Re: [PATCH v3 1/2] uaccess: rust: add strncpy_from_user

[Alice Ryhl <aliceryhl@google.com>]

On Mon, May 05, 2025 at 04:30:05PM +0200, Greg Kroah-Hartman wrote:
> On Mon, May 05, 2025 at 12:17:31PM +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.
> > 
> > Reviewed-by: Danilo Krummrich <dakr@kernel.org>
> > Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
> > Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> > ---
> >  rust/kernel/uaccess.rs | 35 ++++++++++++++++++++++++++++++++++-
> >  1 file changed, 34 insertions(+), 1 deletion(-)
> > 
> > diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
> > index 80a9782b1c6e98ed6eae308ade8551afa7adc188..a7b123915e9aa2329f376d67cad93e2fc17ae017 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 +369,36 @@ 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 (some data may have been
> > +/// copied). 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.
> > +///
> > +/// # 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]
> > +#[expect(dead_code)]
> > +fn raw_strncpy_from_user(ptr: UserPtr, buf: &mut [MaybeUninit<u8>]) -> Result<usize> {
> 
> Nit, the parameters here are backwards from the C version of
> strncpy_from_user(), which is going to cause us no end of grief when
> reviewing code between the two languages :(

I'll swap them.

fn raw_strncpy_from_user(dst: &mut [MaybeUninit<u8>], src: UserPtr) -> Result<usize> {

> Also, it's not your fault, but we don't have any type of __user tag for
> data coming from userspace yet to track this type of thing?  The
> compiler (well sparse) can catch this type of thing in C, any hints on
> what we could do in Rust for the same type of guarantee (i.e. don't
> touch user data before it's been copied, and then we need to treat it as
> "unverified" but that's a different patch series...)

The UserPtr typedef is intended to do that, but since it's only a
typedef to usize, the compiler won't detect it if you mix up a user
pointer with a length. (It will detect mix-ups with pointers since we
use an integer type for UserPtr.)

What we can do is replace the typedef with

#[repr(transparent)]
struct UserPtr(pub usize);

That way, it becomes it's own separate type (this is called the newtype
pattern [1]) so that it can't be mixed up with anything else.

The #[repr(transparent)] annotation makes the compiler treat it like a
bare long for ABI-purposes. I'm not sure if any function ABIs actually
treat a long differently from a struct that just contains a long, but if
such ABIs exist, then the annotation ensures that the long ABI is used
rather than the struct-containing-long ABI.

Alice

[1]: https://doc.rust-lang.org/rust-by-example/generics/new_types.html