linux-kernel - Re: [PATCH v2 2/2] uaccess: rust: add UserSliceReader::strcpy_into

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Message-ID: <CAH5fLggj-Mfhd3311aKUt9go_+FeAuXdPxditW26QL5VtLD3iQ@mail.gmail.com>
Date: Tue, 29 Apr 2025 21:29:07 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] uaccess: rust: add UserSliceReader::strcpy_into_buf

On Tue, Apr 29, 2025 at 8:02 PM Boqun Feng <boqun.feng@...il.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@...gle.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?

Alice