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

[Benno Lossin]

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.

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

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

-- 
Cheers,
Benno