On 19.04.24 19:23, Boqun Feng wrote: > 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? What do you mean by "larger provenance"? >>>>> 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? Ultimately it again comes down to missing field projections :) The type `&UnsafeCell<[u8]>` is less *useful*, since you cannot even get the length of the slice. Also indexing into this type is not easily possible. This is because the only way to get/change the inner value of an `UnsafeCell` is via `get`. Compare this with the slice type. It allows getting the length, indexing into it (ie a form of field projections, if we consider slices as having a variable amount of fields). All those issues would be solved by (good) field projections. Field projections also give a reason for why using `&[UnsafeCell<u8>]` is not really different from `&UnsafeCell<[u8]>`: At any point in time we ought to be able to project `&UnsafeCell<[u8]> -> &[UnsafeCell<u8>]`. So it's fine to just use that from the get-go. >> 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? Good idea, do you want to do that, or should I do it? -- Cheers, Benno
