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

[Sami Tolvanen]

On Thu, Aug 22, 2024 at 11:53 PM Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, Aug 22, 2024 at 12:00:15PM +0000, Benno Lossin wrote:
> > > Here's one example in the android tree where 4 64bit fields are reserved
> > > for future abi changes:
> > >     https://android.googlesource.com/kernel/common/+/refs/heads/android12-5.10/include/linux/fs.h#421
> > >
> > > And here's a different place where a field is being used with many
> > > remaining for future use:
> > >     https://android.googlesource.com/kernel/common/+/refs/heads/android12-5.10/include/linux/sched.h#1379
> > >
> > > And also, we want/need lots of other space reservation at times, look at
> > > how "Others" can get access to reserved areas in structures that need to
> > > be done in an abi-safe way:
> > >     https://android.googlesource.com/kernel/common/+/refs/heads/android12-5.10/include/linux/sched.h#1375
> >
> > Let me correct myself, it's only possible to replace one `KAbiReserved`
> > by one new field. You can have as many fields of type `KAbiReserved` as
> > you want. The thing that you can't do is replace a single `KAbiReserved`
> > field by multiple (well you can, but then you have to change the sites
> > that use it).
>
> That's odd/foolish, why would that be the case?  Isn't that exactly what
> a union is for?  How are you going to know ahead of time what size types
> to save space for?

I believe Benno is referring to the lack of anonymous structures in
Rust. While you can replace a reserved field with a struct that
contains multiple smaller fields, you can't access the fields
transparently from the parent struct like you can in C:

    struct s { struct { u32 a; u32 b; }; };
    struct s s;
    s.a = 0;
    ...

It looks like nightly Rust does have some level of support for unnamed
fields in unions, but the implementation is not yet complete:

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=4f268d308fe6aa7a47566c7080c6e604

Benno, Matt, are you familiar with this feature?

Sami