDevice> = if cfg.vhost_net {
                    Box::new(devices::virtio::vhost::Net::<Tap, vhost::Net<Tap>>::new(host_ip,
                                                                                      netmask,
                                                                                      mac_address,
                                                                                      &mem)
                                       .map_err(|e| Error::VhostNetDeviceNew(e))?)
                } else {
                    Box::new(devices::virtio::Net::<Tap>::new(host_ip, netmask, mac_address)
                                       .map_err(|e| Error::NetDeviceNew(e))?)
                };

                let jail = if cfg.multiprocess {
                    let policy_path: PathBuf = if cfg.vhost_net {
                        cfg.seccomp_policy_dir.join("vhost_net_device.policy")
                    } else {
                        cfg.seccomp_policy_dir.join("net_device.policy")
                    };

                    Some(create_base_minijail(empty_root_path, &policy_path)?)
                } else {
                    None
                };

                devs.push(VirtioDeviceStub {dev: net_box, jail});
            }
        }
    }

    if let Some(wayland_socket_path) = cfg.wayland_socket_path.as_ref() {
        let jailed_wayland_path = Path::new("/wayland-0");

        let wl_box = Box::new(devices::virtio::Wl::new(if cfg.multiprocess {
                                                           &jailed_wayland_path
                                                       } else {
                                                           wayland_socket_path.as_path()
                                                       },
                                                       wayland_device_socket)
                                      .map_err(Error::WaylandDeviceNew)?);

        let jail = if cfg.multiprocess {
            let policy_path: PathBuf = cfg.seccomp_policy_dir.join("wl_device.policy");
            let mut jail = create_base_minijail(empty_root_path, &policy_path)?;

            // Create a tmpfs in the device's root directory so that we can bind mount the
            // wayland socket into it.  The size=67108864 is size=64*1024*1024 or size=64MB.
            jail.mount_with_data(Path::new("none"), Path::new("/"), "tmpfs",
                                 (libc::MS_NOSUID | libc::MS_NODEV | libc::MS_NOEXEC) as usize,
                                 "size=67108864")
                .unwrap();

            // Bind mount the wayland socket into jail's root. This is necessary since each
            // new wayland context must open() the socket.
            jail.mount_bind(wayland_socket_path.as_path(), jailed_wayland_path, true)
                .unwrap();

            // Set the uid/gid for the jailed process, and give a basic id map. This
            // is required for the above bind mount to work.
            let crosvm_user_group = CStr::from_bytes_with_nul(b"crosvm\0").unwrap();
            let crosvm_uid = match get_user_id(&crosvm_user_group) {
                Ok(u) => u,
                Err(e) => {
                    warn!("falling back to current user id for Wayland: {:?}", e);
                    geteuid()
                }
            };
            let crosvm_gid = match get_group_id(&crosvm_user_group) {
                Ok(u) => u,
                Err(e) => {
                    warn!("falling back to current group id for Wayland: {:?}", e);
                    getegid()
                }
            };
            jail.change_uid(crosvm_uid);
            jail.change_gid(crosvm_gid);
            jail.uidmap(&format!("{0} {0} 1", crosvm_uid))
                .map_err(Error::SettingUidMap)?;
            jail.gidmap(&format!("{0} {0} 1", crosvm_gid))
                .map_err(Error::SettingGidMap)?;

            Some(jail)
        } else {
            None
        };
        devs.push(VirtioDeviceStub {
                dev: wl_box,
                jail,
        });
    }

    if let Some(cid) = cfg.cid {
        let vsock_box = Box::new(devices::virtio::vhost::Vsock::new(cid, &mem)
                                     .map_err(Error::VhostVsockDeviceNew)?);

        let jail = if cfg.multiprocess {
            let policy_path: PathBuf = cfg.seccomp_policy_dir.join("vhost_vsock_device.policy");

            Some(create_base_minijail(empty_root_path, &policy_path)?)
        } else {
            None
        };

        devs.push(VirtioDeviceStub {dev: vsock_box, jail});
    }

    #[cfg(feature = "gpu")]
    {
        if cfg.gpu {
            let gpu_box =
                Box::new(devices::virtio::Gpu::new(_exit_evt
                                                       .try_clone()
                                                       .map_err(Error::CloneEventFd)?));
            let jail = if cfg.multiprocess {
                error!("jail for virtio-gpu is unimplemented");
                unimplemented!();
            } else {
                None
            };
            devs.push(VirtioDeviceStub {dev: gpu_box, jail});
        }
    }

    let chronos_user_group = CStr::from_bytes_with_nul(b"chronos\0").unwrap();
    let chronos_uid = match get_user_id(&chronos_user_group) {
        Ok(u) => u,
        Err(e) => {
            warn!("falling back to current user id for 9p: {:?}", e);
            geteuid()
        }
    };
    let chronos_gid = match get_group_id(&chronos_user_group) {
        Ok(u) => u,
        Err(e) => {
            warn!("falling back to current group id for 9p: {:?}", e);
            getegid()
        }
    };

    for &(ref src, ref tag) in &cfg.shared_dirs {
        let (jail, root) = if cfg.multiprocess {
            let policy_path: PathBuf = cfg.seccomp_policy_dir.join("9p_device.policy");
            let mut jail = create_base_minijail(empty_root_path, &policy_path)?;

            //  The shared directory becomes the root of the device's file system.
            let root = Path::new("/");
            jail.mount_bind(&src, root, true).unwrap();

            // Set the uid/gid for the jailed process, and give a basic id map. This
            // is required for the above bind mount to work.
            jail.change_uid(chronos_uid);
            jail.change_gid(chronos_gid);
            jail.uidmap(&format!("{0} {0} 1", chronos_uid))
                .map_err(Error::SettingUidMap)?;
            jail.gidmap(&format!("{0} {0} 1", chronos_gid))
                .map_err(Error::SettingGidMap)?;

            (Some(jail), root)
        } else {
            // There's no bind mount so we tell the server to treat the source directory as the
            // root.  The double deref here converts |src| from a &PathBuf into a &Path.
            (None, &**src)
        };

        let p9_box = Box::new(devices::virtio::P9::new(root, tag).map_err(Error::P9DeviceNew)?);

        devs.push(VirtioDeviceStub {dev: p9_box, jail});
    }

    Ok(devs)
}

fn setup_vcpu_signal_handler() -> Result<()> {
    unsafe {
        extern "C" fn handle_signal() {}
        // Our signal handler does nothing and is trivially async signal safe.
        register_signal_handler(SIGRTMIN() + 0, handle_signal)
            .map_err(Error::RegisterSignalHandler)?;
    }
    block_signal(SIGRTMIN() + 0).map_err(Error::BlockSignal)?;
    Ok(())
}

fn run_vcpu(vcpu: Vcpu,
            cpu_id: u32,
            start_barrier: Arc<Barrier>,
            io_bus: devices::Bus,
            mmio_bus: devices::Bus,
            exit_evt: EventFd,
            kill_signaled: Arc<AtomicBool>) -> Result<JoinHandle<()>> {
    thread::Builder::new()
        .name(format!("crosvm_vcpu{}", cpu_id))
        .spawn(move || {
            let mut sig_ok = true;
            match get_blocked_signals() {
                Ok(mut v) => {
                    v.retain(|&x| x != SIGRTMIN() + 0);
                    if let Err(e) = vcpu.set_signal_mask(&v) {
                        error!(
                            "Failed to set the KVM_SIGNAL_MASK for vcpu {} : {:?}",
                            cpu_id, e
                        );
                        sig_ok = false;
                    }
                }
                Err(e) => {
                    error!(
                        "Failed to retrieve signal mask for vcpu {} : {:?}",
                        cpu_id, e
                    );
                    sig_ok = false;
                }
            };

            start_barrier.wait();

            while sig_ok {
                let run_res = vcpu.run();
                match run_res {
                    Ok(run) => {
                        match run {
                            VcpuExit::IoIn(addr, data) => {
                                io_bus.read(addr as u64, data);
                            }
                            VcpuExit::IoOut(addr, data) => {
                                io_bus.write(addr as u64, data);
                            }
                            VcpuExit::MmioRead(addr, data) => {
                                mmio_bus.read(addr, data);
                            }
                            VcpuExit::MmioWrite(addr, data) => {
                                mmio_bus.write(addr, data);
                            }
                            VcpuExit::Hlt => break,
                            VcpuExit::Shutdown => break,
                            VcpuExit::SystemEvent(_, _) =>
                                //TODO handle reboot and crash events
                                kill_signaled.store(true, Ordering::SeqCst),
                            r => warn!("unexpected vcpu exit: {:?}", r),
                        }
                    }
                    Err(e) => {
                        match e.errno() {
                            libc::EAGAIN | libc::EINTR => {},
                            _ => {
                                error!("vcpu hit unknown error: {:?}", e);
                                break;
                            }
                        }
                    }
                }
                if kill_signaled.load(Ordering::SeqCst) {
                    break;
                }

                // Try to clear the signal that we use to kick VCPU if it is
                // pending before attempting to handle pause requests.
                clear_signal(SIGRTMIN() + 0).expect("failed to clear pending signal");
            }
            exit_evt
                .write(1)
                .expect("failed to signal vcpu exit eventfd");
        })
        .map_err(Error::SpawnVcpu)
}

// Reads the contents of a file and converts them into a u64.
fn file_to_u64<P: AsRef<Path>>(path: P) -> io::Result<u64> {
    let mut file = File::open(path)?;

    let mut buf = [0u8; 32];
    let count = file.read(&mut buf)?;

    let content = str::from_utf8(&buf[..count])
        .map_err(|e| io::Error::new(io::ErrorKind::InvalidData, e))?;
    content.trim().parse().map_err(|e| io::Error::new(io::ErrorKind::InvalidData, e))
}

pub fn run_config(cfg: Config) -> Result<()> {
    if cfg.virtio_dev_info.multiprocess {
        // Printing something to the syslog before entering minijail so that libc's syslogger has a
        // chance to open files necessary for its operation, like `/etc/localtime`. After jailing,
        // access to those files will not be possible.
        info!("crosvm entering multiprocess mode");
    }

    let pci_devices = devices::PciDeviceList::new();

    // Masking signals is inherently dangerous, since this can persist across clones/execs. Do this
    // before any jailed devices have been spawned, so that we can catch any of them that fail very
    // quickly.
    let sigchld_fd = SignalFd::new(libc::SIGCHLD).map_err(Error::CreateSignalFd)?;

    let components = VmComponents {
        pci_devices,
        memory_mb: (cfg.memory.unwrap_or(256) << 20) as u64,
        vcpu_count: cfg.vcpu_count.unwrap_or(1),
        kernel_image: File::open(cfg.kernel_path.as_path())
            .map_err(|e| Error::OpenKernel(cfg.kernel_path.clone(), e))?,
        extra_kernel_params: cfg.params,
        wayland_dmabuf: cfg.virtio_dev_info.wayland_dmabuf,
    };

    let mut control_sockets = Vec::new();
    if let Some(ref path_string) = cfg.socket_path {
        let path = Path::new(path_string);
        let dgram = UnixDatagram::bind(path).map_err(Error::CreateSocket)?;
        control_sockets.push(UnlinkUnixDatagram(dgram));
    };
    let (wayland_host_socket, wayland_device_socket) = UnixDatagram::pair()
        .map_err(Error::CreateSocket)?;
    control_sockets.push(UnlinkUnixDatagram(wayland_host_socket));
    // Balloon gets a special socket so balloon requests can be forwarded from the main process.
    let (balloon_host_socket, balloon_device_socket) = UnixDatagram::pair()
        .map_err(Error::CreateSocket)?;

    let virtio_dev_info = cfg.virtio_dev_info;
    let linux = Arch::build_vm(components,
                               |m, e| create_virtio_devs(virtio_dev_info, m, e,
                                                         wayland_device_socket,
                                                         balloon_device_socket))
        .map_err(Error::BuildingVm)?;
    run_control(linux, control_sockets, balloon_host_socket, sigchld_fd)
}

fn run_control(mut linux: RunnableLinuxVm,
               control_sockets: Vec<UnlinkUnixDatagram>,
               balloon_host_socket: UnixDatagram,
               sigchld_fd: SignalFd) -> Result<()> {
    const MAX_VM_FD_RECV: usize = 1;

    // Paths to get the currently available memory and the low memory threshold.
    const LOWMEM_MARGIN: &'static str = "/sys/kernel/mm/chromeos-low_mem/margin";
    const LOWMEM_AVAILABLE: &'static str = "/sys/kernel/mm/chromeos-low_mem/available";

    // The amount of additional memory to claim back from the VM whenever the system is
    // low on memory.
    const ONE_GB: u64 = (1 << 30);

    let max_balloon_memory = match linux.vm.get_memory().memory_size() {
        // If the VM has at least 1.5 GB, the balloon driver can consume all but the last 1 GB.
        n if n >= (ONE_GB / 2) * 3 => n - ONE_GB,
        // Otherwise, if the VM has at least 500MB the balloon driver will consume at most
        // half of it.
        n if n >= (ONE_GB / 2) => n / 2,
        // Otherwise, the VM is too small for us to take memory away from it.
        _ => 0,
    };
    let mut current_balloon_memory: u64 = 0;
    let balloon_memory_increment: u64 = max_balloon_memory / 16;

    #[derive(PollToken)]
    enum Token {
        Exit,
        Stdin,
        ChildSignal,
        CheckAvailableMemory,
        LowMemory,
        LowmemTimer,
        VmControl { index: usize },
    }

    let stdin_handle = stdin();
    let stdin_lock = stdin_handle.lock();
    stdin_lock
        .set_raw_mode()
        .expect("failed to set terminal raw mode");

    let poll_ctx = PollContext::new().map_err(Error::CreatePollContext)?;
    poll_ctx.add(&linux.exit_evt, Token::Exit).map_err(Error::PollContextAdd)?;
    if let Err(e) = poll_ctx.add(&stdin_handle, Token::Stdin) {
        warn!("failed to add stdin to poll context: {:?}", e);
    }
    poll_ctx.add(&sigchld_fd, Token::ChildSignal).map_err(Error::PollContextAdd)?;
    for (index, socket) in control_sockets.iter().enumerate() {
        poll_ctx.add(socket.as_ref(), Token::VmControl{ index }).map_err(Error::PollContextAdd)?;
    }

    // Watch for low memory notifications and take memory back from the VM.
    let low_mem = File::open("/dev/chromeos-low-mem").ok();
    if let Some(ref low_mem) = low_mem {
        poll_ctx.add(low_mem, Token::LowMemory).map_err(Error::PollContextAdd)?;
    } else {
        warn!("Unable to open low mem indicator, maybe not a chrome os kernel");
    }

    // Used to rate limit balloon requests.
    let mut lowmem_timer = TimerFd::new().map_err(Error::CreateTimerFd)?;
    poll_ctx.add(&lowmem_timer, Token::LowmemTimer).map_err(Error::PollContextAdd)?;

    // Used to check whether it's ok to start giving memory back to the VM.
    let mut freemem_timer = TimerFd::new().map_err(Error::CreateTimerFd)?;
    poll_ctx.add(&freemem_timer, Token::CheckAvailableMemory).map_err(Error::PollContextAdd)?;

    // Used to add jitter to timer values so that we don't have a thundering herd problem when
    // multiple VMs are running.
    let mut rng = thread_rng();
    let lowmem_jitter_ms = Range::new(0, 200);
    let freemem_jitter_secs = Range::new(0, 12);
    let interval_jitter_secs = Range::new(0, 6);

    let mut vcpu_handles = Vec::with_capacity(linux.vcpus.len() as usize);
    let vcpu_thread_barrier = Arc::new(Barrier::new((linux.vcpus.len() + 1) as usize));
    let kill_signaled = Arc::new(AtomicBool::new(false));
    setup_vcpu_signal_handler()?;
    for (cpu_id, vcpu) in linux.vcpus.into_iter().enumerate() {
        let handle = run_vcpu(vcpu,
                              cpu_id as u32,
                              vcpu_thread_barrier.clone(),
                              linux.io_bus.clone(),
                              linux.mmio_bus.clone(),
                              linux.exit_evt.try_clone().map_err(Error::CloneEventFd)?,
                              kill_signaled.clone())?;
        vcpu_handles.push(handle);
    }
    vcpu_thread_barrier.wait();

    let mut scm = Scm::new(MAX_VM_FD_RECV);

    'poll: loop {
        let events = {
            match poll_ctx.wait() {
                Ok(v) => v,
                Err(e) => {
                    error!("failed to poll: {:?}", e);
                    break;
                }
            }
        };
        for event in events.iter_readable() {
            match event.token() {
                Token::Exit => {
                    info!("vcpu requested shutdown");
                    break 'poll;
                }
                Token::Stdin => {
                    let mut out = [0u8; 64];
                    match stdin_lock.read_raw(&mut out[..]) {
                        Ok(0) => {
                            // Zero-length read indicates EOF. Remove from pollables.
                            let _ = poll_ctx.delete(&stdin_handle);
                        },
                        Err(e) => {
                            warn!("error while reading stdin: {:?}", e);
                            let _ = poll_ctx.delete(&stdin_handle);
                        },
                        Ok(count) => {
                            linux.stdio_serial
                                .lock()
                                .unwrap()
                                .queue_input_bytes(&out[..count])
                                .expect("failed to queue bytes into serial port");
                        },
                    }
                }
                Token::ChildSignal => {
                    // Print all available siginfo structs, then exit the loop.
                    loop {
                        let result = sigchld_fd.read().map_err(Error::SignalFd)?;
                        if let Some(siginfo) = result {
                            error!("child {} died: signo {}, status {}, code {}",
                                   siginfo.ssi_pid,
                                   siginfo.ssi_signo,
                                   siginfo.ssi_status,
                                   siginfo.ssi_code);
                        }
                        break 'poll;
                    }
                }
                Token::CheckAvailableMemory => {
                    // Acknowledge the timer.
                    freemem_timer.wait().map_err(Error::TimerFd)?;
                    if current_balloon_memory == 0 {
                        // Nothing to see here.
                        if let Err(e) = freemem_timer.clear() {
                            warn!("unable to clear available memory check timer: {}", e);
                        }
                        continue;
                    }

                    // Otherwise see if we can free up some memory.
                    let margin = file_to_u64(LOWMEM_MARGIN).map_err(Error::ReadLowmemMargin)?;
                    let available = file_to_u64(LOWMEM_AVAILABLE).map_err(Error::ReadLowmemAvailable)?;

                    // `available` and `margin` are specified in MB while `balloon_memory_increment` is in
                    // bytes.  So to correctly compare them we need to turn the increment value into MB.
                    if available >= margin + 2*(balloon_memory_increment >> 20) {
                        current_balloon_memory = if current_balloon_memory >= balloon_memory_increment {
                            current_balloon_memory - balloon_memory_increment
                        } else {
                            0
                        };
                        let mut buf = [0u8; mem::size_of::<u64>()];
                        LittleEndian::write_u64(&mut buf, current_balloon_memory);
                        if let Err(e) = balloon_host_socket.send(&buf) {
                            warn!("failed to send memory value to balloon device: {}", e);
                        }
                    }
                }
                Token::LowMemory => {
                    if let Some(ref low_mem) = low_mem {
                        let old_balloon_memory = current_balloon_memory;
                        current_balloon_memory =
                            min(current_balloon_memory + balloon_memory_increment,
                                max_balloon_memory);
                        if current_balloon_memory != old_balloon_memory {
                            let mut buf = [0u8; mem::size_of::<u64>()];
                            LittleEndian::write_u64(&mut buf, current_balloon_memory);
                            if let Err(e) = balloon_host_socket.send(&buf) {
                                warn!("failed to send memory value to balloon device: {}", e);
                            }
                        }

                        // Stop polling the lowmem device until the timer fires.
                        poll_ctx.delete(low_mem).map_err(Error::PollContextDelete)?;

                        // Add some jitter to the timer so that if there are multiple VMs running
                        // they don't all start ballooning at exactly the same time.
                        let lowmem_dur =
                            Duration::from_millis(1000 + lowmem_jitter_ms.ind_sample(&mut rng));
                        lowmem_timer.reset(lowmem_dur, None).map_err(Error::ResetTimerFd)?;

                        // Also start a timer to check when we can start giving memory back.  Do the
                        // first check after a minute (with jitter) and subsequent checks after
                        // every 30 seconds (with jitter).
                        let freemem_dur =
                            Duration::from_secs(60 + freemem_jitter_secs.ind_sample(&mut rng));
                        let freemem_int =
                            Duration::from_secs(30 + interval_jitter_secs.ind_sample(&mut rng));
                        freemem_timer
                            .reset(freemem_dur, Some(freemem_int))
                            .map_err(Error::ResetTimerFd)?;
                    }
                }
                Token::LowmemTimer => {
                    // Acknowledge the timer.
                    lowmem_timer.wait().map_err(Error::TimerFd)?;

                    if let Some(ref low_mem) = low_mem {
                        // Start polling the lowmem device again.
                        poll_ctx.add(low_mem, Token::LowMemory).map_err(Error::PollContextAdd)?;
                    }
                }
                Token::VmControl { index } => {
                    if let Some(socket) = control_sockets.get(index as usize) {
                        match VmRequest::recv(&mut scm, socket.as_ref()) {
                            Ok(request) => {
                                let mut running = true;
                                let response =
                                    request.execute(&mut linux.vm,
                                                    &mut linux.resources,
                                                    &mut running,
                                                    &balloon_host_socket);
                                if let Err(e) = response.send(&mut scm, socket.as_ref()) {
                                    error!("failed to send VmResponse: {:?}", e);
                                }
                                if !running {
                                    info!("control socket requested exit");
                                    break 'poll;
                                }
                            }
                            Err(e) => error!("failed to recv VmRequest: {:?}", e),
                        }
                    }
                }
            }
        }
        for event in events.iter_hungup() {
            // It's possible more data is readable and buffered while the socket is hungup, so
            // don't delete the socket from the poll context until we're sure all the data is
            // read.
            if !event.readable() {
                match event.token() {
                    Token::Exit => {},
                    Token::Stdin => {
                        let _ = poll_ctx.delete(&stdin_handle);
                    },
                    Token::ChildSignal => {},
                    Token::CheckAvailableMemory => {},
                    Token::LowMemory => {},
                    Token::LowmemTimer => {},
                    Token::VmControl { index } => {
                        if let Some(socket) = control_sockets.get(index as usize) {
                            let _ = poll_ctx.delete(socket.as_ref());
                        }
                    },
                }
            }
        }
    }

    // vcpu threads MUST see the kill signaled flag, otherwise they may
    // re-enter the VM.
    kill_signaled.store(true, Ordering::SeqCst);
    for handle in vcpu_handles {
        match handle.kill(SIGRTMIN() + 0) {
            Ok(_) => {
                if let Err(e) = handle.join() {
                    error!("failed to join vcpu thread: {:?}", e);
                }
            }
            Err(e) => error!("failed to kill vcpu thread: {:?}", e),
        }
    }

    stdin_lock
        .set_canon_mode()
        .expect("failed to restore canonical mode for terminal");

    Ok(())
}