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

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Message-ID: <p6mkebfvhxvtqyz6mtohm2ko3nqe2zdawkgbfi6h2rfv2gxbuz@ktixvjaj44en>
Date: Sat, 3 May 2025 11:28:37 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Sean Christopherson <seanjc@...gle.com>, 
	"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, 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 Sat, May 03, 2025 at 11:50:23AM +0200, Peter Zijlstra wrote:
> > +++ b/arch/x86/entry/entry_64_fred.S
> > @@ -116,7 +116,8 @@ SYM_FUNC_START(asm_fred_entry_from_kvm)
> >  	movq %rsp, %rdi				/* %rdi -> pt_regs */
> >  	call __fred_entry_from_kvm		/* Call the C entry point */
> >  	POP_REGS
> > -	ERETS
> > +
> > +	ALTERNATIVE "", __stringify(ERETS), X86_FEATURE_FRED
> >  1:
> >  	/*
> >  	 * Objtool doesn't understand what ERETS does, this hint tells it that
> > @@ -124,7 +125,7 @@ SYM_FUNC_START(asm_fred_entry_from_kvm)
> >  	 * isn't strictly needed, but it's the simplest form.
> >  	 */
> >  	UNWIND_HINT_RESTORE
> > -	pop %rbp
> > +	leave
> >  	RET
> 
> So this, while clever, might be a problem with ORC unwinding. Because
> now the stack is different depending on the alternative, and we can't
> deal with that.
> 
> Anyway, I'll go have a poke on Monday (or Tuesday if Monday turns out to
> be a bank holiday :-).

Can we just adjust the stack in the alternative?

	ALTERNATIVE "add $64 %rsp", __stringify(ERETS), X86_FEATURE_FRED
1:
	UNWIND_HINT_RESTORE
	pop %rbp
	RET

-- 
Josh