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

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Message-ID: <599892ec-3cf5-4349-984b-7c94f2ba5687@suse.com>
Date: Wed, 11 Sep 2024 13:43:46 +0200
From: Petr Pavlu <petr.pavlu@...e.com>
To: Sami Tolvanen <samitolvanen@...gle.com>, Miroslav Benes <mbenes@...e.cz>
Cc: Masahiro Yamada <masahiroy@...nel.org>,
 Luis Chamberlain <mcgrof@...nel.org>, Miguel Ojeda <ojeda@...nel.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.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

On 8/31/24 02:05, Sami Tolvanen wrote:
> On Fri, Aug 30, 2024 at 9:34 AM Miroslav Benes <mbenes@...e.cz> wrote:
>>
>> yes, this is one of the approaches we use in SLES. We add kabi paddings
>> to some structures in advance (see [1] as a random example) and then use
>> it later if needed.
>>
>> It is not the only approach. Much more often we do not have a padding and
>> use alignment holes ([5]), addition of a new member to the end of a
>> structure ([2] or [3]) and such "tricks" ([4] for a newly fully defined
>> structure).
> 
> Thanks for bringing this up! Sounds like we're also going to need a
> way to completely exclude specific fields from the output then. I
> think we can use a similar union approach, but instead of instructing
> the tool to use another type, we can just indicate that the field
> should be skipped. I'll come up with a solution for v3.

It might have been mentioned previously, not sure, but one more case to
consider is handling of enum declarations. New enumerators can be
typically added without breaking ABI, e.g. 'enum E { OLD1, OLD2, NEW }'.
It would be then great to have some ability to hide them from
gendwarfksyms.

I think neither of the __kabi_reserved or __gendwarfksyms_declonly
mechanism can currently help with that.

-- 
Thanks,
Petr