Message-ID: <ec80d964-2062-4556-a11b-ba764e1f58f7@proton.me>
Date: Sat, 24 Aug 2024 13:27:07 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Matthew Maurer <mmaurer@...gle.com>, Sami Tolvanen <samitolvanen@...gle.com>, Masahiro Yamada <masahiroy@...nel.org>, Luis Chamberlain <mcgrof@...nel.org>, Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Wedson Almeida Filho <wedsonaf@...il.com>, Gary Guo <gary@...yguo.net>, Petr Pavlu <petr.pavlu@...e.com>, Neal Gompa <neal@...pa.dev>, Hector Martin <marcan@...can.st>, Janne Grunau <j@...nau.net>, Asahi Linux <asahi@...ts.linux.dev>, linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org, linux-modules@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v2 16/19] gendwarfksyms: Add support for reserved structure fields

On 23.08.24 01:53, Greg Kroah-Hartman 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?

That's what I interpreted from the links you provided above, there are
multiple invocations of `ANDROID_KABI_RESERVE` and I figured they each
can be used to insert a new field. Or can you replace each one by as
many fields as you want, as long as the size is still fine?

> All we really want to do here is "pad out this structure by X bytes" and
> then later "take X bytes to represent this variable" at a later point in
> time.
> 
> Surely rust can do that, right?  :)

Not with all the other things that you need. I feel like this discussion
is dragging a bit on, so we will just ask the Rust folks if they have
any suggestions and if they don't we will ask for a solution. We can
then get back to this when that's done.
It's not like we need this immediately.

>>> All of this also needs to be possible in any structures that are
>>> exported by rust code if vendors want to have a way to track and ensure
>>> that abis do not change over time, just like they can today in C code.
>>
>> All of those structs need to be `repr(C)`, otherwise they don't
>> have a stable layout to begin with.
> 
> Do we have any way to enforce at build time that exports from rust code
> are in this format to ensure that this will work properly going forward?
> I guess someone is going to have to write the first api in rust that
> actually gets used before we worry about this...

I don't know if we already have a way, but we will need one if people
start writing kabi in Rust.

---
Cheers,
Benno

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