Re: [PATCH v6 1/4] rust: uaccess: add userspace pointers

[Boqun Feng]

On Thu, Apr 18, 2024 at 08:59:17AM +0000, Alice Ryhl wrote:
[...]
> +    /// Reads the entirety of the user slice, appending it to the end of the provided buffer.
> +    ///
> +    /// Fails with `EFAULT` if the read happens on a bad address.
> +    pub fn read_all(mut self, buf: &mut Vec<u8>, flags: Flags) -> Result {
> +        let len = self.length;
> +        buf.reserve(len, flags)?;

(Reportedy by Miguel)

When compile with `make rusttest`, kernel crate is compiled as userspace
program, so we need to explicitly pick where the `reserve` comes from
(Vec or VecExt), the current version will hit the following error:

	error[E0061]: this method takes 1 argument but 2 arguments were supplied
	   --> rust/kernel/uaccess.rs:296:13
	    |
	296 |         buf.reserve(len, flags)?;
	    |             ^^^^^^^    -------
	    |                        | |
	    |                        | unexpected argument of type `Flags`
	    |                        help: remove the extra argument
	    |
	note: method defined here
	   --> /home/boqun/linux-rust/rust/test/sysroot/lib/rustlib/src/rust/library/alloc/src/vec/mod.rs:910:12
	    |
	910 |     pub fn reserve(&mut self, additional: usize) {
	    |            ^^^^^^^

	error[E0277]: the `?` operator can only be applied to values that implement `Try`
	   --> rust/kernel/uaccess.rs:296:9
	    |
	296 |         buf.reserve(len, flags)?;
	    |         ^^^^^^^^^^^^^^^^^^^^^^^^ the `?` operator cannot be applied to type `()`
	    |
	    = help: the trait `Try` is not implemented for `()`

	error: aborting due to 2 previous errors

	Some errors have detailed explanations: E0061, E0277.
	For more information about an error, try `rustc --explain E0061`.

and we need to the following fix

diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
index 39481e374c40..80f7e7ca2f5e 100644
--- a/rust/kernel/uaccess.rs
+++ b/rust/kernel/uaccess.rs
@@ -293,7 +293,7 @@ pub fn read<T: FromBytes>(&mut self) -> Result<T> {
     /// Fails with `EFAULT` if the read happens on a bad address.
     pub fn read_all(mut self, buf: &mut Vec<u8>, flags: Flags) -> Result {
         let len = self.length;
-        buf.reserve(len, flags)?;
+        VecExt::<u8>::reserve(buf, len, flags)?;
 
         // The call to `try_reserve` was successful, so the spare capacity is at least `len` bytes
         // long.

Regards,
Boqun

> +
> +        // The call to `try_reserve` was successful, so the spare capacity is at least `len` bytes
> +        // long.
> +        self.read_raw(&mut buf.spare_capacity_mut()[..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: UserPtr,
> +    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 kernel buffer.
> +    ///
> +    /// Fails with `EFAULT` if the write happens on a bad address, or if the write goes out of bounds
> +    /// of this [`UserSliceWriter`]. This call may modify the associated userspace slice even if it
> +    /// returns an error.
> +    pub fn write_slice(&mut self, data: &[u8]) -> Result {
> +        let len = data.len();
> +        let data_ptr = data.as_ptr().cast::<c_void>();
> +        if len > self.length {
> +            return Err(EFAULT);
> +        }
> +        let Ok(len_ulong) = c_ulong::try_from(len) else {
> +            return Err(EFAULT);
> +        };
> +        // SAFETY: `data_ptr` points into an immutable slice of length `len_ulong`, so we may read
> +        // that many bytes from it.
> +        let res = unsafe { bindings::copy_to_user(self.ptr as *mut c_void, data_ptr, len_ulong) };
> +        if res != 0 {
> +            return Err(EFAULT);
> +        }
> +        self.ptr = self.ptr.wrapping_add(len);
> +        self.length -= len;
> +        Ok(())
> +    }
> +}
> 
> -- 
> 2.44.0.683.g7961c838ac-goog
>