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 >
