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: > >>>>> +/// # Examples > >>>>> +/// > >>>>> +/// ``` > >>>>> +/// let b = KBox::<u64>::new(24_u64, GFP_KERNEL)?; > >>>>> +/// > >>>>> +/// assert_eq!(*b, 24_u64); > >>>>> +/// # Ok::<(), Error>(()) > >>>>> +/// ``` > >>>>> +/// > >>>>> +/// ``` > >>>>> +/// # use kernel::bindings; > >>>>> +/// const SIZE: usize = bindings::KMALLOC_MAX_SIZE as usize + 1; > >>>>> +/// struct Huge([u8; SIZE]); > >>>>> +/// > >>>>> +/// assert!(KBox::<Huge>::new_uninit(GFP_KERNEL | __GFP_NOWARN).is_err()); > >>>>> +/// ``` > >>>> > >>>> It would be nice if you could add something like "KBox can't handle big > >>>> allocations:" above this example, so that people aren't confused why > >>>> this example expects an error. > >>> > >>> I don't think that's needed, it's implied by > >>> `SIZE == bindings::KMALLOC_MAX_SIZE + 1`. > >>> > >>> Surely, we could add it nevertheless, but it's not very precise to just say "big > >>> allocations". And I think this isn't the place for lengthy explanations of > >>> `Kmalloc` behavior. > >> > >> Fair point, nevertheless I find examples a bit more useful, when the > >> intention behind them is not only given as code. > >> > >>>>> +/// > >>>>> +/// ``` > >>>>> +/// # 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. > > >> #[repr(align(64))] > >> struct Token; > >> > >> fn main() { > >> let t = 64 as *mut Token; > >> let t = unsafe { t.read() }; // this is fine. > >> let t = 4 as *mut Token; > >> let t = unsafe { t.read() }; // this is UB, see below for miri's output > >> } > >> > >> Miri complains: > >> > >> error: Undefined Behavior: accessing memory based on pointer with alignment 4, but alignment 64 is required > >> --> src/main.rs:8:22 > >> | > >> 8 | let t = unsafe { t.read() }; // this is UB, see below for miri's output > >> | ^^^^^^^^ accessing memory based on pointer with alignment 4, but alignment 64 is required > >> | > >> = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior > >> = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information > >> = note: BACKTRACE: > >> = note: inside `main` at src/main.rs:8:22: 8:30 > > > > `read` explicitly asks for non-null and properly aligned even if `T` has size > > zero. I mentioned this because for `read` it's explicitly documented. However, the nomicon also says "This is possibly needless pedantry because ptr::read is a noop for a ZST, [...]". > > Dereferencing (ie `*t`) also requires that (I just didn't do it, since > then the `Token` must implement `Copy`). Again, how do you know? The documentation isn't clear about it. > > --- > Cheers, > Benno >
