Re: [PATCH v3 1/4] rust: uaccess: add userspace pointers

[Boqun Feng]

On Mon, Mar 11, 2024 at 10:47:13AM +0000, Alice Ryhl wrote:
> From: Wedson Almeida Filho <wedsonaf@xxxxxxxxx>
> 
[...]
> +
> +/// A reader for [`UserSlice`].
> +///
> +/// Used to incrementally read from the user slice.
> +pub struct UserSliceReader {
> +    ptr: *mut c_void,
> +    length: usize,
> +}
> +
> +impl UserSliceReader {
[...]
> +
> +    /// 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 {

I don't think we want to promote the pub usage of this unsafe function,
right? We can provide a safe version:

	pub fn read_slice(&mut self, to: &[u8]) -> Result

and all users can just use the safe version (with the help of
slice::from_raw_parts_mut() if necessary).

> +        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 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 `len` 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 {

Same here, just remove the `pub`, and users should use write_slice()
(with the help of slice::from_raw_parts() if necessary).

Regards,
Boqun

> +        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) }
> +    }
> +}
> 
> -- 
> 2.44.0.278.ge034bb2e1d-goog
>