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

[Alice Ryhl <aliceryhl@google.com>]

On Tue, Apr 29, 2025 at 01:04:44PM +0200, Greg Kroah-Hartman wrote:
> 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).

It doesn't matter, but it may still be useful to mention.

> > +///
> > +/// # 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 think it's easiest to just 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>

Thanks!

Alice