On Wed, Sep 11, 2024 at 3:26 PM Benno Lossin <benno.lossin@xxxxxxxxx> wrote: > > On 11.09.24 13:02, Danilo Krummrich wrote: > > On Wed, Sep 11, 2024 at 08:36:38AM +0000, Benno Lossin wrote: > >> On 11.09.24 01:25, Danilo Krummrich wrote: > >>> On Tue, Sep 10, 2024 at 07:49:42PM +0000, Benno Lossin wrote: > >>>> On 10.09.24 19:40, Danilo Krummrich wrote: > >>>>> On Sat, Aug 31, 2024 at 05:39:07AM +0000, Benno Lossin wrote: > >>>>>> On 16.08.24 02:10, Danilo Krummrich wrote: > >>>>>>> +/// > >>>>>>> +/// ``` > >>>>>>> +/// # use kernel::bindings; > >>>>>>> +/// const SIZE: usize = bindings::KMALLOC_MAX_SIZE as usize + 1; > >>>>>>> +/// struct Huge([u8; SIZE]); > >>>>>>> +/// > >>>>>>> +/// assert!(KVBox::<Huge>::new_uninit(GFP_KERNEL).is_ok()); > >>>>>>> +/// ``` > >>>>>> > >>>>>> Similarly, you could then say above this one "Instead use either `VBox` > >>>>>> or `KVBox`:" > >>>>>> > >>>>>>> +/// > >>>>>>> +/// # Invariants > >>>>>>> +/// > >>>>>>> +/// The [`Box`]' pointer is always properly aligned and either points to memory allocated with `A` > >>>>>> > >>>>>> Please use `self.0` instead of "[`Box`]'". > >>>>>> > >>>>>>> +/// or, for zero-sized types, is a dangling pointer. > >>>>>> > >>>>>> Probably "dangling, well aligned pointer.". > >>>>> > >>>>> Does this add any value? For ZSTs everything is "well aligned", isn't it? > >>>> > >>>> ZSTs can have alignment and then unaligned pointers do exist for them > >>>> (and dereferencing them is UB!): > >>> > >>> Where is this documented? The documentation says: > >>> > >>> "For operations of size zero, *every* pointer is valid, including the null > >>> pointer. The following points are only concerned with non-zero-sized accesses." > >>> [1] > >> > >> That's a good point, the documentation looks a bit outdated. I found > >> this page in the nomicon: https://doc.rust-lang.org/nomicon/vec/vec-zsts.html > >> The first iterator implementation has an alignment issue. (Nevertheless, > >> that chapter of the nomicon is probably useful to you, since it goes > >> over implementing `Vec`, but maybe you already saw it) > >> > >>> [1] https://doc.rust-lang.org/std/ptr/index.html > >> > >> Might be a good idea to improve/complain about this at the rust project. > > > > Well, my point is how do we know? There's no language specification and the > > documentation is (at least) ambiguous. > > So I went through the unsafe-coding-guidelines issues list and only > found this one: https://github.com/rust-lang/unsafe-code-guidelines/issues/93 > Maybe I missed something. You could also try to ask at the rust zulip in > the t-opsem channel for further clarification. > > I think we should just be on the safe side and assume that ZSTs require > alignment. But if you get a convincing answer and if they say that they > will document it, then I don't mind removing the alignment requirement. Please see the section on alignment in the same page. Just because a pointer is valid does not mean that it is properly aligned.
