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

[Boqun Feng]

On Fri, Apr 19, 2024 at 08:36:11AM +0000, Benno Lossin wrote:
> On 19.04.24 01:04, Boqun Feng wrote:
> > On Thu, Apr 18, 2024 at 03:56:11PM -0700, Boqun Feng wrote:
> >> 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.
> 
> There is `to_raw_parts` [1], but that is unstable. (Note that
> `<[T] as Pointee>::Metadata = usize`, see [2])
> 

Oh, that's good to know, thank you! ;-)

> >>
> > 
> > Hmm... but I guess one of the problems of this approach, is how to
> > construct a `&UnsafeCell<[u8]>` from a pointer and length...
> 
> We could use `from_raw_parts` [3]. But when making the slice the outer
> type, we can use a stable function to convert a pointer and a length to
> a slice [4].
> 

Yes, but there appears no way to get a pointer with larger provenance
from a `&[UnsafeCell<u8>]`, right?

> > 
> > Regards,
> > Boqun
> > 
> >>> Another question would be if page allows for uninitialized bits, in that
> >>> case, we would need `&[Opaque<u8>]`.
> >>>
> >>
> >> Yes, or `&Opaque<[u8>]`.
> 
> I don't think that putting the slice on the inside is what we want. Also

Hmm.. why? So in `&UnsafeCell<[u8]>` vs `&[UnsafeCell<u8>]` case, I
think the former represent "a slice of u8 that can be modified in the
same time" very well, and this is what a pointer-and-length pair usually
represents in kernel, I think. But yes, the latter is OK to me as well,
just hard to play the provenance game I guess?

> note that `Opaque<T>` requires that `T: Sized` and that is not the case
> for `[u8]`.

Oh, you're right. In case of MaybeUninit, it requires `T: Sized`, so
`Opaque<[u8]>` doesn't quite work.

Moving forward, maybe the first step is to see whether `&[Opaque<u8>]`
and `&[UnsafeCell<u8>]` can have a good way to generate a pointer with
proper provenance? Time to ping t-opsem maybe?

Regards,
Boqun

> 
> [1]: https://doc.rust-lang.org/nightly/core/primitive.pointer.html#method.to_raw_parts
> [2]: https://doc.rust-lang.org/nightly/core/ptr/trait.Pointee.html#pointer-metadata
> [3]: https://doc.rust-lang.org/nightly/core/ptr/fn.from_raw_parts.html
> [4]: https://doc.rust-lang.org/nightly/core/slice/fn.from_raw_parts.html
> 
> -- 
> Cheers,
> Benno
>