Re: [PATCH v6 4/4] rust: add abstraction for `struct page`

[Boqun Feng]

On Thu, Apr 18, 2024 at 10:08:40PM +0000, Benno Lossin wrote:
> On 18.04.24 20:52, Boqun Feng wrote:
> > On Thu, Apr 18, 2024 at 08:59:20AM +0000, Alice Ryhl wrote:
> >> +    /// Runs a piece of code with a raw pointer to a slice of this page, with bounds checking.
> >> +    ///
> >> +    /// If `f` is called, then it will be called with a pointer that points at `off` bytes into the
> >> +    /// page, and the pointer will be valid for at least `len` bytes. The pointer is only valid on
> >> +    /// this task, as this method uses a local mapping.
> >> +    ///
> >> +    /// If `off` and `len` refers to a region outside of this page, then this method returns
> >> +    /// `EINVAL` and does not call `f`.
> >> +    ///
> >> +    /// # Using the raw pointer
> >> +    ///
> >> +    /// It is up to the caller to use the provided raw pointer correctly. The pointer is valid for
> >> +    /// `len` bytes and for the duration in which the closure is called. The pointer might only be
> >> +    /// mapped on the current thread, and when that is the case, dereferencing it on other threads
> >> +    /// is UB. Other than that, the usual rules for dereferencing a raw pointer apply: don't cause
> >> +    /// data races, the memory may be uninitialized, and so on.
> >> +    ///
> >> +    /// If multiple threads map the same page at the same time, then they may reference with
> >> +    /// different addresses. However, even if the addresses are different, the underlying memory is
> >> +    /// still the same for these purposes (e.g., it's still a data race if they both write to the
> >> +    /// same underlying byte at the same time).
> >> +    fn with_pointer_into_page<T>(
> >> +        &self,
> >> +        off: usize,
> >> +        len: usize,
> >> +        f: impl FnOnce(*mut u8) -> Result<T>,
> > 
> > I wonder whether the way to go here is making this function signature:
> > 
> >      fn with_slice_in_page<T> (
> >          &self,
> > 	       off: usize,
> > 	       len: usize,
> > 	       f: iml FnOnce(&UnsafeCell<[u8]>) -> Result<T>
> >      ) -> Result<T>
> > 
> > , because in this way, it makes a bit more clear that what memory that
> > `f` can access, in other words, the users are less likely to use the
> > pointer in a wrong way.
> > 
> > But that depends on whether `&UnsafeCell<[u8]>` is the correct
> > abstraction and the ecosystem around it: for example, I feel like these
> > two functions:
> > 
> > 	    fn len(slice: &UnsafeCell<[u8]>) -> usize
> > 	    fn as_ptr(slice: &UnsafeCell<[u8]>) -> *mut u8
> > 
> > should be trivially safe, but I might be wrong. Again this is just for
> > future discussion.
> 
> I think the "better" type would be `&[UnsafeCell<u8>]`. Since there you
> can always access the length.
> 

Hmm.. here is the thing, having `&UnsafeCell<[u8]>` means having a `*mut
[u8]>`, and it should always be safe to get a "length" of `*mut [u8]`,
right? I haven't found any method doing that, but the length should be
just a part of fat pointer, so I think getting that is a defined
behavior. But maybe I'm missing something.

> Another question would be if page allows for uninitialized bits, in that
> case, we would need `&[Opaque<u8>]`.
> 

Yes, or `&Opaque<[u8>]`.

Regards,
Boqun

> But I don't remember how to get a valid raw pointer from
> `&[UnsafeCell<u8>]`.
> 
> -- 
> Cheers,
> Benno
>