Re: [PATCH v2 16/19] gendwarfksyms: Add support for reserved structure fields

[Sami Tolvanen]

On Mon, Aug 19, 2024 at 10:17 PM Benno Lossin <benno.lossin@xxxxxxxxx> wrote:
>
> On 19.08.24 21:38, Sami Tolvanen wrote:
> >
> > This definitely looks cleaner than unions in Rust, but how would this
> > scheme be visible in DWARF? You might also need to expand the annotation
> > to allow replacing one reserved field with multiple smaller ones without
> > using structs.
>
> Hmm that's a good question, I have no idea how DWARF works. The way you
> do it in this patch is just by the name of the field, right?

Correct, it just looks at the name of the union fields.

> If Rust's DWARF output contains exact types names (I just checked this,
> I *think* that this is the case, but I have never used/seen DWARF
> before), we might be able to just create a `KAbiReserved<T, R>` type
> that you search for instead of the attribute. The usage would then be
> like this:
>
>     #[repr(C)]
>     pub struct Struct1 {
>         a: u64,
>         _reserved: KAbiReserved<(), u64>,
>     }
>
> And then when adding a new field, you would do this:
>
>     #[repr(C)]
>     pub struct Struct1 {
>         a: u64,
>         b: KAbiReserved<Struct2, u64>,
>     }
>
>     /* Struct2 as above */
>
> The way `KAbiReserved` is implemented is via a `union` (maybe a bit
> ironic, considering what I said in my other replies, but in this case,
> we would provide a safe abstraction over this `union`, thus avoiding
> exposing users of this type to `unsafe`):
>
>     #[repr(C)]
>     pub union KAbiReserved<T, R> {
>         value: T,
>         _reserved: R,
>     }

I like this approach even better, assuming any remaining issues with
ownership etc. can be sorted out. This would also look identical to
the C version in DWARF if you rename _reserved in the union to
__kabi_reserved. Of course, we can always change gendwarfksyms to
support a different scheme for Rust code if a better solution comes
along later.

Sami