linux-kernel - Re: [PATCH v2 52/62] objtool/klp: Introduce klp diff subcommand for diffing object files

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Message-ID: <20250526182259.GP24938@noisy.programming.kicks-ass.net>
Date: Mon, 26 May 2025 20:22:59 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
	Petr Mladek <pmladek@...e.com>, Miroslav Benes <mbenes@...e.cz>,
	Joe Lawrence <joe.lawrence@...hat.com>,
	live-patching@...r.kernel.org, Song Liu <song@...nel.org>,
	laokz <laokz@...mail.com>, Jiri Kosina <jikos@...nel.org>,
	Marcos Paulo de Souza <mpdesouza@...e.com>,
	Weinan Liu <wnliu@...gle.com>,
	Fazla Mehrab <a.mehrab@...edance.com>,
	Chen Zhongjin <chenzhongjin@...wei.com>,
	Puranjay Mohan <puranjay@...nel.org>
Subject: Re: [PATCH v2 52/62] objtool/klp: Introduce klp diff subcommand for
 diffing object files

On Fri, May 09, 2025 at 01:17:16PM -0700, Josh Poimboeuf wrote:

> Without '-ffunction-sections -fdata-sections', reliable object diffing
> would be infeasible due to toolchain limitations:
> 
>   - For intra-file+intra-section references, the compiler might
>     occasionally generated hard-coded instruction offsets instead of
>     relocations.
> 
>   - Section-symbol-based references can be ambiguous:
> 
>     - Overlapping or zero-length symbols create ambiguity as to which
>       symbol is being referenced.
> 
>     - A reference to the end of a symbol (e.g., checking array bounds)
>       can be misinterpreted as a reference to the next symbol, or vice
>       versa.
> 
> A potential future alternative to '-ffunction-sections -fdata-sections'
> would be to introduce a toolchain option that forces symbol-based
> (non-section) relocations.

Urgh.. So the first issue we can fix with objtool, but the ambiguous
cases are indeed very hard to fix up in post.

Did you already talk to toolchain people about this?