Re: [PATCH v2 00/13] objtool: Detect and warn about indirect calls in __nocfi functions

[Peter Zijlstra]

On Fri, May 02, 2025 at 12:53:36PM -0700, Sean Christopherson wrote:

> > 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.

Just so.

> 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

That part is all fine.


>  arch/x86/entry/entry_64_fred.S | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/entry/entry_64_fred.S b/arch/x86/entry/entry_64_fred.S
> index 29c5c32c16c3..7aff2f0a285f 100644
> --- a/arch/x86/entry/entry_64_fred.S
> +++ 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 :-).