On Fri, Apr 19, 2024 at 07:24:31PM +0000, Benno Lossin wrote: > 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"? > Say you have a `&[UnsafeCell<u8>]` whose length is 64, what's the proper way to get a `*mut u8` (or any other pointer) has the provenance for the whole 64 bytes, so that you can pass it to a memcpy like function? "larger" means the size of the provenance is larger than u8. > >>>>> 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>]`. > Right, to me there is no significant difference between these two. Maybe because I'm full field projected minded ;-) > 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? > A way to get a larger provenance (explained above) is currently my only question, so if you think that's something reasonable to ask, i.e. nothing you know of can help. I will post a message there. Regards, Boqun > -- > Cheers, > Benno >
