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 That's a good point, the documentation looks a bit outdated.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. --- Cheers, Benno
