Re: [PATCH v2 1/2] uaccess: rust: add strncpy_from_user

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Tue, Apr 29, 2025 at 09:02:22AM +0000, 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.
> 
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
>  rust/kernel/uaccess.rs | 34 +++++++++++++++++++++++++++++++++-
>  1 file changed, 33 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
> index 80a9782b1c6e98ed6eae308ade8551afa7adc188..acb703f074a30e60d42a222dd26aed80d8bdb76a 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 +369,35 @@ pub fn write<T: AsBytes>(&mut self, value: &T) -> Result {
>          Ok(())
>      }
>  }
> +
> +/// Reads a nul-terminated string into `buf` and returns the length.
> +///
> +/// This reads from userspace until a NUL byte is encountered, or until `buf.len()` bytes have been
> +/// read. Fails with [`EFAULT`] if a read happens on a bad address. 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(buf.len())` is returned.

I don't know if it matters, but this can fill up the buffer a bit and
still fail, to quote from the strncpy_from_user() documentation:

	 If access to userspace fails, returns -EFAULT (some data may have been copied).

> +///
> +/// # Guarantees
> +///
> +/// When this function returns `Ok(len)`, it is guaranteed that the first `len` of `buf` bytes are
> +/// initialized and non-zero. Furthermore, if `len < buf.len()`, then `buf[len]` is a NUL byte.
> +/// Unsafe code may rely on these guarantees.
> +#[inline]
> +pub fn raw_strncpy_from_user(ptr: UserPtr, buf: &mut [MaybeUninit<u8>]) -> Result<usize> {
> +    // CAST: Slice lengths are guaranteed to be `<= isize::MAX`.
> +    let len = buf.len() as isize;
> +
> +    // SAFETY: `buf` is valid for writing `buf.len()` bytes.
> +    let res = unsafe {
> +        bindings::strncpy_from_user(buf.as_mut_ptr().cast::<c_char>(), ptr as *const c_char, len)
> +    };
> +
> +    if res < 0 {
> +        return Err(Error::from_errno(res as i32));

Nit, this can just be returning EFAULT, but I guess it's safest just to
mirror what was passed back.

I would say to just leave it as "pub" for now, but that's not a big
deal.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>