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

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Message-ID: <20240819193851.GA4809@google.com>
Date: Mon, 19 Aug 2024 19:38:51 +0000
From: Sami Tolvanen <samitolvanen@...gle.com>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Masahiro Yamada <masahiroy@...nel.org>,
	Luis Chamberlain <mcgrof@...nel.org>,
	Miguel Ojeda <ojeda@...nel.org>,
	Matthew Maurer <mmaurer@...gle.com>,
	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

Hi Benno,

On Sat, Aug 17, 2024 at 01:19:55PM +0000, Benno Lossin wrote:
> 
> For this use-case (the one in the patch), I don't really know if we want
> to copy the approach from C. Do we even support exporting kABI from
> Rust? If yes, then we I would recommend we tag it in the source code
> instead of using a union. Here the example from the patch adapted:
> 
>     #[repr(C)] // needed for layout stability
>     pub struct Struct1 {
>         a: u64,
>         #[kabi_reserved(u64)] // this marker is new
>         _reserved: u64,
>     }
> 
> And then to use the reserved field, you would do this:
>     
>     #[repr(C)]
>     pub struct Struct1 {
>         a: u64,
>         #[kabi_reserved(u64)]
>         b: Struct2,
>     }
> 
>     #[repr(C)]
>     pub struct Struct2 {
>         b: i32,
>         v: i32,
>     }
> 
> The attribute would check that the size of the two types match and
> gendwarfksyms would use the type given in "()" instead of the actual
> type.

This definitely looks cleaner than unions in Rust, but how would this
scheme be visible in DWARF? You might also need to expand the annotation
to allow replacing one reserved field with multiple smaller ones without
using structs.

Sami