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 >
