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
