Introduce a new io_buffer module.

It abstracts buffers involved in IO. This is in preparation for allowing
drivers to implement `write_iter` and `read_iter`.

No behaviour change is intended, this is a pure refactor.

Signed-off-by: Wedson Almeida Filho <wedsonaf@google.com>

drivers/android/defs.rs

@@ -4,7 +4,7 @@ use core::ops::{Deref, DerefMut};
 use kernel::{
     bindings,
     bindings::*,
-    user_ptr::{ReadableFromBytes, WritableToBytes},
+    io_buffer::{ReadableFromBytes, WritableToBytes},
 };
 
 macro_rules! pub_no_prefix {

drivers/android/node.rs

@@ -6,6 +6,7 @@ use core::{
     sync::atomic::{AtomicU64, Ordering},
 };
 use kernel::{
+    io_buffer::IoBufferWriter,
     linked_list::{GetLinks, Links, List},
     prelude::*,
     sync::{Guard, LockedBy, Mutex, Ref, SpinLock},

drivers/android/process.rs

@@ -9,6 +9,7 @@ use core::{
 use kernel::{
     bindings, c_types,
     file_operations::{File, FileOpener, FileOperations, IoctlCommand, IoctlHandler, PollTable},
+    io_buffer::{IoBufferReader, IoBufferWriter},
     linked_list::List,
     pages::Pages,
     prelude::*,

drivers/android/rust_binder.rs

@@ -11,6 +11,7 @@ use alloc::{boxed::Box, sync::Arc};
 use core::pin::Pin;
 use kernel::{
     cstr,
+    io_buffer::IoBufferWriter,
     linked_list::{GetLinks, GetLinksWrapped, Links},
     miscdev::Registration,
     prelude::*,

drivers/android/thread.rs

@@ -5,6 +5,7 @@ use core::{alloc::AllocError, mem::size_of, pin::Pin};
 use kernel::{
     bindings,
     file_operations::{File, PollTable},
+    io_buffer::{IoBufferReader, IoBufferWriter},
     linked_list::{GetLinks, Links, List},
     prelude::*,
     sync::{CondVar, Ref, SpinLock},

drivers/android/transaction.rs

@@ -2,7 +2,10 @@
 
 use alloc::sync::Arc;
 use core::sync::atomic::{AtomicBool, Ordering};
-use kernel::{bindings, linked_list::Links, prelude::*, sync::Ref, user_ptr::UserSlicePtrWriter};
+use kernel::{
+    bindings, io_buffer::IoBufferWriter, linked_list::Links, prelude::*, sync::Ref,
+    user_ptr::UserSlicePtrWriter,
+};
 
 use crate::{
     defs::*,

rust/kernel/io_buffer.rs

@@ -0,0 +1,154 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Buffers used in IO.
+
+use crate::KernelResult;
+use alloc::vec::Vec;
+use core::mem::{size_of, MaybeUninit};
+
+/// Represents a buffer to be read from during IO.
+pub trait IoBufferReader {
+    /// Returns the number of bytes left to be read from the io buffer.
+    ///
+    /// Note that even reading less than this number of bytes may fail.
+    fn len(&self) -> usize;
+
+    /// Returns `true` if no data is available in the io buffer.
+    fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
+    /// Reads raw data from the io buffer into a raw kernel buffer.
+    ///
+    /// # Safety
+    ///
+    /// The output buffer must be valid.
+    unsafe fn read_raw(&mut self, out: *mut u8, len: usize) -> KernelResult;
+
+    /// Reads all data remaining in the io buffer.
+    ///
+    /// Returns `EFAULT` if the address does not currently point to mapped, readable memory.
+    fn read_all(&mut self) -> KernelResult<Vec<u8>> {
+        let mut data = Vec::<u8>::new();
+        data.try_reserve_exact(self.len())?;
+        data.resize(self.len(), 0);
+
+        // SAFETY: The output buffer is valid as we just allocated it.
+        unsafe { self.read_raw(data.as_mut_ptr(), data.len())? };
+        Ok(data)
+    }
+
+    /// Reads a byte slice from the io buffer.
+    ///
+    /// Returns `EFAULT` if the byte slice is bigger than the remaining size of the user slice or
+    /// if the address does not currently point to mapped, readable memory.
+    fn read_slice(&mut self, data: &mut [u8]) -> KernelResult {
+        // SAFETY: The output buffer is valid as it's coming from a live reference.
+        unsafe { self.read_raw(data.as_mut_ptr(), data.len()) }
+    }
+
+    /// Reads the contents of a plain old data (POD) type from the io buffer.
+    fn read<T: ReadableFromBytes>(&mut self) -> KernelResult<T> {
+        let mut out = MaybeUninit::<T>::uninit();
+        // SAFETY: The buffer is valid as it was just allocated.
+        unsafe { self.read_raw(out.as_mut_ptr() as _, size_of::<T>()) }?;
+        // SAFETY: We just initialised the data.
+        Ok(unsafe { out.assume_init() })
+    }
+}
+
+/// Represents a buffer to be written to during IO.
+pub trait IoBufferWriter {
+    /// Returns the number of bytes left to be written into the io buffer.
+    ///
+    /// Note that even writing less than this number of bytes may fail.
+    fn len(&self) -> usize;
+
+    /// Returns `true` if the io buffer cannot hold any additional data.
+    fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
+    /// Writes zeroes to the io buffer.
+    ///
+    /// Differently from the other write functions, `clear` will zero as much as it can and update
+    /// the writer internal state to reflect this. It will, however, return an error if it cannot
+    /// clear `len` bytes.
+    ///
+    /// For example, if a caller requests that 100 bytes be cleared but a segfault happens after
+    /// 20 bytes, then EFAULT is returned and the writer is advanced by 20 bytes.
+    fn clear(&mut self, len: usize) -> KernelResult;
+
+    /// Writes a byte slice into the io buffer.
+    ///
+    /// Returns `EFAULT` if the byte slice is bigger than the remaining size of the io buffer or if
+    /// the address does not currently point to mapped, writable memory.
+    fn write_slice(&mut self, data: &[u8]) -> KernelResult {
+        // SAFETY: The input buffer is valid as it's coming from a live reference.
+        unsafe { self.write_raw(data.as_ptr(), data.len()) }
+    }
+
+    /// Writes raw data to the io buffer from a raw kernel buffer.
+    ///
+    /// # Safety
+    ///
+    /// The input buffer must be valid.
+    unsafe fn write_raw(&mut self, data: *const u8, len: usize) -> KernelResult;
+
+    /// Writes the contents of the given data into the io buffer.
+    fn write<T: WritableToBytes>(&mut self, data: &T) -> KernelResult<()> {
+        // SAFETY: The input buffer is valid as it's coming from a live
+        // reference to a type that implements `WritableToBytes`.
+        unsafe { self.write_raw(data as *const T as _, size_of::<T>()) }
+    }
+}
+
+/// Specifies that a type is safely readable from byte slices.
+///
+/// Not all types can be safely read from byte slices; examples from
+/// <https://doc.rust-lang.org/reference/behavior-considered-undefined.html> include `bool`
+/// that must be either `0` or `1`, and `char` that cannot be a surrogate or above `char::MAX`.
+///
+/// # Safety
+///
+/// Implementers must ensure that the type is made up only of types that can be safely read from
+/// arbitrary byte sequences (e.g., `u32`, `u64`, etc.).
+pub unsafe trait ReadableFromBytes {}
+
+// SAFETY: All bit patterns are acceptable values of the types below.
+unsafe impl ReadableFromBytes for u8 {}
+unsafe impl ReadableFromBytes for u16 {}
+unsafe impl ReadableFromBytes for u32 {}
+unsafe impl ReadableFromBytes for u64 {}
+unsafe impl ReadableFromBytes for usize {}
+unsafe impl ReadableFromBytes for i8 {}
+unsafe impl ReadableFromBytes for i16 {}
+unsafe impl ReadableFromBytes for i32 {}
+unsafe impl ReadableFromBytes for i64 {}
+unsafe impl ReadableFromBytes for isize {}
+
+/// Specifies that a type is safely writable to byte slices.
+///
+/// This means that we don't read undefined values (which leads to UB) in preparation for writing
+/// to the byte slice. It also ensures that no potentially sensitive information is leaked into the
+/// byte slices.
+///
+/// # Safety
+///
+/// A type must not include padding bytes and must be fully initialised to safely implement
+/// [`WritableToBytes`] (i.e., it doesn't contain [`MaybeUninit`] fields). A composition of
+/// writable types in a structure is not necessarily writable because it may result in padding
+/// bytes.
+pub unsafe trait WritableToBytes {}
+
+// SAFETY: Initialised instances of the following types have no uninitialised portions.
+unsafe impl WritableToBytes for u8 {}
+unsafe impl WritableToBytes for u16 {}
+unsafe impl WritableToBytes for u32 {}
+unsafe impl WritableToBytes for u64 {}
+unsafe impl WritableToBytes for usize {}
+unsafe impl WritableToBytes for i8 {}
+unsafe impl WritableToBytes for i16 {}
+unsafe impl WritableToBytes for i32 {}
+unsafe impl WritableToBytes for i64 {}
+unsafe impl WritableToBytes for isize {}

rust/kernel/lib.rs

@@ -59,6 +59,7 @@ pub mod sync;
 #[cfg(CONFIG_SYSCTL)]
 pub mod sysctl;
 
+pub mod io_buffer;
 mod types;
 pub mod user_ptr;
 

rust/kernel/pages.rs

@@ -4,7 +4,10 @@
 //!
 //! TODO: This module is a work in progress.
 
-use crate::{bindings, c_types, user_ptr::UserSlicePtrReader, Error, KernelResult, PAGE_SIZE};
+use crate::{
+    bindings, c_types, io_buffer::IoBufferReader, user_ptr::UserSlicePtrReader, Error,
+    KernelResult, PAGE_SIZE,
+};
 use core::{marker::PhantomData, ptr};
 
 extern "C" {
@@ -95,7 +98,7 @@ impl<const ORDER: u32> Pages<ORDER> {
     ///
     /// Callers must ensure that the destination buffer is valid for the given length.
     /// Additionally, if the raw buffer is intended to be recast, they must ensure that the data
-    /// can be safely cast; [`crate::user_ptr::ReadableFromBytes`] has more details about it.
+    /// can be safely cast; [`crate::io_buffer::ReadableFromBytes`] has more details about it.
     pub unsafe fn read(&self, dest: *mut u8, offset: usize, len: usize) -> KernelResult {
         // TODO: For now this only works on the first page.
         let end = offset.checked_add(len).ok_or(Error::EINVAL)?;
@@ -114,7 +117,7 @@ impl<const ORDER: u32> Pages<ORDER> {
     ///
     /// Callers must ensure that the buffer is valid for the given length. Additionally, if the
     /// page is (or will be) mapped by userspace, they must ensure that no kernel data is leaked
-    /// through padding if it was cast from another type; [`crate::user_ptr::WritableToBytes`] has
+    /// through padding if it was cast from another type; [`crate::io_buffer::WritableToBytes`] has
     /// more details about it.
     pub unsafe fn write(&self, src: *const u8, offset: usize, len: usize) -> KernelResult {
         // TODO: For now this only works on the first page.

rust/kernel/sysctl.rs

@@ -12,11 +12,12 @@ use core::mem;
 use core::ptr;
 use core::sync::atomic;
 
-use crate::bindings;
-use crate::c_types;
-use crate::error;
-use crate::types;
-use crate::user_ptr::{UserSlicePtr, UserSlicePtrWriter};
+use crate::{
+    bindings, c_types, error,
+    io_buffer::IoBufferWriter,
+    types,
+    user_ptr::{UserSlicePtr, UserSlicePtrWriter},
+};
 
 /// Sysctl storage.
 pub trait SysctlStorage: Sync {