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

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Message-ID: <2a3d4d7fco4szxyrw33lorkhckjq4styfsaljxxwd3v4o42i5z@qdavomj5i2mu>
Date: Fri, 14 Nov 2025 12:43:10 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Peter Zijlstra <peterz@...radead.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 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.

-- 
Josh