// SPDX-License-Identifier: GPL-2.0

// Copyright (C) 2024 Google LLC.

//! User pointers.
//!
//! C header: [`include/linux/uaccess.h`](srctree/include/linux/uaccess.h)

use crate::{
    bindings,
    error::code::*,
    error::Result,
    types::{AsBytes, FromBytes},
};
use alloc::vec::Vec;
use core::ffi::{c_ulong, c_void};
use core::mem::{size_of, MaybeUninit};

/// A pointer to an area in userspace memory, which can be either read-only or
/// read-write.
///
/// All methods on this struct are safe: attempting to read or write invalid
/// pointers will return `EFAULT`. Concurrent access, *including data races
/// to/from userspace memory*, is permitted, because fundamentally another
/// userspace thread/process could always be modifying memory at the same time
/// (in the same way that userspace Rust's [`std::io`] permits data races with
/// the contents of files on disk). In the presence of a race, the exact byte
/// values read/written are unspecified but the operation is well-defined.
/// Kernelspace code should validate its copy of data after completing a read,
/// and not expect that multiple reads of the same address will return the same
/// value.
///
/// These APIs are designed to make it difficult to accidentally write TOCTOU
/// (time-of-check to time-of-use) bugs. Every time a memory location is read,
/// the reader's position is advanced by the read length and the next read will
/// start from there. This helps prevent accidentally reading the same location
/// twice and causing a TOCTOU bug.
///
/// Creating a [`UserSliceReader`] and/or [`UserSliceWriter`] consumes the
/// `UserSlice`, helping ensure that there aren't multiple readers or writers to
/// the same location.
///
/// If double-fetching a memory location is necessary for some reason, then that
/// is done by creating multiple readers to the same memory location, e.g. using
/// [`clone_reader`].
///
/// # Examples
///
/// Takes a region of userspace memory from the current process, and modify it
/// by adding one to every byte in the region.
///
/// ```no_run
/// use alloc::vec::Vec;
/// use core::ffi::c_void;
/// use kernel::error::Result;
/// use kernel::uaccess::UserSlice;
///
/// pub fn bytes_add_one(uptr: *mut c_void, len: usize) -> Result<()> {
///     let (read, mut write) = UserSlice::new(uptr, len).reader_writer();
///
///     let mut buf = Vec::new();
///     read.read_all(&mut buf)?;
///
///     for b in &mut buf {
///         *b = b.wrapping_add(1);
///     }
///
///     write.write_slice(&buf)?;
///     Ok(())
/// }
/// ```
///
/// Example illustrating a TOCTOU (time-of-check to time-of-use) bug.
///
/// ```no_run
/// use alloc::vec::Vec;
/// use core::ffi::c_void;
/// use kernel::error::{code::EINVAL, Result};
/// use kernel::uaccess::UserSlice;
///
/// /// Returns whether the data in this region is valid.
/// fn is_valid(uptr: *mut c_void, len: usize) -> Result<bool> {
///     let read = UserSlice::new(uptr, len).reader();
///
///     let mut buf = Vec::new();
///     read.read_all(&mut buf)?;
///
///     todo!()
/// }
///
/// /// Returns the bytes behind this user pointer if they are valid.
/// pub fn get_bytes_if_valid(uptr: *mut c_void, len: usize) -> Result<Vec<u8>> {
///     if !is_valid(uptr, len)? {
///         return Err(EINVAL);
///     }
///
///     let read = UserSlice::new(uptr, len).reader();
///
///     let mut buf = Vec::new();
///     read.read_all(&mut buf)?;
///
///     // THIS IS A BUG! The bytes could have changed since we checked them.
///     //
///     // To avoid this kind of bug, don't call `UserSlice::new` multiple
///     // times with the same address.
///     Ok(buf)
/// }
/// ```
///
/// [`std::io`]: https://doc.rust-lang.org/std/io/index.html
/// [`clone_reader`]: UserSliceReader::clone_reader
pub struct UserSlice {
    ptr: *mut c_void,
    length: usize,
}

impl UserSlice {
    /// Constructs a user slice from a raw pointer and a length in bytes.
    ///
    /// Constructing a [`UserSlice`] performs no checks on the provided address
    /// and length, it can safely be constructed inside a kernel thread with no
    /// current userspace process. Reads and writes wrap the kernel APIs
    /// `copy_from_user` and `copy_to_user`, which check the memory map of the
    /// current process and enforce that the address range is within the user
    /// range (no additional calls to `access_ok` are needed).
    ///
    /// Callers must be careful to avoid time-of-check-time-of-use
    /// (TOCTOU) issues. The simplest way is to create a single instance of
    /// [`UserSlice`] per user memory block as it reads each byte at
    /// most once.
    pub fn new(ptr: *mut c_void, length: usize) -> Self {
        UserSlice { ptr, length }
    }

    /// Reads the entirety of the user slice, appending it to the end of the
    /// provided buffer.
    ///
    /// Fails with `EFAULT` if the read encounters a page fault.
    pub fn read_all(self, buf: &mut Vec<u8>) -> Result<()> {
        self.reader().read_all(buf)
    }

    /// Constructs a [`UserSliceReader`].
    pub fn reader(self) -> UserSliceReader {
        UserSliceReader {
            ptr: self.ptr,
            length: self.length,
        }
    }

    /// Constructs a [`UserSliceWriter`].
    pub fn writer(self) -> UserSliceWriter {
        UserSliceWriter {
            ptr: self.ptr,
            length: self.length,
        }
    }

    /// Constructs both a [`UserSliceReader`] and a [`UserSliceWriter`].
    ///
    /// Usually when this is used, you will first read the data, and then
    /// overwrite it afterwards.
    pub fn reader_writer(self) -> (UserSliceReader, UserSliceWriter) {
        (
            UserSliceReader {
                ptr: self.ptr,
                length: self.length,
            },
            UserSliceWriter {
                ptr: self.ptr,
                length: self.length,
            },
        )
    }
}

/// A reader for [`UserSlice`].
///
/// Used to incrementally read from the user slice.
pub struct UserSliceReader {
    ptr: *mut c_void,
    length: usize,
}

impl UserSliceReader {
    /// Skip the provided number of bytes.
    ///
    /// Returns an error if skipping more than the length of the buffer.
    pub fn skip(&mut self, num_skip: usize) -> Result {
        // Update `self.length` first since that's the fallible part of this
        // operation.
        self.length = self.length.checked_sub(num_skip).ok_or(EFAULT)?;
        self.ptr = self.ptr.wrapping_byte_add(num_skip);
        Ok(())
    }

    /// Create a reader that can access the same range of data.
    ///
    /// Reading from the clone does not advance the current reader.
    ///
    /// The caller should take care to not introduce TOCTOU issues, as described
    /// in the documentation for [`UserSlice`].
    pub fn clone_reader(&self) -> UserSliceReader {
        UserSliceReader {
            ptr: self.ptr,
            length: self.length,
        }
    }

    /// Returns the number of bytes left to be read from this reader.
    ///
    /// Note that even reading less than this number of bytes may fail.
    pub fn len(&self) -> usize {
        self.length
    }

    /// Returns `true` if no data is available in the io buffer.
    pub fn is_empty(&self) -> bool {
        self.length == 0
    }

    /// Reads raw data from the user slice into a raw kernel buffer.
    ///
    /// Fails with `EFAULT` if the read encounters a page fault.
    ///
    /// # Safety
    ///
    /// The `out` pointer must be valid for writing `len` bytes.
    pub unsafe fn read_raw(&mut self, out: *mut u8, len: usize) -> Result {
        if len > self.length {
            return Err(EFAULT);
        }
        let Ok(len_ulong) = c_ulong::try_from(len) else {
            return Err(EFAULT);
        };
        // SAFETY: The caller promises that `out` is valid for writing `len` bytes.
        let res = unsafe { bindings::copy_from_user(out.cast::<c_void>(), self.ptr, len_ulong) };
        if res != 0 {
            return Err(EFAULT);
        }
        // Userspace pointers are not directly dereferencable by the kernel, so
        // we cannot use `add`, which has C-style rules for defined behavior.
        self.ptr = self.ptr.wrapping_byte_add(len);
        self.length -= len;
        Ok(())
    }

    /// Reads a value of the specified type.
    ///
    /// Fails with `EFAULT` if the read encounters a page fault.
    pub fn read<T: FromBytes>(&mut self) -> Result<T> {
        let len = size_of::<T>();
        if len > self.length {
            return Err(EFAULT);
        }
        let Ok(len_ulong) = c_ulong::try_from(len) else {
            return Err(EFAULT);
        };
        let mut out: MaybeUninit<T> = MaybeUninit::uninit();
        // SAFETY: The local variable `out` is valid for writing `size_of::<T>()` bytes.
        //
        // By using the _copy_from_user variant, we skip the check_object_size
        // check that verifies the kernel pointer. This mirrors the logic on the
        // C side that skips the check when the length is a compile-time
        // constant.
        let res = unsafe {
            bindings::_copy_from_user(out.as_mut_ptr().cast::<c_void>(), self.ptr, len_ulong)
        };
        if res != 0 {
            return Err(EFAULT);
        }
        // Since this is not a pointer to a valid object in our program,
        // we cannot use `add`, which has C-style rules for defined
        // behavior.
        self.ptr = self.ptr.wrapping_byte_add(len);
        self.length -= len;
        // SAFETY: The read above has initialized all bytes in `out`, and since
        // `T` implements `FromBytes`, any bit-pattern is a valid value for this
        // type.
        Ok(unsafe { out.assume_init() })
    }

    /// Reads the entirety of the user slice, appending it to the end of the
    /// provided buffer.
    ///
    /// Fails with `EFAULT` if the read encounters a page fault.
    pub fn read_all(mut self, buf: &mut Vec<u8>) -> Result<()> {
        let len = self.length;
        buf.try_reserve(len)?;

        // SAFETY: The call to `try_reserve` was successful, so the spare
        // capacity is at least `self.length` bytes long.
        unsafe { self.read_raw(buf.spare_capacity_mut().as_mut_ptr().cast(), len)? };

        // SAFETY: Since the call to `read_raw` was successful, so the next
        // `len` bytes of the vector have been initialized.
        unsafe { buf.set_len(buf.len() + len) };
        Ok(())
    }
}

/// A writer for [`UserSlice`].
///
/// Used to incrementally write into the user slice.
pub struct UserSliceWriter {
    ptr: *mut c_void,
    length: usize,
}

impl UserSliceWriter {
    /// Returns the amount of space remaining in this buffer.
    ///
    /// Note that even writing less than this number of bytes may fail.
    pub fn len(&self) -> usize {
        self.length
    }

    /// Returns `true` if no more data can be written to this buffer.
    pub fn is_empty(&self) -> bool {
        self.length == 0
    }

    /// Writes raw data to this user pointer from a raw kernel buffer.
    ///
    /// Fails with `EFAULT` if the write encounters a page fault.
    ///
    /// # Safety
    ///
    /// The `data` pointer must be valid for reading `len` bytes.
    pub unsafe fn write_raw(&mut self, data: *const u8, len: usize) -> Result {
        if len > self.length {
            return Err(EFAULT);
        }
        let Ok(len_ulong) = c_ulong::try_from(len) else {
            return Err(EFAULT);
        };
        let res = unsafe { bindings::copy_to_user(self.ptr, data.cast::<c_void>(), len_ulong) };
        if res != 0 {
            return Err(EFAULT);
        }
        // Userspace pointers are not directly dereferencable by the kernel, so
        // we cannot use `add`, which has C-style rules for defined behavior.
        self.ptr = self.ptr.wrapping_byte_add(len);
        self.length -= len;
        Ok(())
    }

    /// Writes the provided slice to this user pointer.
    ///
    /// Fails with `EFAULT` if the write encounters a page fault.
    pub fn write_slice(&mut self, data: &[u8]) -> Result {
        let len = data.len();
        let ptr = data.as_ptr();
        // SAFETY: The pointer originates from a reference to a slice of length
        // `len`, so the pointer is valid for reading `len` bytes.
        unsafe { self.write_raw(ptr, len) }
    }

    /// Writes the provided Rust value to this userspace pointer.
    ///
    /// Fails with `EFAULT` if the write encounters a page fault.
    pub fn write<T: AsBytes>(&mut self, value: &T) -> Result {
        let len = size_of::<T>();
        if len > self.length {
            return Err(EFAULT);
        }
        let Ok(len_ulong) = c_ulong::try_from(len) else {
            return Err(EFAULT);
        };
        // SAFETY: The reference points to a value of type `T`, so it is valid
        // for reading `size_of::<T>()` bytes.
        //
        // By using the _copy_to_user variant, we skip the check_object_size
        // check that verifies the kernel pointer. This mirrors the logic on the
        // C side that skips the check when the length is a compile-time
        // constant.
        let res = unsafe {
            bindings::_copy_to_user(self.ptr, (value as *const T).cast::<c_void>(), len_ulong)
        };
        if res != 0 {
            return Err(EFAULT);
        }
        // Since this is not a pointer to a valid object in our program,
        // we cannot use `add`, which has C-style rules for defined
        // behavior.
        self.ptr = self.ptr.wrapping_byte_add(len);
        self.length -= len;
        Ok(())
    }
}