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

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

On Fri, May 30, 2025 at 01:32:44PM +0200, Benno Lossin wrote:
> 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?

That would differ from the C function, strncpy_from_user() and force us
reviewers to try to remember what is supposed to be happening here.  So
I wouldn't recommend that if at all possible please.

thanks,

greg k-h