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

[Boqun Feng <boqun.feng@gmail.com>]

On Tue, Apr 29, 2025 at 09:29:07PM +0200, Alice Ryhl wrote:
> On Tue, Apr 29, 2025 at 8:02 PM Boqun Feng <boqun.feng@gmail.com> 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`.
> >
> > s/`dst`/`buf`
> >
> > ?
> 
> Hm, append is also wrong. Thanks.
> 
> > > +
> > > +        // 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);
> > > +        } else {
> > > +            // SAFETY: Due to the check at the beginning, the buffer is not empty.
> > > +            unsafe { *buf.last_mut().unwrap_unchecked() = 0 };
> > > +        }
> > > +        self.skip(len)?;
> > > +
> >
> > So if the UserSlice content is "abcdefg" (not tailing NUL), and the buf
> > size is 4, after a strcpy_into_buf(), the return would be a CStr "abc"
> > (with a tailing NUL), and the UserSlice would move 4 bytes and become
> > "edg" (not tailing NUL), is this a desired behavior?
> >
> > Alternatively, we can make `dst` always 1 byte less then `buf`, so that
> > in the above case, UserSlice will only move 3 bytes and become "defg",
> > and the return CStr is still "abc" (with a tailing NUL).
> 
> Maybe we just have this method consume the UserSliceReader and avoid
> thinking about what happens if you use it afterwards.
> 
> > The current behavior makes me feel like we can lose some information,
> > for example, if the user-kernel protocol is that "a userslice that
> > contains 4 64-byte strings which don't have a tailing NUL", we cannot do
> > 4 strcpy_into_buf() to get them, right? But of course, the scenario is
> > completely made up, just food for thoughts.
> 
> But then you should probably just read the [u8;64] type four times?
> 

Ah, that makes sense. Seems I was trying to over-task this method ;-)

Regards,
Boqun

> Alice