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

[Benno Lossin]

On 23.08.24 21:17, Sami Tolvanen wrote:
> 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?

No, thanks for pointing that out!

But this will run into the issue that field access for unions is
`unsafe`. So we can't really use it. I also tried to use our current
`KAbiReserved<T, R>` approach and using this as `T`:

    struct Foo {
        _: struct { a: u32, b: u32 }
    }

But that doesn't work.

---
Cheers,
Benno