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

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Message-ID: <udmorf6ait6y2xeplz2wfid3xnyga3ahwv7gd7dl46c456auq3@54ym6kszzudy>
Date: Wed, 4 Jun 2025 17:36:15 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Peter Zijlstra <peterz@...radead.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 Mon, May 26, 2025 at 08:22:59PM +0200, Peter Zijlstra wrote:
> 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?

For now, I want to stick with -ffunction-sections -fdata-sections, as
that's what kpatch has done for 10+ years and it works well.  That's the
only option we have for current compilers anyway.

The above mentioned possibility of diffing without -ffunction-sections
-fdata-sections is theoretical, and needs more exploration.  If it
indeed works then we can try to get toolchain support for that.

-- 
Josh