linux-kernel - Re: [PATCH v2 00/13] objtool: Detect and warn about indirect calls in _

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Message-ID: <aBUiwLV4ZY2HdRbz@google.com>
Date: Fri, 2 May 2025 12:53:36 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org, kys@...rosoft.com, 
	haiyangz@...rosoft.com, wei.liu@...nel.org, decui@...rosoft.com, 
	tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, 
	dave.hansen@...ux.intel.com, pbonzini@...hat.com, ardb@...nel.org, 
	kees@...nel.org, Arnd Bergmann <arnd@...db.de>, gregkh@...uxfoundation.org, 
	jpoimboe@...nel.org, linux-hyperv@...r.kernel.org, 
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, linux-efi@...r.kernel.org, 
	samitolvanen@...gle.com, ojeda@...nel.org, xin@...or.com
Subject: Re: [PATCH v2 00/13] objtool: Detect and warn about indirect calls in
 __nocfi functions

On Fri, May 02, 2025, Peter Zijlstra wrote:
> On Thu, May 01, 2025 at 11:30:18AM -0700, Sean Christopherson wrote:
> 
> > Uh, aren't you making this way more complex than it needs to be? 
> 
> Possibly :-)
> 
> > IIUC, KVM never
> > uses the FRED hardware entry points, i.e. the FRED entry tables don't need to be
> > in place because they'll never be used.  The only bits of code KVM needs is the
> > __fred_entry_from_kvm() glue.
> 
> But __fred_entry_from_kvm() calls into fred_extint(), which then
> directly uses the fred sysvec_table[] for dispatch. How would we not
> have to set up that table?

I missed that the first time around.  From my self-reply:

 : Hrm, and now I see that fred_extint() relies on fred_install_sysvec(), which makes
 : me quite curious as to why IRQs didn't go sideways.  Oh, because sysvec_table[]
 : is statically defined at compile time except for PV crud.
 : 
 : So yeah, I think my the patches are correct, they just the need a small bit of
 : prep work to support dynamic setup of sysvec_table.

> > Lightly tested, but this combo works for IRQs and NMIs on non-FRED hardware.
> 
> So the FRED NMI code is significantly different from the IDT NMI code
> and I really didn't want to go mixing those.
> 
> If we get a nested NMI I don't think it'll behave well.

Ah, because FRED hardwware doesn't have the crazy NMI unblocking behavior, and
the FRED NMI entry code relies on that.

But I don't see why we need to care about NMIs from KVM, while they do bounce
through assembly to create a stack frame, the actual CALL is direct to
asm_exc_nmi_kvm_vmx().  If it's just the unwind hint that's needed, that

The attached patches handle the IRQ case and are passing my testing.

View attachment "0001-x86-fred-Install-system-vector-handlers-even-if-FRED.patch" of type "text/x-diff" (2456 bytes)

View attachment "0002-x86-fred-Play-nice-with-invoking-asm_fred_entry_from.patch" of type "text/x-diff" (1723 bytes)

View attachment "0003-x86-fred-KVM-VMX-Always-use-FRED-for-IRQs-when-CONFI.patch" of type "text/x-diff" (2777 bytes)