linux-kernel - Re: [PATCH 2/4] media: atomisp: Fix startup() section placement with -ffunction-sections

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Message-ID: <20251118085711.GL3245006@noisy.programming.kicks-ass.net>
Date: Tue, 18 Nov 2025 09:57:11 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
	live-patching@...r.kernel.org, Hans de Goede <hansg@...nel.org>,
	Mauro Carvalho Chehab <mchehab@...nel.org>,
	linux-toolchains@...r.kernel.org
Subject: Re: [PATCH 2/4] media: atomisp: Fix startup() section placement with
 -ffunction-sections

On Fri, Nov 14, 2025 at 12:43:10PM -0800, Josh Poimboeuf wrote:
> On Fri, Nov 14, 2025 at 09:56:57AM +0100, Peter Zijlstra wrote:
> > On Wed, Nov 12, 2025 at 03:47:49PM -0800, Josh Poimboeuf wrote:
> > > When compiling the kernel with -ffunction-sections (e.g., for LTO,
> > > livepatch, dead code elimination, AutoFDO, or Propeller), the startup()
> > > function gets compiled into the .text.startup section.  In some cases it
> > > can even be cloned into .text.startup.constprop.0 or
> > > .text.startup.isra.0.
> > > 
> > > However, the .text.startup and .text.startup.* section names are already
> > > reserved for use by the compiler for __attribute__((constructor)) code.
> > > 
> > 
> > Urgh, that's a 'fun' one. Is this not a -ffunction-sections bug? I mean,
> > the compiler should never put regular non-reserved user symbols in a
> > section it has reserved for itself, right?
> 
> Right, so there's no ambiguity *IF* we know in advance whether it was
> compiled with -ffunction-sections.  If so, constructor code goes in
> .text.startup.*, and startup() goes in .text.startup or
> .text.startup.constprop.0 or .text.startup.isra.0.
> 
> So it's not really a compiler bug because it's possible to disambiguate
> those.
> 
> Problem is, we can have some objects compiled with -ffunction-sections,
> and some compiled without, in the same kernel.  So the disambiguation
> isn't possible at link time, since for example .text.startup could be
> startup() with -ffunction-sections, or it could be
> __attribute__((constructor)) without -ffunction-sections.
> 
> I attempted to describe all that in patch 4.

Egads, what a mess :-(