Re: [PATCH v2 2/2] uaccess: rust: add UserSliceReader::strcpy_into_buf

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

On Tue, Apr 29, 2025 at 01:38:26PM +0200, Danilo Krummrich wrote:
> On 2025-04-29 13:09, Greg Kroah-Hartman wrote:
> > On Tue, Apr 29, 2025 at 09:02:23AM +0000, Alice Ryhl wrote:
> > > This patch adds a more convenient method for reading C strings from
> > > userspace. Logic is added to NUL-terminate the buffer when necessary
> > > so
> > > that a &CStr can be returned.
> > > 
> > > Note that we treat attempts to read past `self.length` as a fault, so
> > > this returns EFAULT if that limit is exceeded before `buf.len()` is
> > > reached.
> > > 
> > > Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> > > ---
> > >  rust/kernel/uaccess.rs | 35 +++++++++++++++++++++++++++++++++++
> > >  1 file changed, 35 insertions(+)
> > > 
> > > diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
> > > index acb703f074a30e60d42a222dd26aed80d8bdb76a..7cec1b62bd8b816f523c8be12cb29905740789fc
> > > 100644
> > > --- a/rust/kernel/uaccess.rs
> > > +++ b/rust/kernel/uaccess.rs
> > > @@ -293,6 +293,41 @@ pub fn read_all<A: Allocator>(mut self, buf:
> > > &mut Vec<u8, A>, flags: Flags) -> R
> > >          unsafe { buf.set_len(buf.len() + len) };
> > >          Ok(())
> > >      }
> > > +
> > > +    /// Read a NUL-terminated string from userspace and append it
> > > to `dst`.
> > > +    ///
> > > +    /// Fails with [`EFAULT`] if the read happens on a bad address.
> > 
> > Also returns this error:
> > 
> > > +    pub fn strcpy_into_buf<'buf>(&mut self, buf: &'buf mut [u8]) ->
> > > Result<&'buf CStr> {
> > > +        if buf.is_empty() {
> > > +            return Err(EINVAL);
> > 
> > if the buffer is of 0 length.  Don't know if you want to document that
> > or not.
> > 
> > > +        }
> > > +
> > > +        // SAFETY: The types are compatible and `strncpy_from_user`
> > > doesn't write uninitialized
> > > +        // bytes to `buf`.
> > > +        let mut dst = unsafe { &mut *(buf as *mut [u8] as *mut
> > > [MaybeUninit<u8>]) };
> > > +
> > > +        // We never read more than `self.length` bytes.
> > > +        if dst.len() > self.length {
> > > +            dst = &mut dst[..self.length];
> > > +        }
> > > +
> > > +        let mut len = raw_strncpy_from_user(self.ptr, dst)?;
> > > +        if len < dst.len() {
> > > +            // Add one to include the NUL-terminator.
> > > +            len += 1;
> > > +        } else if len < buf.len() {
> > > +            // We hit the `self.length` limit before `buf.len()`.
> > > +            return Err(EFAULT);
> > 
> > How can this happen?
> 
> See my reply here (if I did not get it wrong):
> 
> https://lore.kernel.org/rust-for-linux/aBCrqJe4two4I45G@pollux/

Ah, I should have read ahead :)

I agree, some comments here would be good.  We want everyone to be able
to easily read and understand this code, off-by-one errors are rough.

thanks,

greg k-h