Re: [PATCH] uaccess: rust: add strncpy_from_user

[Alice Ryhl <aliceryhl@google.com>]

On Thu, Apr 24, 2025 at 08:57:13AM -0700, Boqun Feng wrote:
> On Thu, Apr 24, 2025 at 03:17:48PM +0000, Alice Ryhl wrote:
> > This is needed for ioctls that operate on a user-provided string.
> > 
> > It is somewhat unfortunate that strncpy_from_user does not nul-terminate
> > the string when the end of `buf` is reached. This implies that we can't
> > return a &CStr from the function, since the buffer may not always be
> > nul-terminated.
> > 
> > That said, we could add more convenient helpers on top that add a NUL
> > byte in that case.
> > 
> > This method isn't defined on UserSliceReader because it complicates the
> > semantics. The UserSliceReader type also has its own maximum length, so
> > we would have to limit the read by that length too.
> > 
> > Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> > ---
> >  rust/kernel/uaccess.rs | 27 +++++++++++++++++++++++++++
> >  1 file changed, 27 insertions(+)
> > 
> > diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
> > index 80a9782b1c6e98ed6eae308ade8551afa7adc188..1bd82045e81ea887008e30241bd6de27f096b639 100644
> > --- a/rust/kernel/uaccess.rs
> > +++ b/rust/kernel/uaccess.rs
> > @@ -369,3 +369,30 @@ pub fn write<T: AsBytes>(&mut self, value: &T) -> Result {
> >          Ok(())
> >      }
> >  }
> > +
> > +/// Reads a nul-terminated string into `buf` and returns the length.
> > +///
> > +/// Fails with [`EFAULT`] if the read happens on a bad address. If the end of `buf` is reached,
> > +/// then the buffer will not be nul-terminated.
> > +#[inline]
> > +pub fn strncpy_from_user(ptr: UserPtr, buf: &mut [u8]) -> Result<usize> {
> 
> Sorry maybe there is an email I'm missing, but could you provide more
> context of the usage?
> 
> First the function name is a bit weird, because the 'n' in "strncpy"
> means the parameters should have an 'n' (i.e. length) in it, but there
> is none in the Rust version.

There is a length! It's the length of `buf`. It's pretty normal that C
methods with a pointer and length become a Rust method with a slice.

The distinction between strcpy and strncpy in my eyes is that strcpy
reads until you find a NUL byte, whereas strncpy reads until you find a
NUL byte *or* you read a user-specified number of bytes. This method is
in the latter category.

> Also, we don't need to replicate the
> semantics of C here, we could just do a strncpy_from_user(..., ..., len
> - 1), where `len` is the len of the `buf`, and then we would always have
> a nul-terminated string. But maybe I'm missing something from the usage
> side, so this is not doable?

Well, I guess that probably does make sense. I'll update this to always
add a nul-terminator.

Alice