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

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Message-ID: <myt35uyg7v3pwlck6zd643dzjezfrc2d4t66kn7gdzxwjhysdo@64cprpjpbjlh>
Date: Mon, 27 Oct 2025 17:56:47 +0530
From: Manivannan Sadhasivam <mani@...nel.org>
To: Bjorn Helgaas <helgaas@...nel.org>, 
	Nathan Chancellor <nathan@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, 
	Bjorn Helgaas <helgaas@...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>, 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 03:25:24PM -0700, Nathan Chancellor wrote:
> On Tue, Oct 14, 2025 at 09:47:28PM +0200, Peter Zijlstra wrote:
> > 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.
> 
> Yeah and I had thought about changing of_device_get_match_data() to use
> something like ERR_PTR() in the !OF case but that does not fix the issue
> since the call destination is still going to be invalid. To me, this is
> just one of the sharp edges of compile testing: you give up some code or
> configuration cleanliness/expectations for the flexibility of build
> testing.
> 
> > > 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.
> 
> Yeah, we are lucky that this is all LLVM does with this code.
> 
> For what it's worth, there is plenty of "depends on OF" that appears
> across the tree and I see exactly one instance that has a comment above
> it (none with the comment on the same line). Perhaps these are all
> historical if there was a point where stubs were not provided for !OF. I
> do not mind adding a comment if really so desired but this driver does
> not do anything without CONFIG_OF so it feels like the dependency is
> natural anyways.
> 

@Bjorn: Do you still want to add a comment for the dependency?

- Mani

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