vm-memory/src/bytes.rs

// Portions Copyright 2019 Red Hat, Inc.
//
// Portions Copyright 2018 Amazon.com, Inc. or its affiliates. All Rights Reserved.
//
// Portions 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-BSD-3-Clause file.
//
// SPDX-License-Identifier: Apache-2.0 OR BSD-3-Clause

//! Define the `ByteValued` trait to mark that it is safe to instantiate the struct with random
//! data.

use std::io::{Read, Write};
use std::mem::size_of;
use std::result::Result;
use std::slice::{from_raw_parts, from_raw_parts_mut};
use std::sync::atomic::Ordering;

use crate::atomic_integer::AtomicInteger;
use crate::volatile_memory::VolatileSlice;

/// Types for which it is safe to initialize from raw data.
///
/// # Safety
///
/// A type `T` is `ByteValued` if and only if it can be initialized by reading its contents from a
/// byte array.  This is generally true for all plain-old-data structs.  It is notably not true for
/// any type that includes a reference. It is generally also not safe for non-packed structs, as
/// compiler-inserted padding is considered uninitialized memory, and thus reads/writing it will
/// cause undefined behavior.
///
/// Implementing this trait guarantees that it is safe to instantiate the struct with random data.
pub unsafe trait ByteValued: Copy + Default + Send + Sync {
    /// Converts a slice of raw data into a reference of `Self`.
    ///
    /// The value of `data` is not copied. Instead a reference is made from the given slice. The
    /// value of `Self` will depend on the representation of the type in memory, and may change in
    /// an unstable fashion.
    ///
    /// This will return `None` if the length of data does not match the size of `Self`, or if the
    /// data is not aligned for the type of `Self`.
    fn from_slice(data: &[u8]) -> Option<&Self> {
        // Early out to avoid an unneeded `align_to` call.
        if data.len() != size_of::<Self>() {
            return None;
        }

        // SAFETY: Safe because the ByteValued trait asserts any data is valid for this type, and
        // we ensured the size of the pointer's buffer is the correct size. The `align_to` method
        // ensures that we don't have any unaligned references. This aliases a pointer, but because
        // the pointer is from a const slice reference, there are no mutable aliases. Finally, the
        // reference returned can not outlive data because they have equal implicit lifetime
        // constraints.
        match unsafe { data.align_to::<Self>() } {
            ([], [mid], []) => Some(mid),
            _ => None,
        }
    }

    /// Converts a mutable slice of raw data into a mutable reference of `Self`.
    ///
    /// Because `Self` is made from a reference to the mutable slice, mutations to the returned
    /// reference are immediately reflected in `data`. The value of the returned `Self` will depend
    /// on the representation of the type in memory, and may change in an unstable fashion.
    ///
    /// This will return `None` if the length of data does not match the size of `Self`, or if the
    /// data is not aligned for the type of `Self`.
    fn from_mut_slice(data: &mut [u8]) -> Option<&mut Self> {
        // Early out to avoid an unneeded `align_to_mut` call.
        if data.len() != size_of::<Self>() {
            return None;
        }

        // SAFETY: Safe because the ByteValued trait asserts any data is valid for this type, and
        // we ensured the size of the pointer's buffer is the correct size. The `align_to` method
        // ensures that we don't have any unaligned references. This aliases a pointer, but because
        // the pointer is from a mut slice reference, we borrow the passed in mutable reference.
        // Finally, the reference returned can not outlive data because they have equal implicit
        // lifetime constraints.
        match unsafe { data.align_to_mut::<Self>() } {
            ([], [mid], []) => Some(mid),
            _ => None,
        }
    }

    /// Converts a reference to `self` into a slice of bytes.
    ///
    /// The value of `self` is not copied. Instead, the slice is made from a reference to `self`.
    /// The value of bytes in the returned slice will depend on the representation of the type in
    /// memory, and may change in an unstable fashion.
    fn as_slice(&self) -> &[u8] {
        // SAFETY: Safe because the entire size of self is accessible as bytes because the trait
        // guarantees it. The lifetime of the returned slice is the same as the passed reference,
        // so that no dangling pointers will result from this pointer alias.
        unsafe { from_raw_parts(self as *const Self as *const u8, size_of::<Self>()) }
    }

    /// Converts a mutable reference to `self` into a mutable slice of bytes.
    ///
    /// Because the slice is made from a reference to `self`, mutations to the returned slice are
    /// immediately reflected in `self`. The value of bytes in the returned slice will depend on
    /// the representation of the type in memory, and may change in an unstable fashion.
    fn as_mut_slice(&mut self) -> &mut [u8] {
        // SAFETY: Safe because the entire size of self is accessible as bytes because the trait
        // guarantees it. The trait also guarantees that any combination of bytes is valid for this
        // type, so modifying them in the form of a byte slice is valid. The lifetime of the
        // returned slice is the same as the passed reference, so that no dangling pointers will
        // result from this pointer alias. Although this does alias a mutable pointer, we do so by
        // exclusively borrowing the given mutable reference.
        unsafe { from_raw_parts_mut(self as *mut Self as *mut u8, size_of::<Self>()) }
    }

    /// Converts a mutable reference to `self` into a `VolatileSlice`.  This is
    /// useful because `VolatileSlice` provides a `Bytes<usize>` implementation.
    ///
    /// # Safety
    ///
    /// Unlike most `VolatileMemory` implementation, this method requires an exclusive
    /// reference to `self`; this trivially fulfills `VolatileSlice::new`'s requirement
    /// that all accesses to `self` use volatile accesses (because there can
    /// be no other accesses).
    fn as_bytes(&mut self) -> VolatileSlice {
        // SAFETY: This is safe because the lifetime is the same as self
        unsafe { VolatileSlice::new(self as *mut Self as *mut _, size_of::<Self>()) }
    }
}

macro_rules! byte_valued_array {
    ($T:ty, $($N:expr)+) => {
        $(
            // SAFETY: All intrinsic types and arrays of intrinsic types are ByteValued.
            // They are just numbers.
            unsafe impl ByteValued for [$T; $N] {}
        )+
    }
}

macro_rules! byte_valued_type {
    ($T:ty) => {
        // SAFETY: Safe as long T is POD.
        // We are using this macro to generated the implementation for integer types below.
        unsafe impl ByteValued for $T {}
        byte_valued_array! {
            $T,
            0  1  2  3  4  5  6  7  8  9
            10 11 12 13 14 15 16 17 18 19
            20 21 22 23 24 25 26 27 28 29
            30 31 32
        }
    };
}

byte_valued_type!(u8);
byte_valued_type!(u16);
byte_valued_type!(u32);
byte_valued_type!(u64);
byte_valued_type!(u128);
byte_valued_type!(usize);
byte_valued_type!(i8);
byte_valued_type!(i16);
byte_valued_type!(i32);
byte_valued_type!(i64);
byte_valued_type!(i128);
byte_valued_type!(isize);

/// A trait used to identify types which can be accessed atomically by proxy.
pub trait AtomicAccess:
    ByteValued
    // Could not find a more succinct way of stating that `Self` can be converted
    // into `Self::A::V`, and the other way around.
    + From<<<Self as AtomicAccess>::A as AtomicInteger>::V>
    + Into<<<Self as AtomicAccess>::A as AtomicInteger>::V>
{
    /// The `AtomicInteger` that atomic operations on `Self` are based on.
    type A: AtomicInteger;
}

macro_rules! impl_atomic_access {
    ($T:ty, $A:path) => {
        impl AtomicAccess for $T {
            type A = $A;
        }
    };
}

impl_atomic_access!(i8, std::sync::atomic::AtomicI8);
impl_atomic_access!(i16, std::sync::atomic::AtomicI16);
impl_atomic_access!(i32, std::sync::atomic::AtomicI32);
#[cfg(any(
    target_arch = "x86_64",
    target_arch = "aarch64",
    target_arch = "powerpc64",
    target_arch = "s390x"
))]
impl_atomic_access!(i64, std::sync::atomic::AtomicI64);

impl_atomic_access!(u8, std::sync::atomic::AtomicU8);
impl_atomic_access!(u16, std::sync::atomic::AtomicU16);
impl_atomic_access!(u32, std::sync::atomic::AtomicU32);
#[cfg(any(
    target_arch = "x86_64",
    target_arch = "aarch64",
    target_arch = "powerpc64",
    target_arch = "s390x"
))]
impl_atomic_access!(u64, std::sync::atomic::AtomicU64);

impl_atomic_access!(isize, std::sync::atomic::AtomicIsize);
impl_atomic_access!(usize, std::sync::atomic::AtomicUsize);

/// A container to host a range of bytes and access its content.
///
/// Candidates which may implement this trait include:
/// - anonymous memory areas
/// - mmapped memory areas
/// - data files
/// - a proxy to access memory on remote
pub trait Bytes<A> {
    /// Associated error codes
    type E;

    /// Writes a slice into the container at `addr`.
    ///
    /// Returns the number of bytes written. The number of bytes written can
    /// be less than the length of the slice if there isn't enough room in the
    /// container.
    fn write(&self, buf: &[u8], addr: A) -> Result<usize, Self::E>;

    /// Reads data from the container at `addr` into a slice.
    ///
    /// Returns the number of bytes read. The number of bytes read can be less than the length
    /// of the slice if there isn't enough data within the container.
    fn read(&self, buf: &mut [u8], addr: A) -> Result<usize, Self::E>;

    /// Writes the entire content of a slice into the container at `addr`.
    ///
    /// # Errors
    ///
    /// Returns an error if there isn't enough space within the container to write the entire slice.
    /// Part of the data may have been copied nevertheless.
    fn write_slice(&self, buf: &[u8], addr: A) -> Result<(), Self::E>;

    /// Reads data from the container at `addr` to fill an entire slice.
    ///
    /// # Errors
    ///
    /// Returns an error if there isn't enough data within the container to fill the entire slice.
    /// Part of the data may have been copied nevertheless.
    fn read_slice(&self, buf: &mut [u8], addr: A) -> Result<(), Self::E>;

    /// Writes an object into the container at `addr`.
    ///
    /// # Errors
    ///
    /// Returns an error if the object doesn't fit inside the container.
    fn write_obj<T: ByteValued>(&self, val: T, addr: A) -> Result<(), Self::E> {
        self.write_slice(val.as_slice(), addr)
    }

    /// Reads an object from the container at `addr`.
    ///
    /// Reading from a volatile area isn't strictly safe as it could change mid-read.
    /// However, as long as the type T is plain old data and can handle random initialization,
    /// everything will be OK.
    ///
    /// # Errors
    ///
    /// Returns an error if there's not enough data inside the container.
    fn read_obj<T: ByteValued>(&self, addr: A) -> Result<T, Self::E> {
        let mut result: T = Default::default();
        self.read_slice(result.as_mut_slice(), addr).map(|_| result)
    }

    /// Reads up to `count` bytes from an object and writes them into the container at `addr`.
    ///
    /// Returns the number of bytes written into the container.
    ///
    /// # Arguments
    /// * `addr` - Begin writing at this address.
    /// * `src` - Copy from `src` into the container.
    /// * `count` - Copy `count` bytes from `src` into the container.
    fn read_from<F>(&self, addr: A, src: &mut F, count: usize) -> Result<usize, Self::E>
    where
        F: Read;

    /// Reads exactly `count` bytes from an object and writes them into the container at `addr`.
    ///
    /// # Errors
    ///
    /// Returns an error if `count` bytes couldn't have been copied from `src` to the container.
    /// Part of the data may have been copied nevertheless.
    ///
    /// # Arguments
    /// * `addr` - Begin writing at this address.
    /// * `src` - Copy from `src` into the container.
    /// * `count` - Copy exactly `count` bytes from `src` into the container.
    fn read_exact_from<F>(&self, addr: A, src: &mut F, count: usize) -> Result<(), Self::E>
    where
        F: Read;

    /// Reads up to `count` bytes from the container at `addr` and writes them it into an object.
    ///
    /// Returns the number of bytes written into the object.
    ///
    /// # Arguments
    /// * `addr` - Begin reading from this address.
    /// * `dst` - Copy from the container to `dst`.
    /// * `count` - Copy `count` bytes from the container to `dst`.
    fn write_to<F>(&self, addr: A, dst: &mut F, count: usize) -> Result<usize, Self::E>
    where
        F: Write;

    /// Reads exactly `count` bytes from the container at `addr` and writes them into an object.
    ///
    /// # Errors
    ///
    /// Returns an error if `count` bytes couldn't have been copied from the container to `dst`.
    /// Part of the data may have been copied nevertheless.
    ///
    /// # Arguments
    /// * `addr` - Begin reading from this address.
    /// * `dst` - Copy from the container to `dst`.
    /// * `count` - Copy exactly `count` bytes from the container to `dst`.
    fn write_all_to<F>(&self, addr: A, dst: &mut F, count: usize) -> Result<(), Self::E>
    where
        F: Write;

    /// Atomically store a value at the specified address.
    fn store<T: AtomicAccess>(&self, val: T, addr: A, order: Ordering) -> Result<(), Self::E>;

    /// Atomically load a value from the specified address.
    fn load<T: AtomicAccess>(&self, addr: A, order: Ordering) -> Result<T, Self::E>;
}

#[cfg(test)]
pub(crate) mod tests {
    #![allow(clippy::undocumented_unsafe_blocks)]
    use super::*;

    use std::cell::RefCell;
    use std::fmt::Debug;
    use std::mem::align_of;

    // Helper method to test atomic accesses for a given `b: Bytes` that's supposed to be
    // zero-initialized.
    pub fn check_atomic_accesses<A, B>(b: B, addr: A, bad_addr: A)
    where
        A: Copy,
        B: Bytes<A>,
        B::E: Debug,
    {
        let val = 100u32;

        assert_eq!(b.load::<u32>(addr, Ordering::Relaxed).unwrap(), 0);
        b.store(val, addr, Ordering::Relaxed).unwrap();
        assert_eq!(b.load::<u32>(addr, Ordering::Relaxed).unwrap(), val);

        assert!(b.load::<u32>(bad_addr, Ordering::Relaxed).is_err());
        assert!(b.store(val, bad_addr, Ordering::Relaxed).is_err());
    }

    fn check_byte_valued_type<T>()
    where
        T: ByteValued + PartialEq + Debug + Default,
    {
        let mut data = [0u8; 48];
        let pre_len = {
            let (pre, _, _) = unsafe { data.align_to::<T>() };
            pre.len()
        };
        {
            let aligned_data = &mut data[pre_len..pre_len + size_of::<T>()];
            {
                let mut val: T = Default::default();
                assert_eq!(T::from_slice(aligned_data), Some(&val));
                assert_eq!(T::from_mut_slice(aligned_data), Some(&mut val));
                assert_eq!(val.as_slice(), aligned_data);
                assert_eq!(val.as_mut_slice(), aligned_data);
            }
        }
        for i in 1..size_of::<T>().min(align_of::<T>()) {
            let begin = pre_len + i;
            let end = begin + size_of::<T>();
            let unaligned_data = &mut data[begin..end];
            {
                if align_of::<T>() != 1 {
                    assert_eq!(T::from_slice(unaligned_data), None);
                    assert_eq!(T::from_mut_slice(unaligned_data), None);
                }
            }
        }
        // Check the early out condition
        {
            assert!(T::from_slice(&data).is_none());
            assert!(T::from_mut_slice(&mut data).is_none());
        }
    }

    #[test]
    fn test_byte_valued() {
        check_byte_valued_type::<u8>();
        check_byte_valued_type::<u16>();
        check_byte_valued_type::<u32>();
        check_byte_valued_type::<u64>();
        check_byte_valued_type::<u128>();
        check_byte_valued_type::<usize>();
        check_byte_valued_type::<i8>();
        check_byte_valued_type::<i16>();
        check_byte_valued_type::<i32>();
        check_byte_valued_type::<i64>();
        check_byte_valued_type::<i128>();
        check_byte_valued_type::<isize>();
    }

    pub const MOCK_BYTES_CONTAINER_SIZE: usize = 10;

    pub struct MockBytesContainer {
        container: RefCell<[u8; MOCK_BYTES_CONTAINER_SIZE]>,
    }

    impl MockBytesContainer {
        pub fn new() -> Self {
            MockBytesContainer {
                container: RefCell::new([0; MOCK_BYTES_CONTAINER_SIZE]),
            }
        }

        pub fn validate_slice_op(&self, buf: &[u8], addr: usize) -> Result<(), ()> {
            if MOCK_BYTES_CONTAINER_SIZE - buf.len() <= addr {
                return Err(());
            }

            Ok(())
        }
    }

    impl Bytes<usize> for MockBytesContainer {
        type E = ();

        fn write(&self, _: &[u8], _: usize) -> Result<usize, Self::E> {
            unimplemented!()
        }

        fn read(&self, _: &mut [u8], _: usize) -> Result<usize, Self::E> {
            unimplemented!()
        }

        fn write_slice(&self, buf: &[u8], addr: usize) -> Result<(), Self::E> {
            self.validate_slice_op(buf, addr)?;

            let mut container = self.container.borrow_mut();
            container[addr..addr + buf.len()].copy_from_slice(buf);

            Ok(())
        }

        fn read_slice(&self, buf: &mut [u8], addr: usize) -> Result<(), Self::E> {
            self.validate_slice_op(buf, addr)?;

            let container = self.container.borrow();
            buf.copy_from_slice(&container[addr..addr + buf.len()]);

            Ok(())
        }

        fn read_from<F>(&self, _: usize, _: &mut F, _: usize) -> Result<usize, Self::E>
        where
            F: Read,
        {
            unimplemented!()
        }

        fn read_exact_from<F>(&self, _: usize, _: &mut F, _: usize) -> Result<(), Self::E>
        where
            F: Read,
        {
            unimplemented!()
        }

        fn write_to<F>(&self, _: usize, _: &mut F, _: usize) -> Result<usize, Self::E>
        where
            F: Write,
        {
            unimplemented!()
        }

        fn write_all_to<F>(&self, _: usize, _: &mut F, _: usize) -> Result<(), Self::E>
        where
            F: Write,
        {
            unimplemented!()
        }

        fn store<T: AtomicAccess>(
            &self,
            _val: T,
            _addr: usize,
            _order: Ordering,
        ) -> Result<(), Self::E> {
            unimplemented!()
        }

        fn load<T: AtomicAccess>(&self, _addr: usize, _order: Ordering) -> Result<T, Self::E> {
            unimplemented!()
        }
    }

    #[test]
    fn test_bytes() {
        let bytes = MockBytesContainer::new();

        assert!(bytes.write_obj(std::u64::MAX, 0).is_ok());
        assert_eq!(bytes.read_obj::<u64>(0).unwrap(), std::u64::MAX);

        assert!(bytes
            .write_obj(std::u64::MAX, MOCK_BYTES_CONTAINER_SIZE)
            .is_err());
        assert!(bytes.read_obj::<u64>(MOCK_BYTES_CONTAINER_SIZE).is_err());
    }

    #[repr(C)]
    #[derive(Copy, Clone, Default)]
    struct S {
        a: u32,
        b: u32,
    }

    unsafe impl ByteValued for S {}

    #[test]
    fn byte_valued_slice() {
        let a: [u8; 8] = [0, 0, 0, 0, 1, 1, 1, 1];
        let mut s: S = Default::default();
        s.as_bytes().copy_from(&a);
        assert_eq!(s.a, 0);
        assert_eq!(s.b, 0x0101_0101);
    }
}