linux-kernel - Re: [PATCH v5 2/2] uaccess: rust: add UserSliceReader::strcpy_into

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Message-ID: <CAH5fLghMD2RQynaFdLVa+JAy17o5NW_pe7p3Cj1-A-FQVA--_A@mail.gmail.com>
Date: Tue, 17 Jun 2025 10:55:42 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Benno Lossin <lossin@...nel.org>
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>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	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 v5 2/2] uaccess: rust: add UserSliceReader::strcpy_into_buf

On Tue, Jun 17, 2025 at 9:38 AM Benno Lossin <lossin@...nel.org> wrote:
>
> On Mon Jun 16, 2025 at 2:41 PM CEST, 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.
> >
> > Reviewed-by: Danilo Krummrich <dakr@...nel.org>
> > Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
>
> I have one concern left below, when we fix or resolve that:
>
> Reviewed-by: Benno Lossin <lossin@...nel.org>
>
> > ---
> >  rust/kernel/uaccess.rs | 60 +++++++++++++++++++++++++++++++++++++++++++++++++-
> >  1 file changed, 59 insertions(+), 1 deletion(-)
> >
> > diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
> > index 635a03e0989f3fe99be80987aa47763782de1d3f..106aa05ea1b88868fe48f64ca9c86b20ad7db68e 100644
> > --- a/rust/kernel/uaccess.rs
> > +++ b/rust/kernel/uaccess.rs
> > @@ -291,6 +291,65 @@ pub fn read_all<A: Allocator>(mut self, buf: &mut Vec<u8, A>, flags: Flags) -> R
> >          unsafe { buf.inc_len(len) };
> >          Ok(())
> >      }
> > +
> > +    /// Read a NUL-terminated string from userspace and return it.
> > +    ///
> > +    /// The string is read into `buf` and a NUL-terminator is added if the end of `buf` is reached.
> > +    /// Since there must be space to add a NUL-terminator, the buffer must not be empty. The
> > +    /// returned `&CStr` points into `buf`.
> > +    ///
> > +    /// Fails with [`EFAULT`] if the read happens on a bad address (some data may have been
> > +    /// copied).
> > +    #[doc(alias = "strncpy_from_user")]
> > +    pub fn strcpy_into_buf<'buf>(self, buf: &'buf mut [u8]) -> Result<&'buf CStr> {
> > +        if buf.is_empty() {
> > +            return Err(EINVAL);
> > +        }
> > +
> > +        // 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(dst, self.ptr)?;
> > +        if len < dst.len() {
> > +            // Add one to include the NUL-terminator.
> > +            len += 1;
> > +        } else if len < buf.len() {
> > +            // This implies that `len == dst.len() < buf.len()`.
> > +            //
> > +            // This means that we could not fill the entire buffer, but we had to stop reading
> > +            // because we hit the `self.length` limit of this `UserSliceReader`. Since we did not
> > +            // fill the buffer, we treat this case as if we tried to read past the `self.length`
> > +            // limit and received a page fault, which is consistent with other `UserSliceReader`
> > +            // methods that also return page faults when you exceed `self.length`.
> > +            return Err(EFAULT);
> > +        } else {
> > +            // This implies that `len == buf.len()`.
> > +            //
> > +            // This means that we filled the buffer exactly. In this case, we add a NUL-terminator
> > +            // and return it. Unlike the `len < dst.len()` branch, don't modify `len` because it
> > +            // already represents the length including the NUL-terminator.
> > +            //
> > +            // SAFETY: Due to the check at the beginning, the buffer is not empty.
> > +            unsafe { *buf.last_mut().unwrap_unchecked() = 0 };
>
> What about the case `self.length == 0`? Will `raw_strncpy_from_user`
> return early with a page fault, or will it return with `len == 0`?
> Because if it is the latter, then this will result in UB.

If `self.length == 0`, then you will either:
1. If buf.is_empty() then you return EINVAL at the top.
2. Otherwise, you return EFAULT from the `len < buf.len()` case.

Alice