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
