Re: [PATCH v4 1/2] uaccess: rust: add strncpy_from_user

["Benno Lossin" <lossin@kernel.org>]

On Tue May 27, 2025 at 2:34 PM CEST, Alice Ryhl wrote:
> This patch adds a direct wrapper around the C function of the same name.
> It's not really intended for direct use by Rust code since
> strncpy_from_user has a somewhat unfortunate API where it only
> nul-terminates the buffer if there's space for the nul-terminator. This
> means that a direct Rust wrapper around it could not return a &CStr
> since the buffer may not be a cstring. However, we still add the method
> to build more convenient APIs on top of it, which will happen in
> subsequent patches.
>
> Reviewed-by: Danilo Krummrich <dakr@kernel.org>
> Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>

Reviewed-by: Benno Lossin <lossin@kernel.org>

One question below.

> ---
>  rust/kernel/uaccess.rs | 35 ++++++++++++++++++++++++++++++++++-
>  1 file changed, 34 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
> index 80a9782b1c6e98ed6eae308ade8551afa7adc188..9b1e4016fca2c25a44a8417c7e35e0fcf08aa959 100644
> --- a/rust/kernel/uaccess.rs
> +++ b/rust/kernel/uaccess.rs
> @@ -8,7 +8,7 @@
>      alloc::{Allocator, Flags},
>      bindings,
>      error::Result,
> -    ffi::c_void,
> +    ffi::{c_char, c_void},
>      prelude::*,
>      transmute::{AsBytes, FromBytes},
>  };
> @@ -369,3 +369,36 @@ pub fn write<T: AsBytes>(&mut self, value: &T) -> Result {
>          Ok(())
>      }
>  }
> +
> +/// Reads a nul-terminated string into `dst` and returns the length.
> +///
> +/// This reads from userspace until a NUL byte is encountered, or until `dst.len()` bytes have been
> +/// read. Fails with [`EFAULT`] if a read happens on a bad address (some data may have been
> +/// copied). When the end of the buffer is encountered, no NUL byte is added, so the string is
> +/// *not* guaranteed to be NUL-terminated when `Ok(dst.len())` is returned.
> +///
> +/// # Guarantees
> +///
> +/// When this function returns `Ok(len)`, it is guaranteed that the first `len` bytes of `dst` are
> +/// initialized and non-zero. Furthermore, if `len < dst.len()`, then `dst[len]` is a NUL byte.
> +/// Unsafe code may rely on these guarantees.
> +#[inline]
> +#[expect(dead_code)]
> +fn raw_strncpy_from_user(dst: &mut [MaybeUninit<u8>], src: UserPtr) -> Result<usize> {

We could also return `&[u8]` here instead of the size. Would that
improve the users of this API?

---
Cheers,
Benno

> +    // CAST: Slice lengths are guaranteed to be `<= isize::MAX`.
> +    let len = dst.len() as isize;
> +
> +    // SAFETY: `dst` is valid for writing `dst.len()` bytes.
> +    let res = unsafe {
> +        bindings::strncpy_from_user(dst.as_mut_ptr().cast::<c_char>(), src as *const c_char, len)
> +    };
> +
> +    if res < 0 {
> +        return Err(Error::from_errno(res as i32));
> +    }
> +
> +    #[cfg(CONFIG_RUST_OVERFLOW_CHECKS)]
> +    assert!(res <= len);
> +
> +    Ok(res as usize)
> +}