linux-kernel - Re: [PATCH v2] PCI: rcar-host: Add OF Kconfig dependency to avoid objtool no-cfi warning

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Message-ID: <20251014194728.GD1206438@noisy.programming.kicks-ass.net>
Date: Tue, 14 Oct 2025 21:47:28 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Bjorn Helgaas <helgaas@...nel.org>
Cc: Nathan Chancellor <nathan@...nel.org>,
	Marek Vasut <marek.vasut+renesas@...il.com>,
	Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Krzysztof Wilczy??ski <kwilczynski@...nel.org>,
	Manivannan Sadhasivam <mani@...nel.org>,
	Rob Herring <robh@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	Magnus Damm <magnus.damm@...il.com>, Kees Cook <kees@...nel.org>,
	linux-pci@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
	linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
	kernel test robot <lkp@...el.com>
Subject: Re: [PATCH v2] PCI: rcar-host: Add OF Kconfig dependency to avoid
 objtool no-cfi warning

On Tue, Oct 14, 2025 at 02:13:30PM -0500, Bjorn Helgaas wrote:

> Ugh.  This might be the best solution, but it's a bit problematic
> without a hint about why "depends on OF" is here.  Theoretically there
> are stubs for everything to make COMPILE_TEST work, so I think we're
> about to drop all the dependencies on OF.

Its those stubs are exactly the problem.

> This dependency to avoid a no-cfi warning looks like the kind of thing
> that could someday go away if the tools get smarter.  Maybe we can add
> a Kconfig comment here, but I don't really know enough to write one.
> Something like this?

Its not a CFI warning per-se, the compiler is hitting known UB
(unconditional NULL deref) and is currently emitting a NULL pointer
indirect call, but given how aggressive clang has been on encountering
UB it might just stop code-gen entirely and generate fall-through
warnings (been there done that).

Smarter compiler here is only going to make this worse.