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
